Contents

CHAPTER III

Mechanism of Wounding¹

E. Newton Harvey, Ph. D., J. Howard McMillen, Ph. D., Elmer G. Butler,
Ph. D., and William O. Puckett, Ph. D.

HISTORICAL NOTE

Pictures of rifle bullets in rapid flight have always aroused interest and admiration-interest from the resemblance to moving ships with prominent bow and stern waves and a turbulent wake; admiration that so rapid a movement can be stopped in a photograph and the detail of events clearly visualized. Since the first spark pictures of moving bullets in air, obtained by Mach² in 1887 and Boys³ in 1893, a mass of information has been gathered on trajectories, stability, spin, yaw, and precession of projectiles. This field of inquiry is usually classified as exterior ballistics to distinguish it from what happens within the gun, or interior ballistics.

The events which occur when a bullet strikes and enters the body have received much less attention-in part, owing to the rapidity of changes which take place in an opaque medium and the difficulty of measuring them and, in part, to the complexity of the body and the feeling that few significant generalizations could be made regarding it. Actually, the changes which occur when a high-velocity bullet enters soft tissue are remarkably independent of body structure, and a common series of events can be outlined. The recent technical development of high-speed cameras that can take moving pictures at the rate of 8,000 frames a second and an X-ray apparatus that requires only one-millionth of a second for exposure have eliminated the previous barriers to understanding the mechanism of wounding. It is now possible to analyze events that are all over in a few thousandths of a second.

¹The research on which this chapter is based was carried out under a contract, recommended by the Committee on Medical Research, between the Office of Scientific Research and Development and Princeton University. Work under this contract began on 15 October 1943 and continued to 1 November 1945. On the latter date, the contract was transferred to the Office of the Surgeon General. The work was brought to completion on 28 February 1946. All of the research was conducted in the Biological Laboratories of Princeton University, Princeton, N. J. It is important to record here that the success of the work has been due in great measure to the wholehearted cooperation of the professionally and technically trained persons who, at one time or another, were members of the "Wound Ballistics Research Group." In addition to the authors of this chapter, the following persons took part in the investigation: Mr. Delafield DuBois, Mr. Joseph C. Gonzalez, Mr. Vincent Gregg, Dr. Harry Grundfest, Mr. James J. Hay, Dr. William Kleinberg, Dr. Irvin M. Korr, Mr. Daniel B. Leyerle, Dr. William D. McElroy, Mr. John R. Mycock, Dr. Gerald Oster, Mr. R. G. Stoner, Miss Mary Jane Thompson, Mr. Harold A. Towne, and Dr. Arthur H. Whiteley.
²Mach, von E., and Salcher, P.: Photographische Fixirung der durch Projectile in der Luft eingeleiteten Vorgänge. Der Kais. Acad. der Wiss. zu Wein, 1887 and 1889. (Also in Nature, London 42: 250-251, 1890.)
³Boys, C. V.: On Electric Spark Photographs; or Photography of Flying Bullets, etc., by the Lights of the Electric Spark. Nature, London 47: 415-421, 440-446, 1893.

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Thus, a new field of inquiry has arisen, that of wound ballistics, a study of the mechanics of wounding and related subjects. The field has two aspects. One is a determination of the factors involved in injury and the relation between the severity of the wound and such characteristics of the missile as its mass, velocity, shape, momentum, energy, and power. The attempt is made to relate the ability to wound or to kill with some physical property of the projectile. Such inquiry gives an answer to the question, whether an antipersonnel bomb is more effective if it breaks into a large number of small fragments or a smaller number of relatively large fragments.

The second aspect of wound ballistics involves a study of the nature of the damage to tissues, whether it results from stretching and displacement or from pressure changes accompanying the shot. Of particular interest is the commonly observed injury of organs far away from the bullet path. Such knowledge greatly aids the surgeon in his treatment of the wound and is necessary for the establishment of rules for removal of dead tissue and the amount of debridement necessary for proper recovery. The knowledge of wound ballistics is, therefore, important not only in offense but also in defense.

With the perfection of guns that could shoot high-velocity missiles came the observation that the resulting wounds appeared as though they had been caused by an actual explosion within the body. External signs of injury were often slight, the entrance and exit holes small, but an unbelievable amount of damage occurred within. Hugier (cited by Horsley⁴) noted this explosive effect as early as 1848 in Paris, and it has been emphasized by all subsequent writers. Such action has led to mutual accusation by both sides in warfare that the enemy was using explosive bullets. Not only is the tissue pulped within a large region about the bullet path but intact nerves lose their ability to conduct impulses and bones are found to be broken that have not suffered a direct hit.

It is in this explosive effect that high-velocity missiles differ from those of low velocity. The wounds from a spear or a nearly spent revolver bullet correspond more closely to the expected cylinder of disintegrated tissue, little larger than the spear itself. This type of wound can be compared to what happens when a rod is plunged into soft snow. Snow piles up in front and is pushed ahead and to the side, and when the rod is withdrawn a hole is left whose diameter is little more than that of the rod. The situation is far different with high-velocity missiles. They leave behind a large temporary cavity whose behavior is quite comparable to the gas bubble of an underwater explosion. Later, the cavity collapses, but far-reaching destructive effects have occurred during the expansion. A detailed description of what happens during the cavity formation will be found in this chapter.

Much of the early work on wounding was concerned with an explanation of the explosive effect of high-velocity projectiles. Shots were made into various materials, such as gelatin gel or dough, which served as models to

⁴Horsley, V.: The Destructive Effect of Small Projectiles. Nature, London 50: 104-108, 1894.

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explain what must happen in the body. Kocher (1874-76) at Berne, Switzerland, was a pioneer in this study, which he rightly thought was a hydrodynamic problem. Delorme and Chevasse⁵ in Paris, Bruns⁶ (1892) in Germany, and Horsley in England continued the work.

In 1898, Stevenson⁷ brought out his monograph "Wounds in War," to be followed by La Garde's⁸ "Gunshot Injuries" and by Wilson's⁹ account of casualties during World War I. The monumental "Lehrbuch von Ballistik" by Cranz and Becker,¹⁰ now in its fifth edition, first appeared in 1910. In addition to a valuable description of the small arms in use by various nations at the time of publication, these books consider the theories which have been advanced to explain the explosive effect of bullets.

One of the earliest views was that the "wind" of the bullet (that is, its shock wave), or the air compressed on the face of the bullet, was responsible for the explosion. It is quite certain that this view is incorrect since the explosive effects appear if a mass of flesh is shot in a vacuum. Neither can the explosive effect be connected with the shock wave which appears when tissue is hit, since this wave moves through the tissue at the rate of 4,800 f.p.s. (feet per second) and has passed well beyond the wound region before the explosive expansion occurs.

It is a simple matter also to eliminate such theories as invoke rotation of the bullet, flattening of the bullet, or heating of tissues by the bullet as the cause of the explosion. Steel spheres shot from a smoothbore rifle which do not rotate and do not flatten on impact are known to cause explosive effects. Moreover, the kinetic energy of these spheres is not sufficient, even if all were converted into the energy of steam, to account for the explosion.

There remains, as the correct explanation of the explosive cavity, what early workers called the accelerated particle theory. This view regards the energy of the bullet as being transferred to the soft tissue in front and to each side, thus imparting momentum to these tissue particles, so that they rapidly move away from the bullet path, thus acting like "secondary missiles." Once set in motion, the "inertia of the fluid particles" continues its motion, and a large space or cavity is left behind. As Stevenson puts it, the bullet causes damage not only by crushing and attrition of tissue directly but also indirectly by the fluids moving away from its path. Wilson¹¹ compares this "blasting out" of soft tissues to the effect of the stream of water from a firehose.

Later work has been largely concerned with special aspects of wound

⁵Delorme, E., and Chevasse, Prof.: Étude Comparative des Éffets Produits Par les Balles du Fusil Gras de 11 mm et du Fusil Lebel de 8 mm. Arch. d. Med. et Pharm. Mil. 17: 81-112, 1892.
⁶Bruns, Paul: Ueber die Kriegschirurgische Bedeutung der Neuen Feuerwaffen. Berlin: August Hirschwald, 1892.
⁷Stevenson, W. F.: Wounds in War. New York: Wm. Wood and Co., 1898.
⁸La Garde, L. A.: Gunshot Injuries. 2d ed. New York: Wm. Wood and Co., 1916.
⁹Wilson, Louis B.: Firearms and Projectiles; Their Bearing on Wound Production. In The Medical Department of the U.S. Army in the World War. Washington: Government Printing Office, 1927, vol. XI, pt. 1, pp. 9-56.
¹⁰Cranz, C., and Becker, K.: Handbook of Ballistics. Vol. I, Exterior Ballistics. Translated from 2d German ed. London: His Majesty's Stationery Office, 1921, pp. 442-450.
¹¹Wilson, L. B.: Dispersion of Bullet Energy in Relation to Wound Effects. Mil. Surgeon 49: 241-251, 1921.

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ballistics. Callender and French¹² and Callender¹³ used Plasticine as a model for tissues and studied especially the yaw of bullets and the relation of wound damage to the power delivered. They introduced more modern methods of measuring velocities and also obtained records of the pressure changes during the passage of a bullet through Plasticine.

Black, Burns, and Zuckerman¹⁴ have described the enormous damage done by minute fragments of metal from bombbursts. These fragments move with velocities far higher than those of ordinary rifle bullets. Using the spark shadowgraph method and steel spheres, weighing only 53 mg., they were able to imitate the destructive effect of bomb splinters and obtained spark shadow outlines of rabbit legs during passage of the missile. These shadowgrams indicated a large swelling due to the cavity within.

The present work¹⁵ is an attempt to place wound ballistics on a sound quantitative basis. It regards the phenomena observed in wounding of soft tissue as fundamentally like the phenomena which occur when a high-velocity missile enters a liquid. The subject is treated as a branch of underwater ballistics. By means of high-speed motion pictures, spark shadowgrams, and microsecond roentgenograms, measurements have been made of all the changes occurring during passage of a projectile through various parts of the body, and certain constants have been established relating mass, velocity, shape, and other characteristics of the missile to wound phenomena. By means of these constants, it is now possible to predict exactly what damage may be expected from the impact of a known mass moving with any known velocity. The data on which this survey is based are given in later sections, together with reproductions of the photographs and roentgenograms.

The basic purpose of a study of wounding is to obtain data with which to predict the degree of incapacitation (the weeks of hospitalization) which may result from a hit by a missile of given mass (M) moving with a given velocity (V). The incapacitation will naturally depend on the region of the body which is struck. This region in turn will depend on the tactical situation, for example, trench or open warfare, as determined by the military command, which must also decide the length of hospitalization permissible. The probability of a hit is thus a function of the projected body areas exposed. The probable time of hospitalization will vary with the severity of the wound for a

¹²Callender, G. R., and French, R. W.: Wound Ballistics: Studies on the Mechanism of Wound Production by Rifle Bullets. Mil. Surgeon 77: 177-201, 1935.
¹³Callender, G. R.: Wound Ballistics: Mechanism of Production of Wounds by Small Arms Bullets and Shell Fragments. War Med. 3: 337-350, 1943.
¹⁴Black, A. N., Burns, B. D., and Zuckerman, S.: An Experimental Study of the Wounding Mechanism of High Velocity Missiles. Brit. M. J. 2: 872-874, 1941. ¹⁵Among the difficult problems encountered during the investigation, particularly in its early stages, was that of assembling under the stress of wartime conditions necessary apparatus and supplies. The beginning of the work would have long been delayed had it not been for the generous loan of equipment by the Ballistics Research Laboratory of the Aberdeen Proving Ground, Aberdeen, Md., and the continued cooperation of members of the staff of this laboratory. We are indebted, also, to the Frankford Arsenal, Philadelphia, Pa., for the loan, until our own equipment was available, of a surge generator, which was essential for the taking of microsecond roentgenograms. Certain items of apparatus originally constructed at the Climatic Research Laboratory of the Signal Corps, Fort Monmouth, N.J., was also made available on loan. The staff of the Princeton University Section, Division 2, National Defense Research Committee, had aided greatly throughout the investigation, both with advice and in respect to securing promptly the needed equipment.-Authors' Note.

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particular region and can best be estimated by a military surgeon with considerable field experience. With such knowledge, effectiveness of antipersonnel bombs in terms of casualties can be accurately evaluated, since the distribution of fragment masses and their velocities at various distances from the explosion can be readily determined. This chapter, however, does not propose to estimate time of hospitalization as a result of wounds received from any specific weapon but rather to determine the basic laws governing damage to the various tissues in the body.

METHODS USED IN STUDYING WOUNDING

Army rifles are designed to shoot a 9.6-gram bullet with a velocity of 2,700 f.p.s. and to incapacitate or kill a human target weighing approximately 70 kg. (kilograms). To investigate directly the mechanism of wounding on such a scale would require many large animals and an extensive firing range for the experiments. It is far more economical and fully as instructive to reduce the size of missile and target in proportion. The investigation can then be carried out in any laboratory. For example, a 0.4-gram missile moving 2,700 f.p.s. and striking a 3-kg. animal represents a situation, so far as mass of missile and mass of target are concerned, analogous to those of standard army rifle ammunition and the human body. Therefore, deeply anesthetized cats and dogs have been used for study with steel spheres as missiles (table 25). Fragments of varied shape and corresponding mass and velocity have also been studied.

To supplement the direct experiments on animals, it is highly instructive to study nonliving models. These models simplify the physical conditions and serve to illustrate what can happen in a homogeneous medium. Blocks of gelatin gel, rubber tubes filled with a liquid, or a tank, with Plexiglas sides, filled with water served as targets to record the phenomena connected with the passage of high-velocity missiles. The tank of water, particularly, allows high-speed photography and  complete analysis of all that happens.

Table 25.-TABLE 25.-Data on steel spheres

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FIGURE 47.-Smoothbore .30 caliber gun mounted in front of wooden sabot screen. The apparatus for spark shadowgram method of velocity measurement is at right and part of an impact record at left.

FIGURE 48.-Wooden sabots used to carry a ³/16-inch steel sphere (above) and a ¹/16-inch steel sphere (below). At right is the sabot inserted in a .30 caliber army shell.

FIGURE 49.-General view of the tank of water with lights (behind), sabot screen (top), and high-speed motion picture camera (front) for study of phenomena during a shot into a liquid. The gun pointing vertically downward is attached to a beam above the tank. The bright spot of light on the left side of the front of the tank is a sodium lamp running on 60 cycle a.c. which records ¹/120-second intervals in the film.

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Since many wounds in modern warfare come from steel bomb fragments of small size but of high velocity, a gun was selected which could be used for shooting either fragments or spheres of a mass around 1 gram or less. The gun was a standard caliber .30 Winchester smoothbore which was proof shot with pressures of 65,000 to 68,000 pounds per square inch (fig. 47). The fragment or sphere was carried in a depression in the front of a cylindrical wood sabot about 16 mm. long, split in half longitudinally, and lathe turned to fit the caliber .30 Army standard primed shell (fig. 48). The wooden sabot was satisfactory except for very high velocities (velocities in excess of 3,800 f.p.s.), when it pulverized. In such instances, a similar Textolite plastic sabot was substituted. When the missile emerged from the gun, air resistance separated the two halves of the sabot. These halves were caught by a wooden screen with a hole in the center through which the missile could pass. The shells were filled with fast-burning, 60 mm. mortar powder which was adequate for the sabot and fragments. Variations of velocity were obtained by varying the powder charge from 0.1 gram (1,120 f.p.s.) to 1 gram (4,430 f.p.s.). If care was taken in fitting the sabot, variations in the velocities showed a percentage deviation of only 2.4 for a given powder charge. Figure 49 shows a vertical gun above a water tank with Plexiglas sides to permit high-speed motion picture photography. The lights used for illumination are to the left and the camera to the right.

