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 arousedinterest and admiration-interest from the resemblance to moving ships withprominent bow and stern waves and a turbulent wake; admiration that so rapid amovement can be stopped in a photograph and the detail of events clearlyvisualized. Since the first spark pictures of moving bullets in air, obtained byMach2 in 1887 and Boys3in 1893, a mass of information has been gathered on trajectories,stability, spin, yaw, and precession of projectiles. This field of inquiry isusually classified as exterior ballistics to distinguish it from what happenswithin the gun, or interior ballistics.
The events which occur when a bullet strikes and enters thebody have received much less attention-in part, owing to the rapidity ofchanges which take place in an opaque medium and the difficulty of measuringthem and, in part, to the complexity of the body and the feeling that fewsignificant generalizations could be made regarding it. Actually, the changeswhich occur when a high-velocity bullet enters soft tissue are remarkablyindependent of body structure, and a common series of events can be outlined.The recent technical development of high-speed cameras that can take movingpictures at the rate of 8,000 frames a second and an X-ray apparatus thatrequires only one-millionth of a second for exposure have eliminated theprevious barriers to understanding the mechanism of wounding. It is now possibleto analyze events that are all over in a few thousandths of a second.
1The research on which this chapter is based wascarried out under a contract, recommended by the Committee on Medical Research,between the Office of Scientific Research and Development and PrincetonUniversity. Work under this contract began on 15 October 1943 and continued to 1November 1945. On the latter date, the contract was transferred to the Office ofthe Surgeon General. The work was brought to completion on 28 February 1946. Allof the research was conducted in the Biological Laboratories of PrincetonUniversity, Princeton, N. J. It is important to record here that the success ofthe work has been due in great measure to the wholehearted cooperation of theprofessionally and technically trained persons who, at one time or another, weremembers of the "Wound Ballistics Research Group." In addition to theauthors of this chapter, the following persons took part in the investigation:Mr. Delafield DuBois, Mr. Joseph C. Gonzalez, Mr. Vincent Gregg, Dr. HarryGrundfest, 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. ArthurH. Whiteley.
2Mach, von E., and Salcher, P.: Photographische Fixirung der durchProjectile in der Luft eingeleiteten Vorg?nge. Der Kais. Acad. der Wiss. zuWein, 1887 and 1889. (Also in Nature, London 42: 250-251, 1890.)
3Boys, C. V.: On Electric Spark Photographs; or Photography of FlyingBullets, 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 woundballistics, a study of the mechanics of wounding and related subjects. The fieldhas two aspects. One is a determination of the factors involved in injury andthe relation between the severity of the wound and such characteristics of themissile as its mass, velocity, shape, momentum, energy, and power. The attemptis made to relate the ability to wound or to kill with some physical property ofthe projectile. Such inquiry gives an answer to the question, whether anantipersonnel bomb is more effective if it breaks into a large number of smallfragments or a smaller number of relatively large fragments.
The second aspect of wound ballistics involves a study of thenature of the damage to tissues, whether it results from stretching anddisplacement or from pressure changes accompanying the shot. Of particularinterest is the commonly observed injury of organs far away from the bulletpath. Such knowledge greatly aids the surgeon in his treatment of the wound andis necessary for the establishment of rules for removal of dead tissue and theamount of debridement necessary for proper recovery. The knowledge of woundballistics is, therefore, important not only in offense but also in defense.
With the perfection of guns that could shoot high-velocitymissiles came the observation that the resulting wounds appeared as though theyhad been caused by an actual explosion within the body. External signs of injurywere often slight, the entrance and exit holes small, but an unbelievable amountof damage occurred within. Hugier (cited by Horsley4)noted this explosive effect as early as 1848 in Paris, and it has beenemphasized by all subsequent writers. Such action has led to mutual accusationby both sides in warfare that the enemy was using explosive bullets. Not only isthe tissue pulped within a large region about the bullet path but intact nerveslose their ability to conduct impulses and bones are found to be broken thathave not suffered a direct hit.
It is in this explosive effect that high-velocity missilesdiffer from those of low velocity. The wounds from a spear or a nearly spentrevolver bullet correspond more closely to the expected cylinder ofdisintegrated tissue, little larger than the spear itself. This type of woundcan be compared to what happens when a rod is plunged into soft snow. Snow pilesup in front and is pushed ahead and to the side, and when the rod is withdrawn ahole is left whose diameter is little more than that of the rod. The situationis far different with high-velocity missiles. They leave behind a largetemporary cavity whose behavior is quite comparable to the gas bubble of anunderwater explosion. Later, the cavity collapses, but far-reaching destructiveeffects have occurred during the expansion. A detailed description of whathappens during the cavity formation will be found in this chapter.