The velocity of missiles is fairly constant for a given charge of powder, provided the sabots are carefully made to give uniform fit in the ends of the shells. This statement has been checked by three different methods of measuring velocity. One method makes use of the shock wave of the missile in air. This shock wave is allowed to impinge on a metal plate containing a row of small holes. On passing through the holes, the shock wave is converted into a series of sound waves whose shadow is recorded on a photographic plate by a spark discharge. The velocity of the missile is equal to the velocity of sound in air, divided by the sine of the angle between the envelope of sound wave fronts emerging from the holes and the path of the missile.

The well-known Aberdeen chronograph was also used to measure the velocity. This instrument records, on a strip of paper fixed to a drum rotating at a known speed, the time taken by the missile to pass between two stations.

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As the missile passes each station (two tinfoil sheets), a contact is made, thereby triggering a spark which perforates the revolving paper. The time interval can then be read as distance between the two perforations.

The third instrument used for recording velocities was the Remington chronoscope. This also necessitates two trigger screens. When the bullet passes one screen, a condenser begins to charge from a source of voltage and when the second screen is passed charging is stopped. The electrical charge on the condenser then represents a certain time interval which the missile has taken to pass between the stations and can be read with a ballistic galvanometer.

High-speed moving pictures were taken either with the Western Electric 8 mm. Fastax camera (fig. 50 A), capable of 8,000 frames per second, or with the Eastman 16 mm. high-speed camera (fig. 50 B), capable of 3,000 frames per second. Both cameras use the optical compensation principle, in which the film moves across the lens continuously and a rotating prism throws successive images on the film with the same speed as the film itself. Trigger devices were necessary to fire the gun at the proper moment by means of an electromagnet, as a 100-ft. roll of 16 mm. film takes only 1.5 seconds to pass across the lens. Time intervals were recorded by photographing a sodium lamp running on 60 cycles a.c. (alternating current). For illumination, banks of 2 to 12 150-watt projection spotlights, run on 220 volts instead of the rated 110 volts, were used. The light of these bulbs was directed on the object or, for transmitted light, illuminated evenly a ground glass plate placed on the rear wall of the tank.

The spark shadowgraph technique for shock wave recording depends upon a change in refractive index of the medium resulting from a change in pressure. The change in refractive index can be detected on a photographic plate as a shadow, if a point source of light is used for illumination. The point source of light used for high-velocity missiles in water was a high-voltage spark from the discharge of a condenser (fig. 51). The spark, whose duration is less than a millionth of a second, is about 5 feet in front of the tank of water through which the missile will pass, and the photographic plate is on the rear wall of the tank. When the bullet breaks a contact in a screen, the spark is triggered through a thyratron controlled high-voltage surge across the spark gap. By means of a delay circuit, any time interval after the breaking of the screen can be selected for the spark shadowgram.

For taking roentgenograms with an exposure of a millionth of a second, the Westinghouse X-ray surge generator, or Micronex, was used. This apparatus requires a special X-ray tube, with a large tungsten target and a cold cathode. The discharge of a bank of condensers through the tube supplies the current of thousands of amperes, lasting less than a microsecond. Voltage can be varied from 180 to 360 kv. (kilovolt), by charging the six condensers (each of 0.04 microfarad capacity) in parallel at 30 to 60 kv. and then discharging in series. A control box makes operation automatic, and a trigger and delay circuit times the X-ray surge for any desired moment, measured in microseconds. The entire outfit is shown in figure 52.

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FIGURE 50.-High-speed motion picture cameras. A. Fastax 8 mm. motion picture camera with cover removed to show film looped over sprocket behind rotating prism. The arrow points to a neon lamp which can be used for timing. Two wire grid trigger screens are below camera and a thyratron trigger circuit is at right. B. Eastman 16 mm. high-speed motion picture camera with cover removed to show film reels and rotating prism.

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FIGURE 51.-Apparatus for spark shadowgram technique. The water tank with photographic plate behind it is at right. The spark electrodes are in the cylindrical tube (9 cm. long) above the high-voltage condenser at the left. Mounted on the same platform are the high-voltage transformer, rectifying tube, and accessory parts.

For accurately recording pressure changes in an animal, a calibrated piezoelectric tourmaline crystal was used. As a result of changes in pressure, the crystal develops an electrical charge which can be amplified and applied to a cathode ray oscillograph with a single sweep. The phosphorescence of the electron beam on the face of the oscillograph is then photographed. Trigger screens in the proper position before the target were used to start the sweep, whose duration was varied between 130 microseconds and 45 milliseconds. The time calibration was made with a sine wave oscillator. Great precautions must be taken to shield the circuits from electrical and mechanical disturbances which might cause artefacts in the record.

UNDERWATER BALLISTICS AS A GUIDE TO THE WOUNDING

MECHANISM

In order to predict the severity of a wound, it is necessary to know what happens when a missile enters the body. The missile's retardation and penetration must be determined and all other phenomena measured quantitatively and related to its mass and impact velocity. Since the material of the body is heterogeneous and opaque, the investigation would be greatly simplified if a homogeneous transparent medium could be substituted and used as a model for the establishment of fundamental laws.

Fortunately, this can be done. The nature of the forces which act on a moving missile will depend on its velocity. For fast missiles, such as have been used in this investigation, these forces are chiefly inertial forces. They depend

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FIGURE 52.-Westinghouse Micronex apparatus for X-ray pictures with exposure of one-millionth of a second. The X-ray tube is at left and the large surge generator containing banks of high-voltage condensers in the middle. In front of the surge generator (from left to right) is the trigger-delay circuit, the control box, and the high-voltage transformer.

primarily on the density of the medium rather than on its viscosity or its structure. Except where there are very strong structural bonds, as in bone, ballistic laws for soft tissue must be similar to those for a liquid or a gel.

Most soft tissues contain about 80 percent water, and it has been found that many of the important events in wounding can be reproduced by shooting into a tank of water. Such a shot is pictured in figures 53 and 54, frames from a high-speed moving picture of a steel sphere entering water with a velocity of approximately 3,000 f.p.s. The large explosive temporary cavity is initially cone shaped but later becomes more spherical and pulsates several times before subsiding to a mass of air bubbles. The cavity behind a sphere shot into water elongates as the sphere proceeds through the water. It also expands radially and then shrinks. Along the narrow neck of the cavity not far behind the sphere, the cavity eventually collapses, creating two cavities. The smaller cavity continues to trail behind the sphere, while the larger one begins to pulsate. The time at which the cavity separation or sealing off takes place

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FIGURE 53.-Frames (1,920 per second) from a high-speed motion picture of a³/16-inch steel sphere entering water with a velocity of 3,160 feet per second. The surface of the water is at the top of each frame and the depth of water 55 cm. Note the initial expansion and later contraction of the temporary cavity to minimum volume at frame 25, with subsequent expansion. (Experiment No. 4, of 4 Mar. 1944.)

depends on the size and density of the bullet. After the cavity behind the sphere separates, the larger main cavity moves slowly in the direction of the sphere. As it pulls away from the surface, a narrow neck develops between it and the surface. The neck soon disintegrates leaving the cavity completely isolated. The isolated cavity continues in slow motion in the direction of the sphere and eventually disintegrates. During all of this process, the cavity undergoes a series of pulsations and grows and shrinks in a regular manner. The pulsations may continue for as many as 7 or 8 cycles and disappear as the cavity disintegrates.

The velocity of radial movement of the water away from the sphere track is about one-tenth that of the sphere velocity. The maximum displacement of

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FIGURE 54.-Frames (1,920 per second) from a high-speed motion picture of a ¹/8-inch steel sphere (velocity 2,300 f.p.s.) striking the surface of water to show the splash and cavity formation. The dots at left are 5 cm. apart. (Experiment No. 191, of 23 Mar. 1945.)

the cavity wall is proportional to the square root of the kinetic energy of the sphere at any level, and the maximum volume of the explosive cavity is determined by the initial kinetic energy of the sphere. This is expressed as an expansion coefficient which gives the volume of cavity formed for each unit of energy and is equal to 8.92 X 10^-7 cc./erg. for water. The period of the first few pulsations of the temporary cavity depends on the cube root of the missile energy and can be expressed numerically (pp. 181-189).

A gel behaves like water, as is illustrated in the frames from a high-speed moving picture of a ¹/8-inch steel sphere entering 20 percent gelatin gel with a velocity of 3,800 f.p.s. (fig. 55). The phenomena are nearly the same, even to the splash, although the numerical values of the constants are different. In addition, there is left in gelatin a permanent cavity or track, which is also observed in tissues. The volume of this permanent cavity can be expressed by an excavation coefficient, which gives the volume of cavity formed for each unit of missile energy. The behavior of a rectangular block of gelatin is shown in figure 56.

Rapid retardation of the sphere can be observed in figures 53 and 54, where the tip of the cavity represents the progress of the sphere in equal units of time. This retardation is proportional to the square of the velocity of the sphere, a general law for liquids expressed as a retardation coefficient, a. If the material or size of spheres differ, the various quantities are related in the following way: a= rAC_D/2M, where C_D is the drag coefficient, r the density of the

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FIGURE 55.-Frames (6,000 per second) from a motion picture of a ¹/8-inch steel sphere entering a tank of 20 percent gelatin gel with a velocity of 3,800 feet per second. The scale marks at left are 5 cm. apart. Note the splash and cavity formation which is quite similar to that of water. The permanent cavity of a previous shot shows at right as a vertical line. Frames are numbered below. Temperature: 24° C. (Experiment No. 8G, of 12 June 1944.)

FIGURE 56.-Frames (6,000 per second) from a high-speed motion picture of a gelatin block shot from right to left with a ¹/8-inch steel sphere moving 3,800 feet per second. Note the expansion of the block and the two bubblelike protuberances at entrance and exit sites. The bubbles collapse; then the entrance bubble reappears (frames 18 to 35) and again collapses. The background squares are centimeters. (Reel 2, 31 Dec. 1943.)

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liquid, M the mass, and A the sphere projected cross-sectional area. For water C_D=0.297 and for 20 percent gelatin at 24° C., C_D=0.350.

If the missile is a fragment instead of a sphere, the projected area will change as the fragment turns. Hence, the velocity in the water will vary in an irregular manner. The retardation coefficient, the drag coefficient, and the energy delivered to the water will all differ during the advance of the fragment. Turning of the fragment thus leads to the formation of irregular temporary cavities, as shown in figure 57. The cavity is widest when a fragment moves broadside and smallest when the movement is head on.

The velocity squared law holds for spheres in water until the velocity becomes very small. It is difficult to speak of a penetration distance in water. In a gel, however, after decrease to a certain critical velocity V_c, another retardation law is obeyed. Structural bonds and viscous forces quickly bring the sphere to a stop at a definite penetration distance (pp. 227-230).

The pressure on the front of a sphere moving through water is proportional to the square of the velocity V and is numerically equal to ½rV²C_D. For the shot illustrated in figure 53, the pressure at impact is about 1,500 atmospheres, and the water in front of the sphere is compressed and its refractive index changed. This region of compression at the surface of the water moves away as a spherical shock wave, with a velocity slightly greater than sound in water (4,800 f.p.s.). Spark shadowgrams showing the successive movements of the shock wave are reproduced in figure 58. Each wave consists of an instantaneous rise in pressure to a peak, with an approximately logarithmic fall behind. A pressure time curve for a shock wave is reproduced in figure 59. For the shock wave of figure 59, the peak pressure 10 cm. from the surface is 40 atmospheres and the half decay time about 30 microseconds. The peak intensity of a shock varies directly as the equare of the impact velocity and the projected area of the missile and inversely as the distance from the water surface; it is independent of the density of the missile. Shock waves are reflected from surfaces as either pressure or tension waves, depending on the wave velocity in the material and the density of the material.

Behind the shock wave, the pressure distribution in the water is complicated and continually changing. The very high pressure region in front of the sphere can be visualized by inspection of figure 60, a spark shadowgram of a 3/16-inch steel sphere moving in water behind a grid of lines on a Plexiglas plate. The distortion of the lines in front and at the sides of the sphere is due to a change of refractive index, resulting from compression of the water. Later on, much lower and slower pressure changes, with a phase of decreased pressure, appear around the temporary cavity. A record of these slower pressure changes connected with pulsation of the cavity is shown in figure 61 and the corresponding motion picture of the shot in figure 62.

All the events just cited-shock waves, cavity formation, movements of the medium, and pressure changes-occur when a high-velocity sphere enters soft parts of the body. A retardation coefficient, a drag coefficient, and ex-

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FIGURE 57.-Irregular temporary cavities formed in water by fragments which rotate during penetration. A. A cylinder striking broadside. B. A disk striking broadside. C. A cylinder striking head on, then turning broadside, and finally head on before slowing down. Note that the width of cavity at any level reflects the projection area of the cylinder. Similar cavities occur in tissues when fragments penetrate. Scale marks are 5 cm. apart.

pansion coefficient (of the temporary explosive cavity) and an excavation coefficient (of the permanent cavity) can all be given numerical values.

Among tissues, the numerical constants vary slightly. They differ somewhat from those of water or gel because (1) tissues vary greatly in structural makeup and (2) the body is enclosed in a layer of elastic muscle and skin, rather than the fairly rigid walls of a tank, as in the case of experiments with liquid mediums. Wound ballistics is actually a special branch of underwater ballistics. The remarkable similarity of the phenomena in tissues and in water will be brought out in the following sections.

THE WOUND TRACK OR PERMANENT CAVITY IN MUSCLE

The passage of a high-velocity missile through soft tissues results in the immediate formation of an explosive or temporary cavity many times larger than the missile. After the passage of the missile, the large temporary cavity decreases in volume and a much smaller permanent cavity remains. The size of the permanent cavity is undoubtedly governed by the size of the temporary cavity, which, in turn, is dependent on the size of the missile, as well as on the nature of the tissues involved.

Small, high-velocity steel spheres passing through soft tissue, such as the thigh of a cat, produce rather small entrance and exit holes (fig. 63). The entrance hole produced by a ⁴/32-inch steel sphere striking the thigh with a velocity of 3,000 f.p.s. is shown in figure 63A. The exit hole made by this same

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FIGURE 58.-A series (S68, S31, S71, S90, and S21) of spark shadowgrams of ¹/8-inch spheres taken at successively longer time intervals after the sphere has hit the water surface. Note how the shock wave, moving 4,800 f.p.s. leaves the retarded sphere behind. The striking velocity in all shadowgrams is 3,000 f.p.s. except in the second where it is 1,772 f.p.s.

FIGURE 59.-A pressure-time record of a shock wave resulting from impact on the surface of water of a ³/16-inch steel sphere moving 3,000 f.p.s. The crystal gage was 6 inches from the point of impact, at a 45° angle with the missile path. The time marks are 20 microseconds apart. The peak pressure is 600 pounds per square inch. (Experiment No. 41g, July 1945.)

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FIGURE 60.-Enlargement of a spark shadowgram of the region around a 3/16-inch steel sphere (impact velocity 3,800 f.p.s.) which has penetrated 5 cm. of water. The high pressure near the missile is revealed by the distortion of a 1 mm. grid placed between spark and sphere. (Experiment No. P105.)

FIGURE 61.-A record of pressure changes in a tank of water during penetration of a 3/16-inch steel sphere with a striking velocity of 3,800 f.p.s. The first peak at left is that of the shock wave pressure. The first, second, and third peaks corresponding to three minimum cavity volumes are well marked. Note the subatmospheric pressures below the baseline. The high-frequency (3,000 per second) pressure excursions in the first third of the record are due to vibrations of the steel frame of the tank. One vertical division represents a pressure of 13.1 pounds per square inch. The time record is 1000 cycles. (Experiment No. 3, of 8 Jan. 1946.)

FIGURE 62.-Frames (2,120 per second) from a high-speed motion picture of a 3/16-inch steel sphere entering a tank of water with a velocity of 3,000 f.p.s. The formation behind the sphere of a large cone-shaped temporary cavity which later becomes nearly spherical and pulsates is apparent. Frames are numbered at top and bottom. The first maximum volume is in frame 12, the first minimum in frame 23; the second maximum is in frame 36 and the second minimum in frame 47. The dots to the left are 5 cm. apart. The pressure record for this particular shot is reproduced in figure 61. The crystal gage is visible at the end of the slightly curved line at right of each frame. (Experiment No. 3, of 8 Jan. 1946.)

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sphere is shown in figure 63B. In general, exit holes produced by spheres are smaller than entrance holes, because of the decreased velocity of the sphere after it has traversed the thigh. In many cases, the exit hole in muscle is slitlike as contrasted with the circular entrance hole. This slitlike opening is due to the fact that the muscle fibers split apart along their long axes.

The size and configuration of the entrance and exit holes produced by an irregular fragment is dependent on the orientation of the fragment at the instant it enters or emerges from the tissues (fig. 64). The entrance hole made by a small elongate steel fragment (mass 612 mg.) which struck the thigh with a velocity of approximately 3,000 f.p.s. is shown in figure 64A. Yaw cards showed that the fragment struck the thigh broadside, inflicting a very large wound. Had the missile presented a smaller surface to the tissues at the time of impact, a much less severe wound of entrance would have resulted.

A microsecond roentgenogram showed that this same fragment emerged from the thigh oriented along its long axis. Hence, the exit hole is comparatively small, as is shown in figure 64B.

The approximate size and configuration of the wound track or permanent cavity can be determined in several ways. These include (1) roentgenograms of the tissue made immediately after each shot, (2) exploration and dissection of the wound, and (3) reconstruction of the cavity from thin (1-2 mm.) sections of the tissues.

Study of the wound track from roentgenograms (fig. 65) reveals that the permanent cavity formed by the passage of a steel sphere through the thigh is

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FIGURE 63.-Muscle of cat thigh with entrance and exit holes produced by a ⁴/32 inch steel sphere with a striking velocity of 3,000 feet per second. A. Entrance hole. B. Exit hole.

somewhat fusiform in shape, having its greatest diameter in the central portions of the thigh. This is illustrated by the roentgenogram shown in figure 65A.

This simple configuration of the permanent cavity is quite often modified by the fact that individual muscles are blown apart along fascial planes as a result of the passage of the missile. These newly created spaces tend to become a part of the permanent cavity and to give it an irregular pattern as shown in figure 65B.

This same type of fusiform cavity is produced when a small high-velocity steel sphere is fired through a block of 20 percent gelatin gel (fig. 66A). The permanent cavities formed by the passage of several ⁴/32-inch steel spheres through a block of gelatin gel are shown in figure 66B.

Dissection of the wound track in the thigh reveals that the permanent cavity is largest near the center of the thigh and smallest at the points of entrance and exit of the sphere. This fact is illustrated by the thigh shown in figure 67. Figure 67A shows the entrance hole in the thigh of a cat made by a ⁴/32-inch steel sphere which struck the thigh with a velocity of 3,800 f.p.s. Figure 67B shows the much larger cavity deeper in the tissues of this same thigh. These photographs demonstrate clearly that the small wound of entrance gives no true picture of the amount of damage produced deeper in the tissues.

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FIGURE 64.-Muscle of cat thigh with entrance and exit holes produced by a small steel fragment which struck the thigh broadside. The dimensions of the fragment were 14 x 5 x 2.5 mm.; the mass of the fragment, 612 mg. The impact velocity of the missile was approximately 3,000 feet per second. A. Entrance hole. B. Exit hole. The fragment emerged from the thigh oriented along its long axis.

The most exact method of determining the size and configuration of the permanent cavity is by a study of serial sections of the tissues cut in a plane at right angles to the path of the missile. A representative set of these sections, each approximately 2 mm. thick, is shown in figure 67C.

Study of a number of sets of serial sections reveals that the permanent cavity in the thigh actually consists of a series of fusiform cavities. This manner of cavity formation is related to the anatomy of the thigh muscles. It appears that as a sphere traverses the thigh a permanent fusiform cavity is formed in each of the larger muscles. The permanent cavity left in the intermuscular connective tissue is quite small, probably because of the elastic properties of this type of tissue. Thus, the permanent cavity or wound track in the thigh is really a series of fusiform cavities, individual muscles giving rise to what might be called a scalloped wound.

Essentially, this same type of behavior can be obtained by firing a high-velocity steel sphere through a series of three blocks of gelatin gel, separated by

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FIGURE 65.-Roentgenograms of thigh of cat showing permanent cavity (light area) left in the tissues after the passage of a ⁴/32-inch steel sphere whose impact velocity was 3,000 feet per second. A. Roentgenogram (No. 43) shows the fusiform-shaped permanent cavity.

B. Roentgenogram (No. 200) shows irregular shape of the cavity.

several sheets of cellophane to simulate the intermuscular fascia. The results of this experiment are shown in figure 68. The sphere passed from right to left in the photograph. This photograph, taken immediately after the shot, shows that fusiform cavities are formed in each block, the size of the cavity decreasing as the velocity of the sphere decreased from block to block. It is not proposed that the behavior of the gelatin block system is precisely identical with that of muscle and fascia, but the general characteristics of the cavities in the two cases are quite similar.

The shape and size of the temporary cavity is often modified by the fact that the cavity may come in contact with a rigid structure, such as bone. Then, as the large temporary cavity continues to expand, soft tissues are pulled away from the bone, and these tissues fail to regain their normal position after the collapse of the temporary cavity. This type of behavior is illustrated by the roentgenogram shown in figure 69.

The question of what becomes of the mass of tissues which originally occupied the site of the permanent cavity is a significant one. High-speed

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FIGURE 66.-Blocks of 20 percent gelatin gel. A. Block of 20 percent gelatin gel showing the permanent cavity left after the passage of a ⁴/32-inch steel sphere whose impact velocity was 3,000 f.p.s. Note the similarity of this cavity to that shown in the thigh in figure 65A. B. Block of 20 percent gelatin gel showing the fusiform permanent cavities left after the passage of several ⁴/32-inch steel spheres whose impact velocities were approximately 2,400 f.p.s.

motion pictures and spark shadowgrams show clearly that large amounts of material are lost to the outside during the passage of the missile. This is easily demonstrated by the spark shadowgrams shown in figure 70, of a high-velocity steel sphere passing into a tank of water. The penetration of the missile brings about a marked "splash" at the point of entrance, with the water moving backward at a high velocity. The splash which occurred at the point of exit of a ⁴/32-inch steel sphere in a block of Plasticine is shown in figure 71.

In cases where complete perforation of an object is obtained, large amounts of material are thrown out at both the points of entrance and exit of the sphere.

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FIGURE 67.-Photographs of soft tissues of the thigh of a cat. A. Relatively small entrance hole made by a ⁴/32-inch steel sphere which struck the thigh with a velocity of 3,800 feet per second. B. Tissue shown in A dissected open to show the much larger permanent wound cavity deeper in the tissues of the thigh. C. Serial sections of the soft tissues, cut in a plane at right angles to the path of the missile. Note the permanent cavity and the dark area around it filled with extravasated blood.

This is clearly shown in figure 72, a spark shadowgram of a block of gelatin gel taken immediately after the passage of a ⁴/32-inch steel sphere.

The situation in soft tissues of living animals appears to be very similar to that described for a gel. Figure 73 is a spark shadowgram of the thigh of a cat, taken immediately after the passage of a ⁴/32-inch steel sphere. A definite splash has occurred at the point of entrance of the missile, and materials are flying out at a high velocity. Large amounts of material are also being

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FIGURE 68.-Photograph of three blocks of 20 percent gelatin gel taken after the blocks were traversed by a ⁴/32-inch steel sphere whose initial impact velocity was 3,000 f.p.s. The sphere passed from right to left in the photograph. Note that fusiform cavities have formed in each block, the size of the cavity decreasing as the velocity of the sphere decreased from block to block.

swept out by the missile as it emerges at the left. The loss of materials at the points of entrance and exit of a missile can be demonstrated in shots through the abdomen and excised organs, such as the brain, liver, and kidneys.

THE EXPLOSIVE OR TEMPORARY CAVITY IN MUSCLE

A missile entering soft tissues at a relatively high velocity produces a temporary or explosive cavity of large dimensions. The cavity, at its maximum size, has a cross-sectional diameter many times that of the permanent cavity, which remains after the temporary cavity has collapsed. The temporary cavity persists for a relatively short time, reaching its maximum size in less than a millisecond and lasting for not more than several milliseconds.

The penetration of a small high-velocity steel sphere into a large mass of butcher meat results in the formation of an initially cone-shaped cavity, very similar to the cavity formed by the same type of missile in water (pp. 152-158). Figure 74A is a microsecond roentgenogram showing the large cavity formed in butcher meat by a ⁴/32-inch steel sphere which struck the meat with a velocity of 2,800 f.p.s. and had penetrated a distance of 10.2 cm. when the roentgenogram was made. The sphere eventually perforated the block of meat completely, so that this roentgenogram does not show the final configuration of the temporary cavity. Its chief value lies in demonstrating the striking similarity

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FIGURE 69.-Roentgenogram (No. 171) of the thigh of a cat taken immediately after the passage of a ⁴/32-inch steel sphere whose impact velocity was 3,000 f.p.s. Note the outlines of the permanent cavity (light area) and the manner in which it has "blown out" to the femur. Also note that the femur is fractured, although it was not struck by the sphere.

of the early cavity in animal tissue and that in water, shown by the microsecond roentgenogram in figure 74B.

The greatest mass of muscle in an intact animal is the thigh. In the largest dogs used in this study, the thigh was from 6 to 9 cm. in its greatest dimension. A single microsecond roentgenogram of a thigh can show only one particular stage in the development of the temporary cavity. However, by varying the interval between the time at which the missile struck the thigh and the time at which the roentgenogram was made, it is possible to obtain a series of pictures which together will show successive stages in the development of the cavity. A series of five such microsecond roentgenograms, showing the development of the cavity in the thighs of dogs, is shown in figure 75. In each case, the thigh was struck by a ⁴/32-inch steel sphere whose impact velocity was approximately 2,800 feet per second.

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FIGURE 70-Spark shadowgraph of a ⁴/32-inch steel sphere passing into a tank of water. Note the conical-shaped temporary cavity and the backward "splash" of water at the point where the sphere entered the water.

FIGURE 71.-Photograph of a block of Plasticine showing the cavity made by the passage of a ⁴/32-inch steel sphere whose impact velocity was 3,000 f.p.s. Compare size of cavity with size of the sphere placed in lower left of block. Note the large amount of material thrown out at the point of exit of the sphere. A similar "splash" also occurred at the point of entrance.

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FIGURE 72.-Spark shadowgraph of a rectangular block of 20 percent gelatin gel made immediately after the passage of a ⁴/32-inch steel sphere whose impact velocity was 2,800 f.p.s. The missile passed from right to left in the shadowgraph. Note the manner in which the block has expanded and the large amounts of material being thrown out at both the entrance and exit sites.

Figure 75A is a microsecond roentgenogram showing the temporary cavity 56 microseconds after the sphere struck the thigh. A cone-shaped cavity has formed behind the sphere, whose walls are relatively smooth. It is at this stage of development that the similarity of the temporary cavity in animal tissues and in water is the greatest.

Figure 75B shows the cavity 71 microseconds after the sphere struck the thigh. The sphere has emerged from the thigh and has moved several centimeters from it. The conical cavity is expanding, and its walls are becoming somewhat irregular.

The roentgenogram in figure 75C shows a cavity whose age is 139 microseconds. The sphere has now moved out of the field of the photograph to the right. The cone-shaped cavity has continued to expand, and its walls have become very irregular, probably owing to the irregular stretching and tearing of tissues being displaced by the cavity.

Figure 75D shows the cavity photographed 390 microseconds after the sphere struck the thigh. The cavity has expanded still more and has assumed the shape of a prolate ellipsoid. Observation of many of these cavities indicates that a cavity with this configuration is near its maximum size. The cavity shows marked irregularities on its walls, as well as strands of tissue of different densities, which can be interpreted as areas of stretched and torn tissues. The sphere which produced this cavity had an initial energy of 3.7 X 10⁸ ergs (35 ft.-lb.) and lost approximately 85 percent of this energy in producing the cavity.

Roentgenograms made from 600 to 800 microseconds after the sphere struck the thigh show that the cavity, after reaching its maximum size, col-

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FIGURE 73.-Spark shadowgraph of the thigh of a cat made immediately after the passage of a ⁴/32-inch steel sphere whose impact velocity was 3,000 f.p.s. The missile passed from right to left in the shadowgraph. Note the large amount of materials being ejected at both the points of entrance and exit of the missile. Note the similarity to the gelatin block in figure 72.

lapses. Figure 75E shows a cavity whose age is 819 microseconds. The cavity has practically collapsed, and only a small rounded space remains near the center of the thigh. High-speed motion pictures of the exterior of a thigh, such as those of figure 76, show the temporary swelling, indicative of the internal formation of this cavity.

The temporary cavity in the thigh of a cat, formed by the passage of a ⁴/32-inch steel sphere with an impact velocity of 2,800 f.p.s., is shown in figure 77A. Although this cavity has not reached its maximum size and the sphere did not strike the femur directly, a fracture line has appeared in this bone. Figure 77B is a roentgenogram of this same thigh made before the shot and figure 77C a similar roentgenogram made after the shot. In this latter picture, the permanent cavity is well outlined. This type of "indirect" fracture is dealt with in greater detail on pages 200-204.

All the temporary cavities just described were photographed to show the path of the missile and the cavity in lateral view. Other microsecond roentgenograms show that the cavity formed in soft tissues by a sphere is circular

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FIGURE 74.-Microsecond roentgenograms. A. Roentgenogram (No. 105) of a ⁴/32-inch steel sphere passing through a block of butcher meat. The sphere struck the meat with a velocity of 2,800 f.p.s. Compare with B and note the similarity of the cone-shaped temporary cavity to that formed in water. B. Roentgenogram (No. 25) of a ⁴/32-inch steel sphere passing into a container of water.