Much of the early work on wounding was concerned with anexplanation of the explosive effect of high-velocity projectiles. Shots weremade into various materials, such as gelatin gel or dough, which served asmodels 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) atBerne, Switzerland, was a pioneer in this study, which he rightly thought was ahydrodynamic problem. Delorme and Chevasse5 inParis, Bruns6 (1892) in Germany, and Horsley inEngland continued the work.
In 1898, Stevenson7 broughtout his monograph "Wounds in War," to be followed by La Garde's8"Gunshot Injuries" and by Wilson's9account of casualties during World War I. The monumental "Lehrbuch vonBallistik" by Cranz and Becker,10 now inits fifth edition, first appeared in 1910. In addition to a valuable descriptionof the small arms in use by various nations at the time of publication, thesebooks consider the theories which have been advanced to explain the explosiveeffect of bullets.
One of the earliest views was that the "wind" ofthe bullet (that is, its shock wave), or the air compressed on the face of thebullet, was responsible for the explosion. It is quite certain that this view isincorrect since the explosive effects appear if a mass of flesh is shot in avacuum. Neither can the explosive effect be connected with the shock wave whichappears when tissue is hit, since this wave moves through the tissue at the rateof 4,800 f.p.s. (feet per second) and has passed well beyond the wound regionbefore the explosive expansion occurs.
It is a simple matter also to eliminate such theories asinvoke rotation of the bullet, flattening of the bullet, or heating of tissuesby the bullet as the cause of the explosion. Steel spheres shot from asmoothbore rifle which do not rotate and do not flatten on impact are known tocause explosive effects. Moreover, the kinetic energy of these spheres is notsufficient, even if all were converted into the energy of steam, to account forthe explosion.
There remains, as the correct explanation of the explosivecavity, what early workers called the accelerated particle theory. This viewregards the energy of the bullet as being transferred to the soft tissue infront and to each side, thus imparting momentum to these tissue particles, sothat they rapidly move away from the bullet path, thus acting like"secondary missiles." Once set in motion, the "inertia of thefluid particles" continues its motion, and a large space or cavity is leftbehind. As Stevenson puts it, the bullet causes damage not only by crushing andattrition of tissue directly but also indirectly by the fluids moving away fromits path. Wilson11 compares this "blasting out" of soft tissues to theeffect of the stream of water from a firehose.
Later work has been largely concerned with special aspects ofwound
5Delorme, E., and Chevasse, Prof.: ?tude Comparativedes ?ffets Produits Par les Balles du Fusil Gras de 11 mm et du FusilLebel de 8 mm. Arch. d. Med. et Pharm. Mil. 17: 81-112, 1892.
6Bruns, Paul: Ueber die Kriegschirurgische Bedeutungder Neuen Feuerwaffen. Berlin: August Hirschwald, 1892.
7Stevenson, W. F.: Wounds in War. New York: Wm. Wood andCo., 1898.
8La Garde, L. A.: Gunshot Injuries. 2d ed. New York: Wm. Woodand Co., 1916.
9Wilson, Louis B.: Firearms and Projectiles; Their Bearing onWound Production. In The Medical Department of the U.S. Army inthe 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'sStationery Office, 1921, pp. 442-450.
11Wilson, L. B.: Dispersion of Bullet Energy in Relationto Wound Effects. Mil. Surgeon 49: 241-251, 1921.
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ballistics. Callender and French12 and Callender13 usedPlasticine as a model for tissues and studied especially the yaw of bullets andthe relation of wound damage to the power delivered. They introduced more modernmethods of measuring velocities and also obtained records of the pressurechanges during the passage of a bullet through Plasticine.
Black, Burns, and Zuckerman14 have described theenormous damage done by minute fragments of metal from bombbursts. Thesefragments move with velocities far higher than those of ordinary rifle bullets.Using the spark shadowgraph method and steel spheres, weighing only 53 mg., theywere able to imitate the destructive effect of bomb splinters and obtained sparkshadow outlines of rabbit legs during passage of the missile. These shadowgramsindicated a large swelling due to the cavity within.
The present work15 is an attempt to place wound ballisticson a sound quantitative basis. It regards the phenomena observed in wounding ofsoft tissue as fundamentally like the phenomena which occur when a high-velocitymissile enters a liquid. The subject is treated as a branch of underwaterballistics. By means of high-speed motion pictures, spark shadowgrams, andmicrosecond roentgenograms, measurements have been made of all the changesoccurring during passage of a projectile through various parts of the body, andcertain constants have been established relating mass, velocity, shape, andother characteristics of the missile to wound phenomena. By means of theseconstants, it is now possible to predict exactly what damage may be expectedfrom the impact of a known mass moving with any known velocity. The data onwhich this survey is based are given in later sections, together withreproductions of the photographs and roentgenograms.