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FIGURE 75.-Microsecond roentgenograms of the thigh of a dog showing the temporary cavity formed by a ⁴/32-inch steel sphere whose impact velocity was 2,800 feet per second. A. Roentgenogram (No. 22) was made 56 microseconds after the sphere struck the thigh and shows the cone-shaped temporary cavity formed by the sphere. B. Roentgenogram (No. 18) was made 71 microseconds after the sphere struck the thigh. Note that the sphere has just emerged at the right and that the temporary cavity is expanding. C. Roentgenogram (No. 32) was made 139 microseconds after the sphere struck the thigh. Note the continued expansion of the cavity which results in a stretching and tearing of the tissues. D. Roentgenogram (No. 28) was made 390 microseconds after the sphere struck the thigh. The cavity has assumed the shape of a prolate ellipsoid and is judged to be at its maximum size. E. Roentgenogram (No. 31) was made approximately 819 microseconds after the sphere struck the thigh. Note that the cavity has practically collapsed and only a small cavity remains near the center of the thigh

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FIGURE 76.-A series of prints from a high-speed motion picture (4,500 frames per second) of the leg of an anesthetized cat, with skin intact, shot with a ¹/8-inch steel sphere moving 3,000 f.p.s. The sphere entered from the right and exited from the left side. The foot is up. The temporary swelling, indicating a large cavity within, can be clearly seen. (Reel 11, 6 Jan. 1944.)

when seen in cross section. The latter is well shown in the roentgenogram in figure 78, taken 200 microseconds after the sphere struck the thigh. The small black spot in the center of this photograph marks the point at which the sphere penetrated the X-ray film.

In the case of irregular fragments, the size and configuration of the temporary cavity depends not alone on the energy of the fragment but also on its projected area as it strikes the tissue. The projected area varies along the path of the missile as changes in orientation of the fragment occur. This is illustrated by the microsecond roentgenogram shown in figure 79. The thigh of a cat was struck by a small elongated fragment (originally part of a 75 mm. shell) whose mass was 630 mg. and whose impact velocity was 3,000 f.p.s. The fragment struck the thigh broadside and emerged with the orientation shown in this photograph. The cavity is very large at the point of entry and much smaller near the point of exit of the missile. The femur, struck directly by the missile, was badly shattered.

A second case is shown in figure 80, where a thigh was struck by an elongated fragment made from a small wire nail. The fragment was cylindrical, 11 mm. in length, 2.5 mm. in diameter, and had a mass of 380 mg. Its striking velocity was approximately 3,000 f.p.s. The irregular shape of this cavity indicates that the orientation of the fragment changed slightly as the missile passed through the tissues.

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FIGURE 77.-Roentgenograms of thigh of a cat made before and after thigh was struck by a ⁴/32-inch steel sphere whose impact velocity was 2,800 f.p.s. A. Roentgenogram made 170 microseconds after the shot. Note the expanding temporary cavity and the fracture line appearing in the femur, although this bone was not struck by the sphere. B. Roentgenogram (No. 135) made immediately before the shot. C. Roentgenogram made immediately after the shot. Note the permanent wound track (light areas), which indicate regions where muscles have been separated, and the fractured femur.

The temporary cavities produced by standard .22 caliber ammunition are very similar to those produced by spheres, as long as the bullet remains oriented on its long axis. This is illustrated by the roentgenogram in figure 81. If the bullet wobbles, or in any way changes its orientation, the result is similar to that just described for fragments.

A temporary cavity, very similar to those described in cat thighs, can be obtained by firing a steel sphere through the excised skin of a cat thigh which has been filled either with gelatin gel or with water. The cavity in a gelatin-filled skin is shown in figure 82A and in a water-filled skin in figure 82B. These photographs again emphasize the similarity of the temporary cavities in animal tissues and in the nonliving materials used.

Study and measurement of a large number of temporary cavities show that the total volume of the cavity is proportional to the energy delivered by the missile. Data obtained have made it possible to obtain a value for an expansion coefficient, k. The expansion coefficient, k, in muscle has a value of 80.1 x 10^-9 cm. ³/erg. This can be restated as follows: For every erg of energy lost by a missile in muscle, there is formed a temporary cavity with a volume of 80.1 x 10^-9 cm.³

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FIGURE 78.-Microsecond roentgenogram (No. 232) of the thigh of a cat showing the temporary cavity formed after the passage of a ⁴/32-inch steel sphere whose impact velocity was 3,000 f.p.s. The cavity age is 200 microseconds. The sphere was fired in a line parallel to the X-ray beam, and the temporary cavity (dark area) is seen in cross section. Note the approximately circular shape of the cavity.

The relationship of total cavity volume to energy expended can be demonstrated in another way. Steel spheres of two different masses (⁸/32-inch spheres, mass 1.04 gm., and ⁴/32-inch, mass 0.130 gm.) were fired through the thighs of cats. The striking velocities of the two spheres were adjusted so that each size of sphere would lose approximately the same amount of energy in passing through the tissues. The striking velocity of the ⁸/32-inch sphere was approximately 1,500 f.p.s.; that of the ⁴/32-inch sphere, 3,000 f.p.s. In cases where measured energy losses were approximately equal, the volumes of the temporary cavities produced by the two-sized spheres were likewise approximately equal. An illustration of this equality is shown in figure 83.

The formation of this high explosive cavity results in great displacement and tearing of muscle and connective tissues, rupture of small blood vessels, and stretching and compression of larger blood vessels and nerves. This behavior is sufficient to account for the very serious damage often observed in wounds at a considerable distance from the missile track. A more detailed description will be found on pages 189-200.

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FIGURE 79.-Microsecond roentgenogram (No. 277) of the thigh of a cat showing the temporary cavity formed by the passage of a small, irregular fragment of a 75 mm. shell. The fragment is seen emerging from the thigh at the right. Note the irregular shape of the temporary cavity and the fractured femur.

THE EXPLOSIVE OR TEMPORARY CAVITY IN ABDOMEN, THORAX, AND HEAD

Phenomena quite similar to those which have been discussed for muscle occur when a high-velocity missile enters the abdomen, the thorax, or the head. A temporary cavity, filled largely with water vapor, forms behind the projectile. After expanding to a certain volume, the cavity collapses. During the expansion, tissue is stretched and torn, and, following the pulsation and collapse of the cavity, tissue is violently pushed together with additional injury.

Although the general structural makeup of the abdomen is similar to that of muscle, the thorax and head are quite different. The thorax is largely air filled, because of the large volume occupied by the lungs. Its walls are also more rigid than are those of the abdomen, because of the supporting ribs. The head is made up of a brain, essentially liquid, enclosed in rigid cranial walls. The temporary cavity in thorax or head will, therefore, be modified by various secondary conditions, and the expansion coefficient can be expected to be quite different in the three regions.

The chief changes resulting from a shot through the abdomen of a deeply anesthetized cat are shown in figure 84. The two bulges of the temporary cavity on each side are apparent in frames 2 to 4. These bulges later collapse (frames 5 to 14) and then appear again (frame 15) as small, wrinkled projections

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FIGURE 80.-Microsecond roentgenogram (No. 264) of the thigh of a cat showing the temporary cavity formed by the passage of a small elongate section of a wire nail. Note the irregular shape of this cavity as contrasted with those formed by spheres.

which later merge with the general violent, twisting movements of the abdomen. A similar type of swelling, indicative of a large temporary cavity within, results from a shot through a rubber tube filled with water (fig. 85). The abdomen behaves like this model liquid system.

The large temporary cavity within the abdomen is revealed in the microsecond roentgenogram of figure 86, triggered just as the cavity is beginning to collapse, as indicated by the slight indentation on each side. In this figure and in figure 87, the intestine has been made radiopaque by barium sulfate. A smaller cavity in process of growth is shown in figure 87A, B, and C, which allows comparison of the abdomen before, during, and after the shot. The increased diameter of the intestine is readily apparent in the center microsecond roentgenogram, probably because of the flattening against the abdominal walls. Note that the barium sulfate has leaked out into the body cavity after the shot, indicating extensive perforation and damage to the intestine, a point corroborated by autopsy.

Microsecond roentgenograms, taken at a time when the second protuberances of frame 15 (fig. 84) have appeared, show no second internal cavity. The collapse of the initial temporary cavity seems to be complete. Since entrance and exit holes in the skin are small and a marked splash of material

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FIGURE 81.-Microsecond roentgenogram (No. 126) of the thigh of a small dog showing the conical-shaped temporary cavity formed by the passage of a .22 caliber long rifle bullet. Note the similarity of this cavity to that formed by a sphere as shown in figure 75.

has been observed to move out from each hole, it is very likely that little or no air can rush into the cavity. The cavity is filled mostly with water vapor, and consequently complete collapse will occur, with only a few small gas pockets undergoing pulsation. In this respect, a shot into the abdomen differs from a shot into a tank of water where the partially air filled temporary cavity (fig. 62) undergoes a series of marked pulsations. If a steel fragment instead of a sphere is shot through the abdomen, irregular temporary cavities appear (fig. 88).

During a shot through the thorax, very little movement is evident (fig. 89). The lack of movement is connected in part with the air-filled lungs, which do not fulfill conditions for cavity formation, and in part to the strong rib-reinforced walls of the thorax. In roentgenograms (fig. 90) giving views before, during, and after the shot, no clearly visible cavity is apparent. Because of the large amount of air in the lungs and the difficulty of distinguishing cavity from air, a clear-cut temporary cavity is hardly to be expected. It is apparent, however, that the heart has been displaced upward and to the right as a result of the shot, so that some type of temporary cavity is presumably formed.

The pressures which accompany a high-velocity missile moving through tissue are enormous (pp. 211-223). Therefore, it is not surprising to find that a steel sphere fired into the head can produce a temporary cavity in brain tissue, despite the apparent strength of the cranium which must resist the pressure. The cavity formed by a missile in the brain of an intact cranium is of finite size, partly because brain tissue is forced through regions of less resistance (such as the frontal sinuses and the various foramina of the skull) and partly because of the stretching of the cranium itself. When the energy delivered is very great, skull bones are actually torn apart along suture lines.

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FIGURE 82.-Microsecond roentgenograms of skin of cat thigh, showing temporary cavities formed after passage of a ⁴/32-inch steel sphere whose impact velocity was 2,800 feet per second. A. Roentgenogram (No. 147) shows cavity in skin filled with 20 percent gelatin gel. Note the similarity of this cavity to those formed in the thigh as shown in figures 75 and 77. B. Roentgenogram (No. 150) shows cavity in skin filled with water. Note the similarity of this cavity to those cavities formed in animal tissues.

The temporary cavity within the skull is apparent in the microsecond roentgenogram of figure 91, a dog's head perforated by a ¹/8-inch steel sphere moving 4,000 f.p.s. Figure 92 is a similar microsecond roentgenogram of the head of a cat showing views before, during, and after the shot. A cavity similar to that in the dog's head is apparent in the microsecond roentgenogram of the cat.

The explosive effect of a high-velocity missile within the cranium increases with increased energy. With very high velocities, there is complete shattering of the skull, usually along suture lines. This effect is illustrated in figure 93. Movement of brain tissue during expansion of the temporary cavity pushes the bone apart.

To demonstrate the necessity of a liquid medium for the development of these pressure effects, the brain of a cat was removed through the foramen magnum and the air-filled head was then shot with a ¹/8-inch steel sphere moving 3,800 feet per second. A photograph of the cleaned skull of this cat is reproduced in figure 94. It will be noted that no shattering has occurred, the only damage being rather neat entrance and exit holes. Without a liquid medium, the high pressure necessary to blow skull bones apart cannot be built up.

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FIGURE 83.-Microsecond roentgenograms of thigh of a cat. A. Roentgenogram (No. 261) shows temporary cavity formed by a ⁴/32-inch steel sphere whose impact velocity was approximately 3,000 f.p.s. The energy lost by this sphere was approximately equal to that lost by the ⁸/32-inch sphere as shown in B. Note the similarity of the two cavities. B. Roentgenogram (No. 262) shows the temporary cavity formed by the passage of an ⁸/32-inch steel sphere whose impact velocity was approximately 1,500 f.p.s. Compare with A.

MOVEMENTS FOLLOWING COLLAPSE OF THE EXPLOSIVE CAVITY

In the preceding pages, the explosive cavity in soft tissue, with its volume many times greater than the volume of material swept out by the missile, was clearly demonstrated. It was reasonable to suppose that when the cavity collapsed such violent motion would not immediately stop. Investigation of the movement in soft tissue after the cavity has collapsed bears out this conjecture. The motion continues for a considerable length of time, long after the missile has passed by. Once again, it is instructive to examine the action in water and gelatin gel before proceeding to animals.

In water, the collapsing cavity closes in, entrapping the air that rushes in after the bullet. When the cavity is compressed to its minimum volume, it springs open again and the process is repeated. The cavity thus undergoes a series of pulsations. For a ¹/8-inch steel sphere traveling with an impact velocity of 3,000 f.p.s., the first few pulsations have a period of about 8 milliseconds. The period is greatest for the spheres of greater energy. The period in seconds for all spheres was found to equal the product of 9.85 X 10^-6 and the cube root

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FIGURE 84.-Frames (2,880 per second) from a high-speed motion picture showing volume changes and movements in the abdomen of a cat resulting from the passage of a ⁶/32-inch steel sphere whose impact velocity was 3,800 f.p.s. The squares painted on the shaved abdomen are 1 inch apart. (Experiment No. 1, of 20 Nov. 1945.)

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FIGURE 85.-Frames (about 2,400 per second) from a high-speed motion picture showing volume changes in a rubber tube filled with water, resulting from the passage of a small high-velocity steel sphere. Note the similarity in behavior to that of the abdomen shown in figure 84.

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FIGURE 86.-Microsecond roentgenogram (No. 183) of the temporary cavity formed in the abdomen of a cat after the passage of ⁴/32-inch steel sphere with an impact velocity of 3,200 f.p.s. Cavity age 400 microseconds.

FIGURE 87.-Roentgenograms of abdomen of a cat. The alimentary tract has been made radiopaque with barium sulfate. A. Roentgenogram (No. 186) made before the shot. B. Microsecond roentgenogram (No. 186) showing the large temporary cavity formed after the passage of a ⁴/32-inch steel sphere with an impact velocity of 3,200 feet per second. C. Roentgenogram (No. 186) made immediately after the shot. Note distribution of opaque material as compared with that shown in A.

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FIGURE 88.-Microsecond roentgenogram (No. 267) of the abdomen of a cat showing the temporary cavity formed by the passage of a small cylinder of steel (11 X 2.5 mm.) weighing 420 mg. Its striking velocity was 3,000 f.p.s. Note the irregular shape of the cavity.

of the impact energy in ergs. The periodicity of the cavity is clearly illustrated in figure 62, the first minimum appearing in frame 23 and the second in frame 47. The pulsations in water for a ⁶/32-inch sphere traveling with a velocity of 3,000 f.p.s. have been observed to last at least one twenty-fifth of a second. The pulsations in water occur because air is trapped within the missile track. As air rushes into the cavity, the cavity is sealed off by Bernouilli forces.

In gelatin gel, the cavity also appears to pulsate about an air bubble, but in this case the pulsations are directed along the track of the missile. A typical pulsation cavity is shown in figure 95. The cavity closes in from the top and bottom to form two internal nipples, as can be seen in frame 11. Eventually the cavity breaks up in two segments, as shown in frame 22 (see also fig. 55).

When missiles pass through soft structures, such as the abdomen of a cat, violent motion of the tissues occurs. The larger the energy of the shot, the greater the action on the abdomen. Some concept of the violence of this movement can be obtained from inspection of figure 84. In frames 10 and 13 of figure 84, the abdomen is considerably indented where the bullet perforated. This is also shown in figure 96. Some of the expansive movement is directly upward toward the thoracic cavity. However, the motion in the abdomen is

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FIGURE 89.-Frames from a high-speed motion picture of the thorax of a cat, traversed by a ⁴/32-inch steel sphere with an impact velocity of 3,200 f.p.s. Compare with figure 84 and note the absence of pronounced movements and volume changes in the thorax.

FIGURE 90.-Roentgenograms of the thorax of a cat. A. Roentgenogram (No. 189) taken immediately before the shot. B. Microsecond roentgenogram (No. 189) made 370 microseconds after the thorax was struck by a ⁴/32-inch steel sphere whose impact velocity was 3,200 f.p.s. Note that the temporary cavity does not show up well in the thorax. C. Roentgenogram (No. 189) made immediately after the shot. Note the manner in which the heart is displaced, although the missile did not strike the heart.

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FIGURE 91.-Microsecond roentgenogram of the head of an anesthetized dog, showing a large temporary cavity within the cranium produced by the passage of a ¹/8-inch steel sphere. At impact with the head, the sphere had a velocity of approximately 4,000 f.p.s. Entrance of the sphere was at the left in the roentgenogram, exit at the right. (Experiment No. 248.)

not like that of the pulsating cavity in the water tank but rather like the distortion waves which are set up in a block of gel when it is given a sharp blow. The microsecond roentgenograms show a complete absence of an oscillation bubble, as was seen in water.

The shot into the thigh of a cat also produces a violent action. The high-speed motion picture frames in figure 76, showing a cat leg, reveal this. When the leg is skinned, waves resembling waves on a water surface are produced, as in the "bullet-view" moving pictures of figures 97 and 98. These waves travel down the thigh with velocities ranging from 4.1 to 5.2 meters per second. It is not clear whether this wave was the regular muscular contraction wave (velocity between 6 and 12 meters per second) or rather a mechanical disturbance.

Unlike the abdomen, the cavity in the thigh pulsates on a partially air filled cavity. When the moving pictures are studied, these pulsations can be observed and timed. For example, a sphere traveling with a velocity of about 3,000 f.p.s. was observed to start pulsations having a period of about 3 milliseconds. Microsecond roentgenograms show an air bubble in the thigh at a late stage. Figure 99B is a microsecond roentgenogram taken 3.5 milliseconds after the missile passed through the leg. This is at a time when the second expansion of the cavity occurs and the entrapped bubble of air is plainly visible.

It is of interest to conjecture on what would happen to parts of the body when struck by a missile, if these parts were not confined by such structures as

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FIGURE 92.-Roentgenograms of the head of a cat. A. Roentgenogram (No. 76) made before the shot. B. Microsecond roentgenogram (No. 76) shows an early cavity. The ¹/8-inch sphere struck with a velocity of 3,800 f.p.s. C. Roentgenogram (No. 76) made after the shot.

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FIGURE 93.-Skull from head of cat struck in right temporal region by a ¹/8-inch steel sphere with an impact velocity of approximately 3,800 feet per second. (Experiment No. 240.)

skin, abdominal wall, or skull. The disintegration of the tissue will presumably be greater when it is unconfined. In figure 100 is shown the bare muscles of the thigh as they are struck by a missile. The muscles are extensively separated, and the bullet hole shows clearly, although the path of the bullet was in the plane of the picture. In figure 101 is shown a pig spleen when struck by a missile. This picture was taken with two mirrors; the one above provides a top view, while the one on the left shows the entrance hole. The tissue flies apart in all directions.

NATURE AND EXTENT OF DAMAGE AROUND THE WOUND

TRACK

The chief emphasis in this section will be on wounds of the thigh. Some attention, however, will be given to wounds of the abdomen and thorax. The nature of the damage produced in the thighs of anesthetized dogs and cats by high-velocity missiles is representative of that occurring in muscular and connective tissues. In consideration of such a wound, it is necessary to distin-

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FIGURE 94.-Photographs of the skull of a cat, showing entrance and exit holes produced by a ¹/8-inch steel sphere with an impact velocity of 3,800 f.p.s. Head of cat severed from body and brain removed before the shooting. A. Entrance site in left temporal region. B. Exit site in right temporal region.

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FIGURE 95.-Frames (1,920 per second) from a motion picture (No. 147) of the cavity in gelatin produced by a ⁴/32-inch steel sphere with impact velocity of about 3,000 f.p.s. The distance between two black marks on left is 5 cm. In frames 7 to 11, the top and the bottom of the cavity are moving toward each other producing internal nipples which later (frame 15) separate again. The single cavity breaks into two cavities (frame 22) which again join and slowly collapse to produce a permanent missile track, similar to the one which can be seen on the left.

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FIGURE 96.-Microsecond roentgenograms of abdomen of a cat. A. Roentgenogram (No. 463) shows barium sulfate in stomach. B. Roentgenogram (No. 464) taken 5.5 milliseconds after a ⁶/32-inch steel sphere (impact velocity 3,800 f.p.s.) passed through at the level of the narrow line. The time corresponds to the collapse of the temporary cavity. Note that the abdomen is still enlarged at the stomach level and narrow at the missile level but that there is practically no cavity within; the collapse is complete.

guish between damage to soft tissues, such as muscle and connective tissues, and damage to the more specialized structures of the thigh, such as the femur, nerves, and larger blood vessels. Only those in the first category will be described here, while damage to the more specialized structures will be considered later (pp. 200-211).

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FIGURE 97.-Frames (2,280 per second) from a motion picture (No. 154) of the skinned leg of a cat struck by a ⁴/32-inch sphere moving with a velocity of about 2,000 f.p.s. The right side of each frame shows the entrance hole and subsequent changes as the sphere strikes head on; the left side is the reflection in a mirror viewed perpendicular to the missile path. Note that the entrance hole in frame 1 is obscured by spray in the next three frames. A definite bulge appears on the profile of the right hand picture in frame 7 and travels down the muscle like a wave (velocity 4-5 meters per second), passing out of view about frame 19.

FIGURE 98.-Frames (2,160 per second) from a motion picture (No. 153) of a skinned cat thigh struck by a ⁴/32-inch steel sphere (0.3 gram) traveling with a velocity of about 3,000 f.p.s. The right side of each frame shows the entrance hole and subsequent changes as the sphere strikes head on; the left side is a reflection in a mirror viewed at right angles to the bullet path. Note in frame 2 that the explosive cavity produces a bulge on the side of the thigh. The gyrations of the entrance hole are clearly shown in frames 1, 2, and 3.

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FIGURE 99.-Microsecond roentgenograms of the thigh of a cat. A. Roentgenogram before a shot. B. Roentgenogram (No. 474) taken 3.5 milliseconds after a ¹/8-inch steel sphere with an impact velocity of 3,000 f.p.s. has perforated the thigh. The entrapped air bubble causes the thigh to pulsate a few times.

Obviously, soft tissues directly in the path of a missile are badly damaged. These tissues are reduced to a pulp and much of the material is actually thrown out of the thigh during the expansion of the temporary cavity, as discussed previously (pp. 167-180). The loss of this material leaves an excavation, the permanent cavity.

It has been shown earlier that the expansion of the temporary cavity results in a stretching and tearing of the tissues for a considerable distance away from the missile track. With the collapse of the temporary cavity, these tissues regain their original positions and, except for darkened areas of extravasated blood, may have a fairly normal appearance, macroscopically.

A more complete assessment of the exact type of damage suffered by these soft tissues can be had from a histologic study. In each case to be described, a considerable volume of tissue adjacent to the wound cavity was fixed and sectioned at thicknesses ranging from 20 to 50 microns.

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FIGURE 100.-Frames from a high-speed motion picture (No. 152) of the muscles of the thigh of a cat blown apart by a ¹/8-inch steel sphere traveling with a velocity of about 3,000 f.p.s. The sphere is passing from left to right. Note how the muscle which is first struck by the missile swings out at right angle to the missile path so that the entrance hole is clearly visible.

Tissues bordering the wound cavity in the thigh suffer two primary types of damage: (1) That affecting the muscle fibers and (2) that affecting the intermuscular and intramuscular connective tissues and small blood vessels. Damage to the muscle fibers is manifested by a coagulation and swelling of the fibers

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FIGURE 101.-Frames (1,800 per second) from a motion picture (No. 20, 26 May 1944) of a ¹/8-inch steel sphere (striking velocity 3,000 f.p.s.) passing through a pig spleen. The squares on background are centimeters. A mirror above shows the top view and one to the left shows the entrance view. Note the marked splash of material at entrance and exit with complete disintegration of tissue.

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in a region extending for some distance from the wound cavity. The muscle fibers (fig. 102) in this region are unique in their staining properties and often swell to twice the diameter of normal fibers. Swollen fibers are well shown by the photomicrograph in figure 102A. These fibers should be compared with normal undamaged fibers, photographed at the same magnification and shown in figure 102B. More distal to the wound cavity, "muscle clots" are formed, accompanied by other phenomena of cellular disorganization. Still further distally, however, the muscle fibers exhibit a remarkably small amount of damage despite the fact that they have been moved considerably by the expansion of the temporary cavity. The three regions just mentioned are visible in the photomicrograph in figure 102C. Normal undamaged fibers are seen at the left of the section, muscle clots in the central region, and swollen fibers to the right.

Vascular damage is extensive for a considerable distance from the permanent wound cavity. Multiple ruptures of the capillaries occur, and the muscle fibers are widely separated by accumulations of extravasated blood. This is illustrated by the photomicrograph in figure 103. These areas of hemorrhage may extend for considerable distances along fascial lines. Histologic sections show that the larger blood vessels, even though they lie close to the wound cavity, are undamaged. Bleeding around the wound appears to be a matter of capillary bleeding, unless a larger blood vessel is struck directly.

It should be emphasized that these observations are based on materials fixed within an hour or so after the shot. No attempt has been made to conduct survival studies or to follow the course of wound healing.

Because of their structural characteristics, it is very difficult to determine the exact type of damage suffered by the diffuse intermuscular connective tissues. The are elastic, and, as a result, the permanent cavity formed in them is quite small. Examination of areas around the wound shows that the individual muscles are often widely separated and stripped from their surrounding connective tissues. It appears quite likely that a great deal of the expansion caused by a missile follows these intermuscular fascial planes and causes damage in these tissues at considerable distances from the wound cavity.

Because of the heterogeneous nature of the tissues and organs involved, wounds of the abdomen are much more difficult to evaluate accurately. If the missile passes through the intestinal mass, regions of the intestine directly in the path of the missile are usually completely severed or exhibit large tears. A chief factor in causing damage in the abdomen is the rapidly expanding temporary cavity which momentarily blows apart the components of the intestinal mass, as illustrated by high-speed motion pictures and microsecond roentgenograms on pages 182 through 185. This cavity may produce large tears in the mesenteries with damage to such organs as the pancreas and spleen. Breaks in many of the mesenteric blood vessels occur, causing severe hemorrhage into the peritoneal cavity.

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FIGURE 102.-Photomicrographs showing damaged and undamaged muscle fibers adjacent to the wound cavity in the thigh of a dog, produced by a ⁴/32-inch steel sphere with an impact velocity of 3,035 feet per second. A. Photomicrograph showing damaged muscle fibers. Compare with uninjured muscle in B and note that the muscle fibers are approximately two times the diameter of normal muscle fibers. Note the large amount of extravasated blood between the fibers. B. Photomicrograph showing apparently undamaged muscle at a distance from the wound cavity. Compare with the damaged muscle in A. C. Photomicrograph of section of muscle adjacent to the wound cavity. Undamaged regions of the fibers are shown at the left, areas with "muscle clots" in the center, and badly swollen portions of the fibers to the right.

Perforations of the intestine are often observed at points quite distant from the path of the missile. These are undoubtedly due to rapid pressure changes associated with the temporary cavity, acting on gas contained in the intestine. A short period of lowered pressure in the cavity around the intestine causes the intestine to explode at points where these gas pockets are present, as explained on pages 211-223.

Damage to thoracic structures was restricted primarily to lung tissue, as in none of the experiments were the heart or great vessels struck directly. The wound track in lung tissue was never large, probably because of the sponginess and elasticity of this type of tissue. The thorax, on autopsy, usually contained a considerable amount of blood, a result of hemorrhages of the smaller

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FIGURE 102b.-Photomicrographs showing damaged and undamaged muscle fibers adjacent to the wound cavity in the thigh of a dog, produced by a ⁴/32-inch steel sphere with an impact velocity of 3,035 feet per second. A. Photomicrograph showing damaged muscle fibers. Compare with uninjured muscle in B and note that the muscle fibers are approximately two times the diameter of normal muscle fibers. Note the large amount of extravasated blood between the fibers. B. Photomicrograph showing apparently undamaged muscle at a distance from the wound cavity. Compare with the damaged muscle in A. C. Photomicrograph of section of muscle adjacent to the wound cavity. Undamaged regions of the fibers are shown at the left, areas with "muscle clots" in the center, and badly swollen portions of the fibers to the right.

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FIGURE 103.-Photomicrograph showing swollen muscle fibers of a dog's thigh in cross section. Note the large amount of extravasated blood between the fibers.

pulmonary vessels. In all the animals studied, the lungs were greatly collapsed, much more so than is usually observed after pneumothorax (pp. 171-180).

DAMAGE TO BONE BY HIGH-VELOCITY MISSILES

Damage to bone can be discussed under two headings: (1) Damage to the long bones, particularly the femur and humerus; and (2) damage to flat bones, such as those which comprise the skull.

The most obvious type of fracture of a long bone is one which results from a missile striking the bone directly. In none of the experiments was a deliberate attempt made to strike either the femur or the humerus. However, an occasional stray shot did hit the bone, and a number of microsecond roentgenograms were obtained of thighs in which this was the case.

Figure 104 is a microsecond roentgenogram of the thigh of a cat, made immediately after the passage of a ⁴/32-inch steel sphere whose impact velocity was 3,000 f.p.s. The sphere struck the femur directly. The fact that the bone has been hit has not markedly affected the expansion of the temporary cavity. In fact, it appears from this roentgenogram that the femur also "explodes," in a manner very similar to the soft tissues around it.

A second case is shown in the microsecond roentgenogram in figure 105 where the femur was struck by a small fragment (originally part of a 75 mm. shell). In this case, the fragment was broken into two pieces as a result of its impact with the bone. One piece has remained in the thigh, the second has emerged. Figure 106 is a microsecond roentgenogram of a beef rib, made immediately after the passage of an ⁸/32-inch steel sphere whose impact velocity was 2,800 f.p.s. The behavior of the bone is very remindful of the manner of formation of the temporary cavity in soft tissues.