The basic purpose of a study of wounding is to obtain datawith which to predict the degree of incapacitation (the weeks ofhospitalization) which may result from a hit by a missile of given mass (M) movingwith a given velocity (V). The incapacitation will naturally depend onthe region of the body which is struck. This region in turn will depend on thetactical situation, for example, trench or open warfare, as determined by themilitary command, which must also decide the length of hospitalizationpermissible. The probability of a hit is thus a function of the projected bodyareas exposed. The probable time of hospitalization will vary with theseverity 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 ofProduction of Wounds by Small Arms Bullets and Shell Fragments. War Med. 3: 337-350,1943.
14Black, A. N., Burns, B. D., and Zuckerman, S.: AnExperimental Study of the Wounding Mechanism of High Velocity Missiles. Brit. M.J. 2: 872-874, 1941. 15Among the difficult problems encountered during theinvestigation, particularly in its early stages, was that of assembling underthe stress of wartime conditions necessary apparatus and supplies. The beginningof the work would have long been delayed had it not been for the generous loanof equipment by the Ballistics Research Laboratory of the Aberdeen ProvingGround, Aberdeen, Md., and the continued cooperation of members of the staff ofthis 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 itemsof apparatus originally constructed at the Climatic Research Laboratory of theSignal Corps, Fort Monmouth, N.J., was also made available on loan. The staff ofthe Princeton University Section, Division 2, National Defense ResearchCommittee, had aided greatly throughout the investigation, both with advice andin respect to securing promptly the needed equipment.-Authors' Note.
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particular region and can best be estimated by a militarysurgeon with considerable field experience. With such knowledge, effectivenessof antipersonnel bombs in terms of casualties can be accurately evaluated, sincethe distribution of fragment masses and their velocities at various distancesfrom the explosion can be readily determined. This chapter, however, does notpropose to estimate time of hospitalization as a result of wounds received fromany specific weapon but rather to determine the basic laws governing damage tothe various tissues in the body.
METHODS USED IN STUDYING WOUNDING
Army rifles are designed to shoot a 9.6-gram bullet with avelocity of 2,700 f.p.s. and to incapacitate or kill a human target weighingapproximately 70 kg. (kilograms). To investigate directly the mechanism ofwounding on such a scale would require many large animals and an extensivefiring range for the experiments. It is far more economical and fully asinstructive to reduce the size of missile and target in proportion. Theinvestigation can then be carried out in any laboratory. For example, a 0.4-grammissile 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 thoseof standard army rifle ammunition and the human body. Therefore, deeplyanesthetized cats and dogs have been used for study with steel spheres asmissiles (table 25). Fragments of varied shape and corresponding mass andvelocity have also been studied.
To supplement the direct experiments on animals, it is highlyinstructive to study nonliving models. These models simplify the physicalconditions and serve to illustrate what can happen in a homogeneous medium.Blocks of gelatin gel, rubber tubes filled with a liquid, or a tank, withPlexiglas sides, filled with water served as targets to record the phenomenaconnected with the passage of high-velocity missiles. The tank of water,particularly, allows high-speed photography and complete analysis of all thathappens.
Table 25.-TABLE 25.-Data on steel spheres
Diameter | Mass |
| Energy (? MV2) (ergs, 108X) at- | ||||
Inches | Centimeters | Grains | Grams |
| 4,000 f.p.s. | 500 f.p.s. | 4,000 f.p.s. |
1/16 | 0.159 | 0.251 | 0.0163 | 0.0248 | 0.199 | 0.0194 | 1.21 |
3/32 | .238 | .842 | .0549 | .0836 | .670 | .0654 | 4.08 |
1/8 | .317 | 2.00 | .1300 | .1981 | 1.58 | .154 | 9.65 |
3/16 | .476 | 6.77 | .4397 | .6700 | 5.35 | .522 | 32.60 |
? | .635 | 16.05 | 1.0413 | 1.588 | 12.70 | 1.240 | 77.30 |
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FIGURE 49.-General view of the tank of water withlights (behind), sabot screen (top), and high-speed motion picture camera(front) for study of phenomena during a shot into a liquid. The gun pointingvertically downward is attached to a beam above the tank. The bright spot oflight on the left side of the front of the tank is a sodium lamp running on 60cycle a.c. which records 1/120-second intervals in the film.
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Since many wounds in modern warfare come from steel bombfragments of small size but of high velocity, a gun was selected which could beused 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 shotwith pressures of 65,000 to 68,000 pounds per square inch (fig. 47). Thefragment or sphere was carried in a depression in the front of a cylindricalwood sabot about 16 mm. long, split in half longitudinally, and lathe turned tofit the caliber .30 Army standard primed shell (fig. 48). The wooden sabot wassatisfactory except for very high velocities (velocities in excess of 3,800f.p.s.), when it pulverized. In such instances, a similar Textolite plasticsabot was substituted. When the missile emerged from the gun, air resistanceseparated the two halves of the sabot. These halves were caught by a woodenscreen with a hole in the center through which the missile could pass. Theshells were filled with fast-burning, 60 mm. mortar powder which was adequatefor the sabot and fragments. Variations of velocity were obtained by varying thepowder charge from 0.1 gram (1,120 f.p.s.) to 1 gram (4,430 f.p.s.). If care wastaken in fitting the sabot, variations in the velocities showed a percentagedeviation of only 2.4 for a given powder charge. Figure 49 shows a vertical gunabove a water tank with Plexiglas sides to permit high-speed motion picturephotography. The lights used for illumination are to the left and the camera tothe right.
The velocity of missiles is fairly constant for a givencharge of powder, provided the sabots are carefully made to give uniform fit inthe ends of the shells. This statement has been checked by three differentmethods of measuring velocity. One method makes use of the shock wave of themissile in air. This shock wave is allowed to impinge on a metal platecontaining a row of small holes. On passing through the holes, the shock wave isconverted into a series of sound waves whose shadow is recorded on aphotographic plate by a spark discharge. The velocity of the missile is equal tothe velocity of sound in air, divided by the sine of the angle between theenvelope of sound wave fronts emerging from the holes and the path of themissile.
The well-known Aberdeen chronograph was also used to measurethe velocity. This instrument records, on a strip of paper fixed to a drumrotating at a known speed, the time taken by the missile to pass between twostations.
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As the missile passes each station (two tinfoil sheets), acontact is made, thereby triggering a spark which perforates the revolvingpaper. The time interval can then be read as distance between the twoperforations.
The third instrument used for recording velocities was theRemington chronoscope. This also necessitates two trigger screens. When thebullet passes one screen, a condenser begins to charge from a source of voltageand when the second screen is passed charging is stopped. The electrical chargeon the condenser then represents a certain time interval which the missile hastaken to pass between the stations and can be read with a ballisticgalvanometer.
High-speed moving pictures were taken either with the WesternElectric 8 mm. Fastax camera (fig. 50 A), capable of 8,000 frames per second, orwith the Eastman 16 mm. high-speed camera (fig. 50 B), capable of 3,000 framesper second. Both cameras use the optical compensation principle, in which thefilm moves across the lens continuously and a rotating prism throws successiveimages on the film with the same speed as the film itself. Trigger devices werenecessary to fire the gun at the proper moment by means of an electromagnet, asa 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 cyclesa.c. (alternating current). For illumination, banks of 2 to 12 150-wattprojection spotlights, run on 220 volts instead of the rated 110 volts, wereused. The light of these bulbs was directed on the object or, for transmittedlight, illuminated evenly a ground glass plate placed on the rear wall of thetank.
The spark shadowgraph technique for shock wave recordingdepends upon a change in refractive index of the medium resulting from a changein pressure. The change in refractive index can be detected on a photographicplate as a shadow, if a point source of light is used for illumination. Thepoint source of light used for high-velocity missiles in water was ahigh-voltage spark from the discharge of a condenser (fig. 51). The spark, whoseduration is less than a millionth of a second, is about 5 feet in front of thetank of water through which the missile will pass, and the photographic plate ison the rear wall of the tank. When the bullet breaks a contact in a screen, thespark is triggered through a thyratron controlled high-voltage surge across thespark gap. By means of a delay circuit, any time interval after the breaking ofthe screen can be selected for the spark shadowgram.
For taking roentgenograms with an exposure of a millionth ofa second, the Westinghouse X-ray surge generator, or Micronex, was used. Thisapparatus requires a special X-ray tube, with a large tungsten target and a coldcathode. The discharge of a bank of condensers through the tube supplies thecurrent of thousands of amperes, lasting less than a microsecond. Voltage can bevaried from 180 to 360 kv. (kilovolt), by charging the six condensers (each of0.04 microfarad capacity) in parallel at 30 to 60 kv. and then discharging inseries. A control box makes operation automatic, and a trigger and delay circuittimes the X-ray surge for any desired moment, measured in microseconds. Theentire outfit is shown in figure 52.
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For accurately recording pressure changes in an animal, acalibrated piezoelectric tourmaline crystal was used. As a result of changes inpressure, the crystal develops an electrical charge which can be amplified andapplied to a cathode ray oscillograph with a single sweep. The phosphorescenceof 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 thesweep, whose duration was varied between 130 microseconds and 45 milliseconds.The time calibration was made with a sine wave oscillator. Great precautionsmust be taken to shield the circuits from electrical and mechanical disturbanceswhich 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 necessaryto know what happens when a missile enters the body. The missile's retardationand penetration must be determined and all other phenomena measuredquantitatively and related to its mass and impact velocity. Since the materialof the body is heterogeneous and opaque, the investigation would be greatlysimplified if a homogeneous transparent medium could be substituted and used asa model for the establishment of fundamental laws.