The question whether bone fragments may be driven out into the soft tissues and act as secondary missiles is a significant one. The present observa-

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FIGURE 104.-Microsecond roentgenogram (No. 11) of the thigh of a cat made immediately after the passage of a ⁴/32-inch steel sphere with an impact velocity of 3,000 f.p.s. The sphere struck the femur directly. Note the "explosive" behavior of the femur.

ions indicate that fragments fly out into the temporary cavity and, with the collapse of the cavity, are forced back to approximately their former position. Dissection of wounds, where such extensive shattering of a bone has occurred, rarely discloses fragments at any distance from the bone. This finding is supported by the roentgenogram of a cat thigh, shown in figure 107, which was made shortly after the femur was struck by a ⁴/32-inch steel sphere whose impact velocity was 3,000 f.p.s. The sphere was fired parallel to the X-ray beam so as to pass into the plane of the paper. Although the bone is badly shattered, the fragments are closely clumped together and seem to retain a connection with the parent bone, possibly being held there by the fibrous periosteum. They are free to move but actually are not separated from the bone.¹⁶

A second and less severe type of fracture is that produced by a missile which passes near but does not strike the bone directly. This can be termed an indirect fracture. Roentgenograms of a large number of thighs show that the femur can be broken even though the missile passed as far as 2 or 3 centimeters from the bone. A roentgenogram of this type of fracture is shown in figure 69. The wound cavity appears as a light area to the right of the femur.

¹⁶The wounds of battle casualties frequently contain bone fragments along the course of the permanent wound track. This is especially true in penetrating wounds of the head where small bone fragments derived from the skull are commonly found in the permanent wound track in the brain. In penetrating wounds of the thorax where there have been fractures of the vertebras, ribs, or sternum, bone fragments are frequently embedded in lung tissue. In wounds of the extremities caused by high-velocity shell fragments, fragments of long bones are frequently embedded in the soft tissue adjacent to the permanent wound track. This does not indicate that bone fragments are important as secondary wounding agents, but it does show that bone fragments are not always retained in close approximation to the parent bone.-J. C. B.

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FIGURE 105.-Microsecond roentgenogram (No. 276) of the thigh of a cat made immediately after the passage of a fragment of a 75 mm. shell with an impact velocity of 3,000 f.p.s. The fragment broke into two pieces as a result of striking the bone. One piece has been retained in the the thigh; the second has exited and does not show in the picture.

FIGURE 106.-Microsecond roentgenogram (No. 144) of a beef rib made immediately after the rib was struck by an ⁸/32-inch steel sphere with an impact velocity of 2,800 f.p.s. Note the "explosive" nature of the fracture.

It is also clear that the cavity has expanded toward the femur and that the bone is fractured, as if it had received a heavy blow from the direction of the cavity.

Figure 108 is a roentgenogram of the thigh of a dog made after the thigh was struck by an ⁸/32-inch steel sphere with an impact velocity of 4,000 f.p.s. The femur has been fractured although the sphere passed at a considerable distance from it.

A second case is illustrated by the roentgenogram shown in figure 109. In this case, the thigh was struck midway between the femur and the sciatic nerve. The nerve in this case has been made radiopaque by the injection of iodophenylundecylate. The femur shows a simple fracture. This type of fracture should be compared with the marked comminution of that shown in figure 107, which resulted from a direct hit on the bone (see also roentgenograms on pp. 173-181).

The incidence of the indirect type of fracture appears to be related to the

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striking energy of the missile. In the case of ⁴/32-inch steel spheres, it was found that no fractures of this type occurred at velocities ranging from 1,000 to 2,400 f.p.s. At 2,800 to 3,000 f.p.s., fractures were found in 20 percent of the cases and at the highest velocities used, 4,500 to 4,800 f.p.s., in 45 percent of all the cases.

It is significant that, of the total number of indirect fractures, 80 percent were of the femur and only 20 percent of the humerus. These data are based on 172 cats in which both forelimbs and hind limbs were shot. A probable explanation of this result is that the humerus is architecturally better able to stand the high pressures imposed on it by the missile than is the femur. Also, the humerus appears to be better protected by the surrounding muscle and fascia than is the femur.

The explanation of the indirect type of fracture is found in the rapidly expanding temporary cavity. As this cavity expands, high pressures are brought to bear against the rigid bone. The situation is similar to that of striking the bone a hard blow with a hammer. Figure 110 illustrates this point nicely. This figure shows a microsecond roentgenogram of the thigh of a cat made immediately after the passage of a ⁴/32-inch steel sphere whose impact velocity was 2,800 f.p.s. The temporary cavity is expanding, and careful examination of the femur shows that a clean fracture line has appeared in the bone. A second and similar case is shown in figure 111.

FIGURE 107.-Roentgenogram (No. 288) of the thigh of a cat made after the femur was struck by a ⁴/32-inch steel sphere with an impact velocity of 3,000 f.p.s. Note the shattered femur and the manner in which the fragments are clustered around it. The sciatic nerve also shows in this figure as the dark line to the right of the femur.

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FIGURE 108.-Roentgenogram (No. 108) of the thigh of a dog made after the thigh was struck by an ⁸/32-inch steel sphere with a velocity of 4,000 f.p.s. The femur has been shattered, although not struck directly by the sphere.

Studies on skull damage were made chiefly with ⁴/32-inch steel spheres. Damage to the skull varied from the presence of neat holes, at the points of entrance and exit, to extensive fractures, sometimes resulting in complete shattering of the skull into a large number of separate fragments. Splitting along suture lines was often a prominent type of damage.

The degree of skull damage was found to increase with missile velocity and probably depends on the striking energy of the spheres. This is illustrated by the series of skulls shown in figure 112. Figure 112A demonstrates the neat type of hole which ordinarily occurs when the skull is hit by a ⁴/32-inch steel sphere with an impact velocity of approximately 1,100 f.p.s. The more extensive damage which occurs at higher velocities is shown in figure 112B, a case where the skull was struck with a sphere having a velocity of approximately 4,000 f.p.s. The extensive splitting along sutures and shattering which frequently occurred at the higher velocities is illustrated in figure 112C, a skull struck with a sphere whose impact velocity was approximately 4,600 f.p.s. In most of these latter cases, the skull is completely shattered and must be recovered piece by piece.

Much of this extreme damage to the skull undoubtedly results from pressure developed within the skull at the time a temporary cavity is formed in the brain immediately after passage of the missile. A complete account of the role of the temporary cavity in head wounding has already been presented (pp. 177-180).

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DAMAGE TO BLOOD VESSELS AND NERVES NEAR WOUND TRACK

It has been pointed out (pp. 189-200) that bleeding from a wound in the soft tissues of the thigh resulted primarily from the rupture of capillaries and small blood vessels. It has been a matter of frequent observation that the larger blood vessels, particularly the arteries, passing in or near the wound cavity were apparently undamaged. These vessels are very elastic, and the assumption was made that, unless they lay directly in the path of the missile, they were merely blown aside during the expansion of the temporary cavity and sprang back to their original positions with its collapse.

The correctness of this assumption is confirmed by microsecond X-ray studies (fig. 113). Figure 113A shows a roentgenogram of the thigh of a cat in which the femoral artery and its tributaries have been made radiopaque with barium sulfate. An attempt was made to fill the femoral vein, but too much blood remained in this vessel to give a complete injection. Figure 113B is a microsecond roentgenogram of the same thigh made immediately after the passage of a ⁴/32-inch steel sphere with a velocity of 3,200 f.p.s. The large temporary cavity, resulting from the passage of the missile, is seen in cross section. It is evident that, although the sphere passed at a considerable distance from the vessels, they have been forced aside and follow the contour of the margin of the cavity. Figure 113C shows the same thigh immediately

FIGURE 109.-Roentgenogram (No. 229) of the thigh of a cat made after the passage of a ⁴/32-inch steel sphere with a velocity of 3,000 f.p.s. Note that the femur has been fractured, although not struck directly by the missile. The sciatic nerve to the left has been injected with iodophenylundecylate. Compare with figure 107.

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FIGURE 110.-Microsecond roentgenogram (No. 135) of the thigh of a cat showing the temporary cavity formed after the passage of a ⁴/32-inch steel sphere whose impact velocity was 2,800 f.p.s. Note the fracture line appearing in the femur.

FIGURE 111.-Microsecond roentgenogram (No. 276) of the thigh of a cat showing the temporary cavity formed after the passage of a small steel fragment. Note fracture line in the femur.

after the shot. The location of the permanent cavity is well defined. The blood vessels have moved back to their original position as shown in figure 113A. Subsequent dissection disclosed that both the artery and the vein were undamaged. The magnitude of the blow suffered by these vessels was such as to fracture the femur.

Unlike arteries and veins, large nerves, as the sciatic nerve of the cat, are often severely damaged as a result of being displaced by the temporary cavity. This displacement may cause a stretching and a compression of the nerve sufficient to block its ability to conduct impulses, even though there is no detectable break in the continuity of the nerve.

The sciatic nerve can be made radiopaque by injecting it with either iodobenzene or iodophenylundecylate (fig. 114). The exact manner in which these substances follow the nerve is not well understood and, in many cases, only a single small channel in the rather broad nerve is outlined.

Microsecond roentgenograms show that the nerve is greatly displaced as the temporary cavity expands. Figure 114C is a microsecond roentgenogram of the thigh shown in figure 114B, made immediately after the passage of a ⁴/32-inch steel sphere with a velocity of 3,200 f.p.s. The cavity is seen in cross section. The roentgenogram shows that the nerve has been pushed aside

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FIGURE 112.-Photographs of cat skulls. A. Neat type of hole formed by the passage of a ⁴/32-inch steel sphere with an impact velocity of 1,100 feet per second. B. More extensive damage to the skull produced by a ⁴/32-inch steel sphere which struck with a velocity of 4,000 feet per second. Note the extensive splitting along suture lines. C. Extreme damage produced by a ⁴/32-inch steel sphere which struck with a velocity of 4,600 feet per second.

and follows around the margin of the cavity. Because of the extreme rapidity with which this displacement occurs, the situation is comparable to striking the nerve a sharp blow. Figure 114D shows this same nerve immediately after the shot. Subsequent dissection showed no break in the continuity of the nerve and nothing to suggest gross anatomic damage to the nerve.

In a number of cases where the nerve had been subjected to compression and stretching by the expansion of the temporary cavity, conduction, as determined by electrical stimulation, was blocked. In general, it was necessary for the missile to pass within 1 centimeter of the nerve in order to block conduction. Nerves at a greater distance showed normal conduction.

Nerves in which conduction was blocked as a result of a "near miss" showed no externally detectable break in continuity. However, histologic examination of the nerves showed structural changes which accounted for the loss of conduction. Figure 115 is a photomicrograph of a longitudinal section

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FIGURE 113.-Roentgenograms of the thigh of a cat. A. Roentgenogram (No. 245) showing femoral artery and vein injected with barium sulfate. B. Microsecond roentgenogram showing displacement of the blood vessels by the large temporary cavity formed after the passage of a ⁴/32-inch steel sphere with an impact velocity of 3,200 f.p.s. The dark circular temporary cavity is shown in cross section with missile hole in center. C. Roentgenogram made immediately after the shot. Note that the vessels have returned to their original positions and that the femur is fractured. The permanent cavity shows as a dark area to the right of the blood vessels.

of an undamaged control sciatic nerve of a cat. Figure 116 is a similar photomicrograph of a nerve in which conduction was blocked. This figure shows that the nerve fibers have been widely separated and that many fibers are completely severed, with their ends badly frayed. A critical study of many of the fibers at very high magnifications indicated that the axis cylinders of many of them were broken, but the myelin sheath and neurilemma showed no signs of damage. Figure 117 shows a section from another nerve. In this

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FIGURE 114.-Roentgenograms of the thigh of a cat. A. Roentgenogram (No. 228) showing the sciatic nerve made radiopaque with iodobenzene. B. Roentgenogram (No. 232) made immediately before a shot. C. Microsecond roentgenogram showing the displacement of the sciatic nerve by the temporary cavity (seen in cross section; missile hole in center) which is formed after the passage of a ⁴/32-inch steel sphere whose impact velocity was 3,200 f.p.s. D. Roentgenogram made immediately after the shot. Note relation of the permanent cavity (small dark area) to the nerve.

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FIGURE 115.-Photomicrograph of a section of an undamaged control sciatic nerve of a cat.

FIGURE 116.-Photomicrograph of a section of the sciatic nerve of a cat, showing type of damage produced by a ⁴/32-inch steel sphere which did not strike the nerve directly. Note the separated and broken nerve fibers.

FIGURE 117.-Photomicrograph of a section of the sciatic nerve of a cat, showing type of damage produced by a ⁴/32-inch steel sphere which did not strike the nerve. Note the manner in which the nerve fibers are kinked.

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case, the nerve fibers have been thrown into prominent kinks, as though they had undergone abnormal stretching. In all of the cases described here, the nerve sheath (epineurium) appeared undamaged.

PRESSURE CHANGES ACCOMPANYING THE PASSAGE OF MISSILES

When a high-velocity missile strikes the body and passes through soft tissues, three kinds of pressure change appear: (1) Shock wave pressures, or sharp high-pressure pulses, formed when the missile hits the body surface; (2) very high pressure regions immediately in front and to each side of the moving missile; (3) relatively slow low-pressure changes connected with the behavior of the large explosive temporary cavity formed behind the missile.

Some characteristics of shock waves in water have already been considered (pp. 152-158). Attention was also directed to the high-pressure regions around the moving sphere, whose effects are seen in figure 60. Shock wave pressures and cavity pressure changes in the body can be investigated in two ways: (1) The pressures can be accurately recorded by a proper type of gage, or (2) their existence can be visualized in models simulating conditions found in the body. For accurate recording, a calibrated tourmaline piezoelectric crystal gage was placed in the stomach of the deeply anesthetized animal which was then shot through the posterior part of the abdomen. The method is described on pages 147-152. In order to record shock wave pressures, the amplifier gain was low and the sweep rapid, calibrated in microseconds. To record pressure changes around the temporary cavity, the gain was high and the sweep relatively slow, calibrated in milliseconds.

For visualizing the shock wave pressures, the spark shadowgram method described on page 150 was used. The tissue, placed on the surface of a tank of Ringer's solution, was shot with a high-velocity missile and the spark triggered to catch the shock waves as they moved from tissue to solution; or the tissue was suspended in the solution and the behavior of shock waves on reflection or transmission recorded.

Shock waves in tissue arise at the impact of the missile with the skin or other tissue surface. The velocity of shock waves in tissue is approximately the same as in water, 4,800 f.p.s. The chief difference in behavior of shock waves in the body, as compared with water, is associated with the heterogeneity of the tissues. The wave is dispersed on transmission through, or on reflection from, surfaces. Instead of a single clean wave, there appears a mass of wavelets with a series of high-pressure peaks. Figure 118 shows a shock wave in water partially reflected and partially transmitted by a slab of gelatin gel suspended in the tank. The gelatin is sufficiently homogeneous to give good reflection. Figure 119A shows waves which have arisen in, and passed out of, a mass of thigh muscle suspended at the surface of Ringer's solution. Figure 119B shows a shock wave which has originated from a thigh muscle surface. In both cases, the dispersion of the wave is apparent.

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FIGURE 118.-Shadowgram (P 132) of a wave reflected from and transmitted through a 1¼-inch block of 20 percent gelatin gel suspended in water. The original wave, from a ³/16-inch steel sphere with an impact velocity of about 2,500 f.p.s. can be seen at each side of the block.

FIGURE 119.-Shadowgrams of a shock wave. A. Shadowgram (S 115) of wave whose origin is at the upper surface of a fresh skinned thigh of a cat. The wave has been dispersed and passed from the tissue to water. B. Shadowgram (S 143) of a wave arising at the surface of water and reflected from a fresh skinned thigh of a cat suspended in water. Note the convection trails below the muscle tissue.

Reflection and transmission also occur from a piece of cat's stomach spread on a frame, as illustrated in figure 120. The behavior of a shock wave at the body wall is illustrated in figure 121, which shows a piece of the abdominal wall (skin and muscle) of a cat stretched on the surface of a tank of Ringer's solution. The tank has then been penetrated by a horizontal shot (to right). The shock

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FIGURE 120.-Shadowgram (P 130) of a wave in Ringer's solution after reflection from and transmission through a piece of cat stomach stretched on a frame. The wave arose from a ³/16-inch steel sphere with an impact velocity of 3,000 feet per second.

FIGURE 121.-Spark shadowgram (P 201) of a shock wave reflected from the inner surface of cat's abdominal wall, which is resting on the surface of Ringer's solution. The cone-shaped cavity behind the ³/16-inch steel sphere, which hit the tank horizontally with a velocity of 3,000 f.p.s., is at right. The reflected wave is not so regular as the incident wave and has a light band forward, indicating that it has been inverted to a tension wave on reflection. The low pressure resulting from inversion is indicated by the cavitation, which appears as fine black dots. The secondary waves following the shock wave are due to vibration of the steel sphere. The missile is 6 cm. below the surface of the water.

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wave can be seen toward the left and has been reflected from the undersurface of the body wall. Note that a light band precedes the dark band of the shock wave, indicating that, on reflection, a pressure pulse has been changed to a tension pulse. Such reversal occurs whenever the acoustic impedance (defined as the density multiplied by the wave velocity) of the reflecting medium is less than that of the medium in which the wave was moving. At an air surface, the pressure wave is always reflected as a tension wave.

Reflection from bone, in this case the surface of a human skull suspended in water, is illustrated in figure 122, while figure 123 depicts a row of beef

FIGURE 122.-A. Shadowgram (P 158) of wave in water reflected from the outer surface and also transmitted by a top section of water-soaked human skull. The original wave from a ¹/8-inch steel sphere, with impact velocity of about 3,000 f.p.s., can be seen near the lower right corner of the bone. B. A wave transmitted through a skull section.

FIGURE 123.-A and B. Shadowgrams (S 147 and S 148) showing reflection and transmission of water shock waves by a rack of beef ribs seen in cross section, imitating the thorax. The ¹/8-inch steel sphere had an impact velocity of 3,000 f.p.s. Note the reflection of waves (A and B) and the origin of new wavelets in the space between the ribs (B).

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FIGURE 124.-Shadowgrams of a beef femur and a steel bar almost completely immersed in water. The top of each has been struck by a ¹/8-inch steel sphere moving 3,000 f.p.s. Note that the shock waves within the steel bar have passed into the water but that no such waves pass out of the bone. A. Shadowgram (S 129) of beef femur. B. Shadowgram (P 128) of steel bar.

ribs (seen in cross section) tied together so as to represent the skeleton of the thoracic wall. Reflection from each bone is clearly apparent, as well as the secondary wavelets formed when a shock wave moves through the opening between ribs.

Shock waves do not appear to pass into water when a bone is hit directly by a high-velocity missile. Figure 124A is part of a beef femur whose upper end has been struck. No waves are visible moving from the bone to water, as appear when a bar of steel, shown in figure 124B, is substituted for the bone.

Tourmaline crystal pressure records of four shock waves in the abdomen of a cat are reproduced in figure 125. It will be observed that these records differ from a shock wave in water in that the descending limb of the pressure peak is steep and the shock waves themselves are often multiple. In all these records, the pressures stop abruptly at a certain point. This is an artifact due to blocking of the piezoelectric amplifiers by the surge of current through the microsecond X-ray apparatus used to record conditions within the abdomen at the time the pressure record is made. Such a microsecond roentgenogram is reproduced in figure 126.

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FIGURE 125.-Four records of shock wave pressures (crystal in stomach) from four different cats shot through the abdomen with a ³/16-inch steel sphere moving 3,800 f.p.s. Time in 100 kilocycles. One vertical division in A (cat 360) is 193 pounds per square inch; in B (cat 362), 188 pounds per square inch; in C (cat 333), 162 pounds per square inch; and in D (cat 331), 171 pounds per square inch.

FIGURE 126.-Microsecond roentgenogram of the abdomen of a cat showing the crystal pressure gage in position, taken during the passage of a ³/16-inch steel sphere (at right) with striking velocity of 3,800 f.p.s. The trigger screen for the X-ray surge is at left. Note the large temporary cavity which has expanded around the barium sulfate filled stomach. The pressure record for this figure is reproduced in figure 125C.

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FIGURE 127.-Spark shadowgram (P 199) of a shock wave complex from a ³/16-inch steel sphere which has struck the water surface with a velocity of 3,800 f.p.s. and moved through a gas-filled segment of the colon of a cat. Two other gas-filled segments of the intestines of a cat are to right and left. Capital letters indicate the origin of waves and small letters the wave front. Depending upon the position of the crystal, various types of pressure record would be obtained.

It will be noted from figure 125 that the first pressure pulse of a series may not be so high as the succeeding pulses. This can be explained in part by the reflection of shock waves in the abdomen and in part by the presence of gas pockets in the alimentary tract. Whenever a missile perforates a gas pocket and enters tissue on the opposite side, a new shock wave will be generated. Since the new wave is nearer the crystal, its peak will be higher than the original one started at the body wall. It is not possible, therefore, to present a typical record of shock waves in the abdomen, since so much depends upon reflection and distribution of gas in any particular case.

The manner in which a series of shock waves could appear within the abdomen is illustrated in the spark shadowgram of figure 127, which shows three loops of a cat's colon, each containing an air pocket, suspended in Ringer's solution in the form of a triangle. A shot was fired through one loop of colon and shock wave A-a was formed at the liquid surface. This wave was reflected from each of the other loops of colon and shock waves B-b and C-c were formed. When the shot had passed the gas mass and

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FIGURE 128.-A crystal record of pressure changes in the stomach of a cat when the thigh is shot with a ³/16-inch steel sphere moving 3,800 f.p.s. Time in 100 kilocycles.

hit the further side of the middle piece of colon, another large shock wave was formed, D-d. If a crystal had been placed at X, it would have recorded a medium, followed by a weak and then a strong shock wave, giving a multiple record somewhat like that of figure 125.

When the crystal is in the stomach and the animal is shot through the thigh, about 14 cm. from the crystal, the type of pressure record shown in figure 128 is obtained. There results a jumble of small pressure peaks about 5 microseconds apart and of an intensity of about 10 to 20 pounds per square inch. The pressure record is very similar to what might be expected from the appearance of the shock shadowgram shown in figure 119B.

The relatively slow pressure changes in the cat's abdomen, recorded from a crystal gage in the stomach, are reproduced in figure 129. The timing is in milliseconds. The first peaks mark the shock wave, whose pressure is so great as to rise completely off the record. From measurements of the high-speed motion picture of the shot, taken simultaneously with the pressure record and reproduced in figure 130, it is found that the second maximum pressure corresponds to the collapse of the temporary cavity. The subatmospheric pressure between the shock wave and the second pressure peak corresponds to the maximum of the temporary cavity, visible as two prominent bulges, as shown in frames 3 and 11.

After the large temporary cavity collapses, microsecond roentgenograms show no second expansion, such as occurs after a shot into a tank of water. Although the motion picture of the cat's abdomen does show indications of new wrinkled bulges on each side of the abdomen, these second bulges merge with the subsequent distortion of the abdomen. The two small pressure oscillations in the pressure record appear to have no counterpart in the external movements of the abdomen, visible in the motion picture. The pressure record is, in fact, quite flat during the long period of wavelike abdominal movements.

In the respect just mentioned, a shot into the body differs from a shot into water in a tank, where pulsations of the gas making up the temporary

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FIGURE 129.-A crystal record of pressure changes in the abdomen of a cat shot with a ³/16-inch steel sphere moving 3,000 f.p.s. The time curve is 1000 cycles, and the vertical pressure divisions are 13.1 pounds per square inch. The shock wave at left moves off the screen and is indicated by the line of white ink. There follows a subatmospheric-pressure period which corresponds to the expansion of the temporary cavity and then a pressure peak (3.5 milliseconds after the shock wave) that is represented by frame 11 of figure 130. The later pressure oscillations are believed to be due to pulsation of gas pockets in the intestines. (24 Jan. 1946, cat 364.)

cavity is a striking phenomenon and the pressure changes during these pulsations are found to agree exactly with the expansion (decreased pressure) and contraction (increased pressure) of the cavity, illustrated in figure 60.

It is very probable that the opening made by the shot in the body wall closes almost immediately, so that little air can rush in behind the missile. In an animal, therefore, the initial large temporary cavity may be considered as almost entirely filled with water vapor. When the cavity collapses, only small pockets of gas are left, comparable in volume and scarcely distinguishable from the gas pockets already present in the intestine. In the pressure record of figure 129, the pulsation of these gas pockets is represented by the small pressure oscillations, spaced 2 to 3 milliseconds apart. They are quite comparable to the pulsation observed in small submerged balloons when a sphere is shot into a tank of water.

Small balloons, filled with air and suspended in a tank of water, are instructive for visualizing pressure changes around the temporary cavity resulting from a shot into the tank (fig. 131). As can be seen, the balloons are at first contracted by the high pressure of the shock wave, but very quickly they expand to a large size, as a result of the decreased pressure during expansion of the cavity. In addition to the expansion and contraction of the balloons, synchro-

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FIGURE 130.-Frames (2,360 per second) from a high-speed motion picture of the abdomen of a cat, shaved and marked with black lines 1 inch apart. A ³/16-inch steel sphere has perforated the abdomen from the left. Frame 3 is that of the maximum temporary cavity and corresponds to the maximum subatmospheric pressure of figure 129. Frame 11 corresponds to the first pressure peak after the shock wave.

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FIGURE 131.-Frames (2,160 per second) from a motion picture (166) of a ¹/8-inch steel sphere (impact velocity 3,000 f.p.s.) striking the surface of water (at top of each frame) in which are suspended small rubber balloons. The development of the temporary cavity is well shown. Pressure changes in the water are indicated by contraction and expansion of the balloons. The vertical dots on left are 5 cm. apart.

nized with the volume changes of the cavity, they also pulsate with their own period (about 500 a second), and in this respect they serve as models for the behavior of small gas pockets in the body.

The importance of gas pockets in tissues in relation to pressure changes has been emphasized in the foregoing discussion. That these gas pockets are important in wounding can be determined by suspending in Ringer's solution small masses of tissue, with and without gas pockets, and then shooting into the solution near the tissue masses. Excised hearts of frogs have been used to investigate the mechanism of wounding by this method.

When isolated frog hearts containing no gas are fixed in position in a tank of Ringer's solution, it has been determined that damage from a shot into the solution occurs only when the hearts are rapidly stretched on their moorings by the expansion of the temporary cavity. They suffer no damage from shock waves beyond the boundary of the temporary cavity. The arrangement of such an experiment is illustrated in figure 132. Only the hearts engulfed by the cavity, or greatly stretched by it, were damaged.

In order to eliminate the cavity formation, a piece of armorplate was placed on the water surface and struck by a very high velocity missile. By this method, shock waves of great intensity can be produced, but only a minute cavity forms underneath the armorplate. Water movement is thereby reduced to a minimum. It was found that these high-intensity shock waves did not

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affect frog hearts suspended underneath. However, if gas has first been injected into a heart, as in A and F of figure 133, the sudden expansion of this gas from negative pressures in the water around the minute cavity was found to cause damage. The A heart (near the small cavity) was seriously injured, while the F heart (farther away) suffered no severe damage.

These, and other similar experiments, indicate that it is the subatmospheric pressure around the temporary cavity, recorded in the crystal records of figures 60 and 129, that causes the damage and that this damage results from the expansion of gas pockets rather than from the high pressures connected with the shock wave. Damage by gas expansion may be spoken of as secondary damage, whereas damage from expansion of the temporary cavity itself is primary damage. In both cases, the destructive effects are due to severe tearing of tissue.

A striking demonstration of gas effects is illustrated in figure 134 which shows a loop of cat intestine, with an air bubble within the right end, suspended

FIGURE 132.-Frames (2,400 per second) from a motion picture (168) of six frog hearts tied to two vertical strings in a tank of Ringer's solution. The surface of the water is at the top edge of each frame. A ¹/8-inch steel sphere, with impact velocity of 3,100 f.p.s., penetrated the solution between the vertical rows of hearts, and the development of the temporary cavity is clearly seen. The effect on the hearts is described in the text. The vertical dots on left are 5 cm. apart. The circle in upper left is a mirror reflection of a sodium lamp flashing 120 times a second.

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FIGURE 133.-Frames (above 2,000 per second) from a motion picture (SW 5) of six frog hearts tied to horizontal strings (invisible) below a sheet of armorplate which has been struck (without perforation) by a ¹/8-inch steel sphere moving about 4,000 f.p.s. A small cavity appears under the plate in frames 1 to 6. The upper left (A) and lower right (F) hearts contain air, and the expansion of the air in the upper left heart is clearly visible. The effect on the hearts is described in the text. The horizontal lines at left are 5 cm. apart. A circular mirror in lower middle of each frame reflects a sodium lamp flashing 120 times a second.

in a tank of Ringer's solution. When a shot is fired through the ring of intestine, the gas bubble at the right can be seen to first contract and then expand markedly. When the intestine was later examined, the mucosa and submucosa were found to have been perforated in the gas-containing region, although the muscularis layer was intact. Such effects are exactly comparable to damage to the human body from underwater blast. This damage is restricted to gas pockets in the alimentary canal, leading to intestinal perforation, or to gas in the lungs, where severe hemorrhage occurs. Although secondary damage from gas is important in rifle shots, it never equals the primary damage which results from the expansion and tearing caused by the formation of the temporary cavity.

RETARDATION OF MISSILES BY SOFT TISSUE AND TISSUELIKE SUBSTANCES

The slowing down or retardation of a missile as it traverses tissue is an important factor in determining how and where the missile delivers its energy to the tissue. In order to understand the mechanism of wounding, it is essential to know the law of force which retards the missile. Here, the studies of retardation in water and in 20 percent gelatin gel are very helpful. It has

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FIGURE 134.-Frames (2,280 per second) from a motion picture (173) of a loop of cat colon with an air pocket in the right end, suspended in a tank of Ringer's solution. A ¹/8-inch steel sphere with a velocity of 3,100 f.p.s. entered the water and passed through the center of the loop of colon without hitting it. The large temporary cavity can be seen to expand and touch the colon. Note especially the slight constriction of the air pocket in frame 1 and its expansion in frames 2 to 5. The distance between horizontal lines at left is 5 cm.; the timing mirror is also at left in each frame.

been found that the retardation, dV/dt, is proportional to the square of the missile's velocity, V. This is usually written dV/dt=-aV². a is called the retardation coefficient, V is the instantaneous velocity of the missile, and T the time. The retardation of the missile at high velocities is produced almost entirely by the inertia of the water and gel which was originally in the missile's path and which is forced aside. Since the inertia depends only on the density, it is to be expected that soft tissues, gelatin, and water will behave in nearly the same manner. This proves to be the case-all three offering a resistance to the missile which is proportional to the square of the missile's velocity.