Fortunately, this can be done. The nature of the forces whichact on a moving missile will depend on its velocity. For fast missiles, such ashave been used in this investigation, these forces are chiefly inertial forces.They depend
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primarily on the density of the medium rather than on itsviscosity 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 liquidor a gel.
Most soft tissues contain about 80 percent water, and it hasbeen found that many of the important events in wounding can be reproduced byshooting 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 avelocity of approximately 3,000 f.p.s. The large explosive temporary cavity isinitially cone shaped but later becomes more spherical and pulsates severaltimes before subsiding to a mass of air bubbles. The cavity behind a sphere shotinto water elongates as the sphere proceeds through the water. It also expandsradially and then shrinks. Along the narrow neck of the cavity not far behindthe sphere, the cavity eventually collapses, creating two cavities. The smallercavity continues to trail behind the sphere, while the larger one begins topulsate. The time at which the cavity separation or sealing off takes place
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depends on the size and density of the bullet. After thecavity behind the sphere separates, the larger main cavity moves slowly in thedirection of the sphere. As it pulls away from the surface, a narrow neckdevelops between it and the surface. The neck soon disintegrates leaving thecavity completely isolated. The isolated cavity continues in slow motion in thedirection of the sphere and eventually disintegrates. During all of thisprocess, the cavity undergoes a series of pulsations and grows and shrinks in aregular manner. The pulsations may continue for as many as 7 or 8 cycles anddisappear as the cavity disintegrates.
The velocity of radial movement of the water away from thesphere track is about one-tenth that of the sphere velocity. The maximumdisplacement of
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the cavity wall is proportional to the square root of thekinetic energy of the sphere at any level, and the maximum volume of theexplosive cavity is determined by the initial kinetic energy of the sphere. Thisis expressed as an expansion coefficient which gives the volume of cavity formedfor each unit of energy and is equal to 8.92 X 10-7 cc./erg. for water. Theperiod of the first few pulsations of the temporary cavity depends on the cuberoot of the missile energy and can be expressed numerically (pp. 181-189).
A gel behaves like water, as is illustrated in the framesfrom a high-speed moving picture of a 1/8-inch steel sphere entering 20 percentgelatin gel with a velocity of 3,800 f.p.s. (fig. 55). The phenomena are nearlythe same, even to the splash, although the numerical values of the constants aredifferent. 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 beexpressed by an excavation coefficient, which gives the volume of cavity formedfor each unit of missile energy. The behavior of a rectangular block of gelatinis shown in figure 56.
Rapid retardation of the sphere can be observed in figures 53and 54, where the tip of the cavity represents the progress of the sphere inequal units of time. This retardation is proportional to the square of thevelocity of the sphere, a general law for liquids expressed as a retardationcoefficient, a. If the material or size of spheres differ, the variousquantities are related in the following way: a=rACD/2M, whereCD is thedrag coefficient, r the density of the
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liquid, M the mass, and A the sphere projectedcross-sectional area. For water CD=0.297 and for 20 percent gelatin at 24? C.,CD=0.350.
If the missile is a fragment instead of a sphere, theprojected area will change as the fragment turns. Hence, the velocity in thewater will vary in an irregular manner. The retardation coefficient, the dragcoefficient, and the energy delivered to the water will all differ during theadvance of the fragment. Turning of the fragment thus leads to the formation ofirregular temporary cavities, as shown in figure 57. The cavity is widest when afragment moves broadside and smallest when the movement is head on.
The velocity squared law holds for spheres in water until thevelocity becomes very small. It is difficult to speak of a penetration distancein water. In a gel, however, after decrease to a certain critical velocity Vc,another retardation law is obeyed. Structural bonds and viscous forcesquickly 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 isproportional to the square of the velocity V and is numerically equal to?rV2CD.For the shot illustrated in figure 53, the pressure at impact is about 1,500atmospheres, and the water in front of the sphere is compressed and itsrefractive index changed. This region of compression at the surface of the watermoves away as a spherical shock wave, with a velocity slightly greater thansound in water (4,800 f.p.s.). Spark shadowgrams showing the successivemovements of the shock wave are reproduced in figure 58. Each wave consists ofan instantaneous rise in pressure to a peak, with an approximately logarithmicfall 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 40atmospheres and the half decay time about 30 microseconds. The peak intensity ofa shock varies directly as the equare of the impact velocity and the projectedarea of the missile and inversely as the distance from the water surface; it isindependent of the density of the missile. Shock waves are reflected fromsurfaces as either pressure or tension waves, depending on the wave velocity inthe material and the density of the material.