The retardation coefficient of a ¹/8-inch steel sphere in water, gelatin gel, and muscle has been measured and is as follows:

Water                                          0.091 cm.^-1
20 percent gelatin gel (24°C.)         .106 cm.^-1
Cat muscle (living)                          .136 cm.^-1

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Living cat muscle, therefore, is only 50 percent more retarding than water and only 28 percent more retarding than gelatin gel.

These retardation coefficients for missiles were calculated from the loss of velocity which a sphere experienced in going through the thigh of a deeply anesthetized cat. The length of the missile track was also used in the calculation. The retardation coefficients in water and gelatin gel were calculated from the position-time relationship as measured from the high-speed motion pictures. The retardation coefficient, a, is equal to (r / 2) (A / M) CD where A is the projection area, M the mass, r the density of the medium, and CD the drag coefficient. The measured values of CD for these three substances were: Tissue, 0.45; 20 percent gelatin gel at 24° C., 0.35; and water, 0.30.

A sphere or nontumbling fragment loses its energy rapidly in traversing soft tissues and waterlike substances. The energy, E, falls off exponentially with penetration distance, s, as follows: E=E_oe^-2as. A ¹/8-inch steel sphere loses half its impact energy after penetrating 2.22 cm. of muscle and nine-tenths of its energy after penetrating 8.3 centimeters. A ¹/16-inch sphere will lose these same percentages of energy in just half these distances, while a ¼-inch sphere will require twice the distance.

In the case of high-velocity missiles, certain characteristics of the explosive or temporary cavity are related to the energy dissipated by the missile. It is possible, therefore, to determine how and where the missile lost its energy simply by inspecting a microsecond roentgenogram of the temporary cavity. It turns out that the diameter of the temporary cavity, D (measured perpendicular to the missile path) is proportional to the square root of the space rate of energy change or D=(8 k a E/π), where k is a constant having an experimentally measured value of 8.92 X 10^-7 cm. ³/erg for water and 0.80 X 10^-7 cm.³/erg for living muscle of a cat thigh.

This decrease in energy is clearly observable in a cavity produced in water by its decrease in diameter as the missile is slowed down (fig. 135). This is also shown in figure 75, where a cavity in the thigh can be seen to be wide near entrance and narrow near exit.

The rate at which energy is lost and the cavity diameter also increase with the ratio A/M, which is the ratio of projection area to mass of the missile. This signifies that a missile of large projection area and small mass will lose energy rapidly and will produce a wide, but short cavity. When two spheres of different masses having the same projection area and velocity are allowed to enter the water, the light sphere loses energy rapidly, producing a short, but wide cavity. This is shown in figure 136 (S25, S59) where the dissipation of energy by an aluminum (left) and a steel (right) sphere is contrasted.

When a fragment is shot, tumbling of the fragment changes the projection area, and this change is reflected in the shape of the cavity. Several cavities, formed by tumbling missiles, are shown in figures 56 and 137. This phenomenon is also shown in figure 88, where a cavity in the abdomen of a cat was formed by a tumbling cylindrical fragment (a section of a wire nail).

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FIGURE 135.-A frame from a high-speed motion picture of the cavity formed by a steel sphere, showing that its diameter (measured perpendicular to the bullet path) falls off as the energy of the sphere decreases. The diameter is proportional to velocity of the missile at any instant. Water surface is at left.

The retardation suffered by small steel spheres when traversing human skin was also measured. This was done by mounting several layers of skin in the path of a small steel sphere and measuring the velocity before and after impact. Figure 138 shows the skin pocket mounted in the middle of a shock wave velocity recorder. The inclination of the lines of dashes gives the before and after velocity of the missile. The missile is traveling from right to left.

For equivalent thicknesses, the retardation coefficient for skin was 40 percent larger than that of muscle. The velocity lost by a ¹/8-inch steel sphere when perforating 8 cm. of skin was found to be 0.182(V_o-170)s + 170, where the velocity is expressed in feet per second. The 170 f.p.s. represents the velocity required to enter the skin without penetrating it. For other missiles, the relationship just cited may be extrapolated to give 0.30 AM^-1(V_o-170)s+170. The effect which skin exerts on certain missiles has been calculated from this formula, and the results are as follows:

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FIGURE 136.-Spark shadowgraphs of cavities formed by an aluminum sphere and a steel sphere. Both were traveling with velocities of about 3,000 f.p.s. Note the short, wide cavity of the aluminum sphere (A), indicating that it lost energy at a more rapid rate than the steel sphere (B). A. Shadowgraph (S 25) of ¹/8-inch aluminum sphere. B. Shadowgraph (S 29) of ¹/8-inch steel sphere.

PENETRATION OF MISSILES INTO SOFT TISSUE AND BONE

When wound damage to various internal organs of the body, vascular channels, and nerves is to be considered, the question of how deeply a missile can penetrate different types of tissues becomes a highly important one. Various soft tissues, but more particularly bone, often overlay and serve as a protective layer to important structures underneath. This section presents data which have been secured regarding the problem of penetration.

The distance which a missile travels into soft tissue before being brought to rest depends not only on its impact velocity, V_o, but also on its projection area, A, its mass, M, and its shape factor, F. Such an inference can be drawn from studies on penetration in a tissuelike substance as 20 percent gelatin gel. That the law of penetration for tissue should be the same as the law for gelatin follows from the observation that they both obey the same retardation law.

The penetration, P, into 20 percent gelatin gel at 24° C. by steel spheres is given by P=a^-1 1n(V_o/74)=5.72 A^-1 1n(V_o/74)=59.5R ln(V_o/74) where A

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FIGURE 137.-Spark shadowgraphs of cavities produced by tumbling cylindrical slugs. The width of cavity indicates when they presented the largest projection area and hence delivered the most energy to the water. A. Shadowgraph (P 10) of cavity formed by a ³/32- by ¹⁷/32-inch steel cylinder with an entrance velocity between 2,000 and 3,000 f.p.s. It presented a small area just after entering but near the end of its flight seems to be turning rapidly. B. Shadowgraph (P 90) of cavity formed by a ⁵/16- by ¹⁹/32-inch aluminum cylinder. C. Shadowgraph (P 11) of cavity formed by a cylinder like the one shown in A.

is the projection area in cm.², a the retardation coefficient in cm.-¹, M the mass of the sphere in grams, V_o the impact velocity in meters per second, and R the radius of the sphere in centimeters. The penetration into gelatin gel of eight aluminum spheres having the same velocity but different radii, R, is shown in figure 139 where it is evident that spheres of larger radii penetrate a greater distance.

The penetration of small spheres into living soft tissue was determined by shooting into the thighs of deeply anesthetized dogs. It was assumed that the penetration law was the same as for gelatin gel and that only the constants which appear in the formula needed to be ascertained. The penetration formula for soft tissue was found to be:

P=a - 1 1n(V_o/84)=4.45A^-1M 1n(V_o/84=46.3R 1n(V_o/84).

Larger spheres having the same velocity undergo the greatest penetration. This is shown in the roentgenogram of figure 140 where several ²/32- and ³/32-

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FIGURE 138.-Spark shadowgram (27 Mar. 1945) of several layers of skin just after perforation by a ⁶/32-inch steel sphere. The missile moved from right to left. A plate with small holes converted the shock wave to a sound wave. The inclination of these waves to the missile path is used to measure the impact and residual velocities of the missile. Before impact, the velocity was 3,030 f.p.s. and after, 2,805 f.p.s. Note the debris flying back from entrance side and moving forward on exit side of skin.

inch steel spheres are shown embedded in the thigh of a dog. The spheres had nearly the same velocity, but the lighter ²/32-inch spheres succeeded in going only about two-thirds the distance of the ³/32-inch spheres. For spheres having exactly the same velocity, the penetration distance is inversely proportional to their radius. For spheres having the same radius, the penetration varies as the log of the impact velocity. This is illustrated in figure 141, where two ⁴/32-inch spheres having impact velocities at 2,400 and 1,220 f.p.s. are shown imbedded in butcher meat. The faster ball is further advanced in the tissue.

When missiles other than spheres are considered, it is necessary to distinguish between a tumbling and a nontumbling missile. In a tumbling missile or fragment, the projection area may undergo considerable change in magnitude during flight, and the penetration of the same shaped fragment may vary considerably for different shots. For nontumbling fragments in soft tissue, it is supposed that the formula for penetration would be P=6.67FMA^-1 1n(V_o/V_t), where F is a shape factor (one for a sphere) and V_t is a constant, which probably does not differ much from the one for a sphere having an equivalent projection area. In figure 142 is shown a fragment which has traveled broad-

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FIGURE 139.-Photograph of tank filled with 20 percent gelatin gel, showing the comparative penetration of varying size spheres and .22 caliber long and short bullets. The spheres were aluminum and had ⁸/32-, ⁶/32-, ⁴/32-, and ³/32-inch diameters. Their impact velocity was about 2,800 f.p.s. The largest spheres penetrate the greatest distance. Scale in centimeters.

side and has been stopped after traversing only 6.1 cm. of a cat's abdomen. A sphere of the same mass, or the same fragment traveling end on, would have passed entirely through the abdomen without any difficulty. It is apparent that missiles other than spheres or spin stabilized bullets will have a considerable range of penetration distances, depending on their behavior during flight.

Spongy bone opposes the motion of a spherical missile with a force which acts in a different way from the one for soft tissues. In soft tissues, the force is proportional to the square of the velocity, while in bone the force is independent of the velocity. When the end of a beef femur was cut and spherical missiles shot into the spongy bone, it was found that the penetration was given by P=8.15^-5R² (V_o-200)², where R is the radius of the sphere in inches, V_o the impact velocity in feet per second, and P the penetration in inches. The penetration is greatest for large spheres and increases with the square of the radius. The soft spongy bone of the femur stops missiles more readily than soft tissue. A ⁴/32-inch steel sphere traveling with a velocity of 2,000 f.p.s. in tissue will penetrate 23.3 cm., while the same sphere in bone will travel only 2.65 cm. before being stopped. It may be assumed that the penetration into bony tissues harder than those found in the femur will be correspondingly smaller. The spongy bone of the femur was used in these tests, because it afforded a large mass of fairly uniform bony material. Figure 143 shows three ⁴/32-inch spheres that have penetrated different distances into the end of a beef femur. The bone was sawed along a plane parallel to the axis to give a flat plane of entry. The sphere of highest velocity penetrated the greatest distance.

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FIGURE 140.-Roentgenogram (No. 124) of the left thigh of a dog, showing the greater penetration of two ³/32-inch steel spheres in contrast to that of three ²/32-inch steel spheres. The spheres entered the lateral surface with a velocity of about 1,190 f.p.s. The pins indicate the entrance holes for two of the shots.

FIGURE 141.-Roentgenogram (No. 119) showing butcher meat penetrated by two ⁴/32-inch steel spheres of different velocities. The faster sphere (velocity 2,400 f.p.s) travels further than the slower one (velocity 1,220 f.p.s.). The spheres entered the meat at the left, and the nails at the bottom are 5 cm. apart.

CASUALTIES IN RELATION TO MISSILE MASS AND VELOCITY

An investigation was made to determine the mass and velocity relationship for a missile which is just capable of producing a casualty. The type of casualty chosen was that which would result when a certain vulnerable region in the body was pierced by the missile.

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FIGURE 142.-Roentgenogram (No. 324) of a fragment (to right of vertebral column) which has penetrated about 6.1 cm. into the abdomen of a cat. It traveled with its largest projection area foremost (broadside). A sphere of the same mass would have passed completely through the abdomen. The fragment (from a 75 mm. shell) weighed 0.345 grams and entered from the left with an impact velocity of from 2,000 to 3,000 feet per second.

The total projection area of an erect man and the projection area of the vulnerable region in the body was measured by using anatomic drawings of body sections. The vulnerable regions included the organs, cavities, canals, and those nerves and blood vessels which have a diameter greater than 0.25 centimeter. The total projection area from the anterior aspect was 5.3 square feet, and the vulnerable projection area was 43 percent of this. Hands and feet were not included in the survey.

The thickness of the protective layer, made up of skin, bone, and soft tissue, was measured for each section of the vulnerable region. The average thickness of bone and soft tissue on the front and back of the body was 0.6 cm. and 3.3 cm., respectively. The vulnerable region was found to be better protected by soft tissue and bone from missiles coming from the rear than from those coming from directly in front.

The data on velocity losses in living cat muscles and in fresh human skin were used, in conjunction with penetration measurements on spongy beef bone, to calculate the minimum energy required to perforate the protective layer and pierce the vulnerable region. The calculation was made for ¹/16-, ¹/8-, and ¹/4-inch steel spheres. These perforation energies for the ¹/8-inch sphere varied from 2 to 216 ft.-lb. and depended on the composition and thickness of the protective layer immediately above the region being considered.

The probability that a hit by a given missile will result in a casualty was determined from the ratio of vulnerable projection area to total projection area, where the vulnerable projection area is a projection of those vulnerable regions which the missile is capable of piercing. This probability for any one missile was observed to rise rapidly with the missile's energy and velocity as soon as the threshold energy and velocity were attained. After passing an optimum energy, the probability of wounding increased at a smaller rate until a maximum was reached. This optimum energy was chosen as an index of the energy required of the missile in order to produce the type wound being

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considered. The average optimum energy for ²/32-, ⁴/32-, and ⁸/32-inch steel spheres, when calculated for missiles striking a man from directly in front or directly behind, was 15 foot-pounds. The average probability of wounding which this optimum energy gave was 60 percent of the maximum possible probability or was 0.25 in absolute units.

It was pointed out that the relationship between the mass and velocity of all missiles which produce a casualty of a given type depends on two factors: (1) The severity of the wound which causes this casualty, and (2) the probability that a hit on the body will produce such a wound. In the present analysis, it has been assumed that there is a large group of wounds which have the same severity and the probability of the occurrence of such wounds has been evaluated; the resulting relationship between the mass and velocity which was evaluated was too complex to present in any other way except pictorially.

The mass-velocity data showed that the energy necessary to wound a man increases as the mass of the missile is increased. This is true for the optimum energies and for those energies which give probabilities of wounding equal to 25, 50, and 75 percent of the maximum probability. This increase in energy with mass is shown to be generally true for any analysis in which penetration plays the predominant role.

FIGURE 143.-Roentgenogram (31 Jan. 1945) showing ⁴/32-inch steel spheres that have penetrated various depths into the spongy end of a beef femur. The impact velocities of these spheres were 1,670, 2,790, and 3,110 f.p.s., the fastest one having penetrated the greatest distance. The spheres entered the upper flat surface. The shaft of the bone is represented in cross section by the dark circular area.

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