Behind the shock wave, the pressure distribution in the wateris complicated and continually changing. The very high pressure region in frontof the sphere can be visualized by inspection of figure 60, a spark shadowgramof a 3/16-inch steel sphere moving in water behind a grid of lines on aPlexiglas plate. The distortion of the lines in front and at the sides of thesphere is due to a change of refractive index, resulting from compression of thewater. Later on, much lower and slower pressure changes, with a phase ofdecreased pressure, appear around the temporary cavity. A record of these slowerpressure changes connected with pulsation of the cavity is shown in figure 61and 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-velocitysphere enters soft parts of the body. A retardation coefficient, a dragcoefficient, and ex-
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pansion coefficient (of the temporary explosive cavity) andan excavation coefficient (of the permanent cavity) can all be given numericalvalues.
Among tissues, the numerical constants vary slightly. Theydiffer somewhat from those of water or gel because (1) tissues vary greatly instructural makeup and (2) the body is enclosed in a layer of elastic muscle andskin, rather than the fairly rigid walls of a tank, as in the case ofexperiments with liquid mediums. Wound ballistics is actually a special branchof underwater ballistics. The remarkable similarity of the phenomena in tissuesand 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 tissuesresults in the immediate formation of an explosive or temporary cavity manytimes larger than the missile. After the passage of the missile, the largetemporary cavity decreases in volume and a much smaller permanent cavityremains. The size of the permanent cavity is undoubtedly governed by the size ofthe 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 softtissue, 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 spherestriking the thigh with a velocity of 3,000 f.p.s. is shown in figure 63A. Theexit 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 waveresulting from impact on the surface of water of a 3/16-inch steel sphere moving3,000 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 peakpressure is 600 pounds per square inch. (Experiment No. 41g, July 1945.)
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sphere is shown in figure 63B. In general, exit holesproduced by spheres are smaller than entrance holes, because of the decreasedvelocity of the sphere after it has traversed the thigh. In many cases, the exithole in muscle is slitlike as contrasted with the circular entrance hole. Thisslitlike opening is due to the fact that the muscle fibers split apart alongtheir long axes.
The size and configuration of the entrance and exit holesproduced by an irregular fragment is dependent on the orientation of thefragment at the instant it enters or emerges from the tissues (fig. 64). Theentrance hole made by a small elongate steel fragment (mass 612 mg.) whichstruck the thigh with a velocity of approximately 3,000 f.p.s. is shown infigure 64A. Yaw cards showed that the fragment struck the thigh broadside,inflicting a very large wound. Had the missile presented a smaller surface tothe tissues at the time of impact, a much less severe wound of entrance wouldhave resulted.
A microsecond roentgenogram showed that this same fragmentemerged from the thigh oriented along its long axis. Hence, the exit hole iscomparatively small, as is shown in figure 64B.
The approximate size and configuration of the wound track orpermanent cavity can be determined in several ways. These include (1)roentgenograms of the tissue made immediately after each shot, (2) explorationand dissection of the wound, and (3) reconstruction of the cavity from thin (1-2mm.) 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 spherethrough the thigh is
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FIGURE 63.-Muscle of cat thigh with entrance and exitholes produced by a 4/32 inch steel sphere with a striking velocity of 3,000 feet persecond. A. Entrance hole. B. Exit hole.
somewhat fusiform in shape, having its greatest diameter inthe central portions of the thigh. This is illustrated by the roentgenogramshown in figure 65A.
This simple configuration of the permanent cavity is quiteoften modified by the fact that individual muscles are blown apart along fascialplanes as a result of the passage of the missile. These newly created spacestend to become a part of the permanent cavity and to give it an irregularpattern as shown in figure 65B.
This same type of fusiform cavity is produced when a smallhigh-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-inchsteel spheres through a block of gelatin gel are shown in figure 66B.
Dissection of the wound track in the thigh reveals that thepermanent cavity is largest near the center of the thigh and smallest at thepoints of entrance and exit of the sphere. This fact is illustrated by thethigh shown in figure 67. Figure 67A shows the entrance hole in the thigh of acat made by a 4/32-inch steel sphere which struckthe thigh with avelocity of 3,800 f.p.s. Figure 67B shows the much larger cavity deeper in thetissues of this same thigh. These photographs demonstrate clearly that thesmall wound of entrance gives no true picture of the amount of damage produceddeeper in the tissues.
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The most exact method of determining the size andconfiguration of the permanent cavity is by a study of serial sections of thetissues cut in a plane at right angles to the path of the missile. Arepresentative set of these sections, each approximately 2 mm. thick, is shownin figure 67C.
Study of a number of sets of serial sections reveals that thepermanent cavity in the thigh actually consists of a series of fusiformcavities. This manner of cavity formation is related to the anatomy of the thighmuscles. It appears that as a sphere traverses the thigh a permanent fusiformcavity is formed in each of the larger muscles. The permanent cavity left in theintermuscular connective tissue is quite small, probably because of the elasticproperties of this type of tissue. Thus, the permanent cavity or wound track inthe thigh is really a series of fusiform cavities, individual muscles givingrise to what might be called a scalloped wound.
Essentially, this same type of behavior can be obtained byfiring a high-velocity steel sphere through a series of three blocks of gelatingel, separated by
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B. Roentgenogram (No. 200) shows irregular shape of the cavity.
several sheets of cellophane to simulate the intermuscularfascia. The results of this experiment are shown in figure 68. The sphere passedfrom right to left in the photograph. This photograph, taken immediately afterthe shot, shows that fusiform cavities are formed in each block, the size of thecavity 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 preciselyidentical with that of muscle and fascia, but the general characteristics of thecavities in the two cases are quite similar.
The shape and size of the temporary cavity is often modifiedby the fact that the cavity may come in contact with a rigid structure, such asbone. Then, as the large temporary cavity continues to expand, soft tissues arepulled away from the bone, and these tissues fail to regain their normalposition after the collapse of the temporary cavity. This type of behavior isillustrated by the roentgenogram shown in figure 69.
The question of what becomes of the mass of tissues whichoriginally 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 thepermanent cavity left after the passage of a 4/32-inch steel sphere whose impactvelocity was 3,000 f.p.s. Note the similarity of this cavity to that shown inthe thigh in figure 65A. B. Block of 20 percent gelatin gel showing the fusiformpermanent cavities left after the passage of several 4/32-inch steelspheres whose impact velocities were approximately 2,400 f.p.s.
motion pictures and spark shadowgrams show clearly that largeamounts 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 ahigh-velocity steel sphere passing into a tank of water. The penetration of themissile brings about a marked "splash" at the point of entrance, withthe water moving backward at a high velocity. The splash which occurred at thepoint of exit of a 4/32-inch steel sphere in a block of Plasticine isshown in figure 71.
In cases where complete perforation of an object is obtained, large amountsof material are thrown out at both the points of entrance and exit of thesphere.
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This is clearly shown in figure 72, a spark shadowgram of ablock of gelatin gel taken immediately after the passage of a 4/32-inchsteel sphere.
The situation in soft tissues of living animals appears to bevery similar to that described for a gel. Figure 73 is a spark shadowgram of thethigh of a cat, taken immediately after the passage of a 4/32-inchsteel sphere. A definite splash has occurred at the point of entrance of themissile, and materials are flying out at a high velocity. Large amounts ofmaterial are also being
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swept out by the missile as it emerges at the left. The lossof materials at the points of entrance and exit of a missile can be demonstratedin shots through the abdomen and excised organs, such as the brain, liver, andkidneys.
THE EXPLOSIVE OR TEMPORARY CAVITY IN MUSCLE
A missile entering soft tissues at a relatively high velocityproduces a temporary or explosive cavity of large dimensions. The cavity, at itsmaximum size, has a cross-sectional diameter many times that of the permanentcavity, which remains after the temporary cavity has collapsed. The temporarycavity persists for a relatively short time, reaching its maximum size in lessthan a millisecond and lasting for not more than several milliseconds.
The penetration of a small high-velocity steel sphere into alarge mass of butcher meat results in the formation of an initially cone-shapedcavity, 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 largecavity formed in butcher meat by a 4/32-inch steel sphere which struckthe meat with a velocity of 2,800 f.p.s. and had penetrated a distance of 10.2cm. when the roentgenogram was made. The sphere eventually perforated the blockof meat completely, so that this roentgenogram does not show the finalconfiguration of the temporary cavity. Its chief value lies in demonstrating thestriking similarity
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of the early cavity in animal tissue and that in water, shownby 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 itsgreatest dimension. A single microsecond roentgenogram of a thigh can showonly one particular stage in the development of the temporary cavity. However,by varying the interval between the time at which the missile struck the thighand the time at which the roentgenogram was made, it is possible to obtain aseries of pictures which together will show successive stages in the developmentof the cavity. A series of five such microsecond roentgenograms, showing thedevelopment of the cavity in the thighs of dogs, is shown in figure 75. In eachcase, the thigh was struck by a 4/32-inch steel sphere whose impactvelocity was approximately 2,800 feet per second.
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Figure 75A is a microsecond roentgenogram showing thetemporary cavity 56 microseconds after the sphere struck the thigh. Acone-shaped cavity has formed behind the sphere, whose walls are relativelysmooth. It is at this stage of development that the similarity of the temporarycavity in animal tissues and in water is the greatest.
Figure 75B shows the cavity 71 microseconds after the spherestruck the thigh. The sphere has emerged from the thigh and has moved severalcentimeters from it. The conical cavity is expanding, and its walls are becomingsomewhat irregular.
The roentgenogram in figure 75C shows a cavity whose age is139 microseconds. The sphere has now moved out of the field of the photograph tothe right. The cone-shaped cavity has continued to expand, and its walls havebecome very irregular, probably owing to the irregular stretching and tearing oftissues being displaced by the cavity.
Figure 75D shows the cavity photographed 390 microsecondsafter the sphere struck the thigh. The cavity has expanded still more and hasassumed the shape of a prolate ellipsoid. Observation of many of these cavitiesindicates that a cavity with this configuration is near its maximum size. Thecavity shows marked irregularities on its walls, as well as strands of tissue ofdifferent densities, which can be interpreted as areas of stretched and torntissues. The sphere which produced this cavity had an initial energy of 3.7 X 108ergs (35 ft.-lb.) and lost approximately 85 percent of this energy in producingthe cavity.
Roentgenograms made from 600 to 800 microseconds after thesphere struck the thigh show that the cavity, after reaching its maximum size,col-
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lapses. Figure 75E shows a cavity whose age is 819microseconds. The cavity has practically collapsed, and only a small roundedspace remains near the center of the thigh. High-speed motion pictures of theexterior 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 thepassage of a 4/32-inch steel sphere with an impact velocity of 2,800f.p.s., is shown in figure 77A. Although this cavity has not reached its maximum sizeand the sphere did not strike the femur directly, a fracture line has appearedin this bone. Figure 77B is a roentgenogram of this same thigh made before theshot and figure 77C a similar roentgenogram made after the shot. In this latterpicture, 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 photographedto show the path of the missile and the cavity in lateral view. Othermicrosecond roentgenograms show that the cavity formed in soft tissues by asphere is circular
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when seen in cross section. The latter is well shownin the roentgenogram in figure 78, taken 200 microseconds after the spherestruck 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 andconfiguration of the temporary cavity depends not alone on the energy of thefragment but also on its projected area as it strikes the tissue. The projectedarea varies along the path of the missile as changes in orientation of thefragment occur. This is illustrated by the microsecond roentgenogram shown infigure 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 impactvelocity was 3,000 f.p.s. The fragment struck the thigh broadside and emergedwith the orientation shown in this photograph. The cavity is very large at thepoint 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 athigh was struck by an elongated fragment made from a small wire nail. Thefragment was cylindrical, 11 mm. in length, 2.5 mm. in diameter, and had a massof 380 mg. Its striking velocity was approximately 3,000 f.p.s. The irregularshape of this cavity indicates that the orientation of the fragment changedslightly as the missile passed through the tissues.
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The temporary cavities produced by standard .22caliber ammunition are very similar to those produced by spheres, as long as thebullet remains oriented on its long axis. This is illustrated by theroentgenogram in figure 81. If the bullet wobbles, or in any way changes itsorientation, the result is similar to that just described for fragments.
A temporary cavity, very similar to thosedescribed in cat thighs, can be obtained by firing a steel sphere through theexcised skin of a cat thigh which has been filled either with gelatin gel orwith water. The cavity in a gelatin-filled skin is shown in figure 82A and in awater-filled skin in figure 82B. These photographs again emphasize thesimilarity of the temporary cavities in animal tissues and in the nonlivingmaterials used.
Study and measurement of a large number oftemporary cavities show that the total volume of the cavity is proportional tothe energy delivered by the missile. Data obtained have made it possible toobtain a value for an expansion coefficient, k. The expansioncoefficient, 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 inmuscle, there is formed a temporary cavity with a volume of 80.1 x 10-9cm.3
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The relationship of total cavity volume toenergy expended can be demonstrated in another way. Steel spheres of twodifferent 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 velocitiesof the two spheres were adjusted so that each size of sphere would loseapproximately the same amount of energy in passing through the tissues. Thestriking velocity of the 8/32-inch sphere was approximately 1,500f.p.s.; that of the 4/32-inch sphere, 3,000 f.p.s. In cases wheremeasured energy losses were approximately equal, the volumes of the temporarycavities produced by the two-sized spheres were likewise approximately equal. Anillustration of this equality is shown in figure 83.
The formation of this high explosive cavityresults in great displacement and tearing of muscle and connective tissues,rupture of small blood vessels, and stretching and compression of larger bloodvessels and nerves. This behavior is sufficient to account for the very seriousdamage often observed in wounds at a considerable distance from the missiletrack. A more detailed description will be found on pages 189-200.
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THE EXPLOSIVE OR TEMPORARY CAVITY IN ABDOMEN, THORAX, AND HEAD
Phenomena quite similar to those which havebeen 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 cavitycollapses. During the expansion, tissue is stretched and torn, and, followingthe pulsation and collapse of the cavity, tissue is violently pushed togetherwith additional injury.
Although the general structural makeup of theabdomen 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 thelungs. Its walls are also more rigid than are those of the abdomen, because ofthe 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 expansioncoefficient can be expected to be quite different in the three regions.
The chief changes resulting from a shotthrough the abdomen of a deeply anesthetized cat are shown in figure 84. The twobulges of the temporary cavity on each side are apparent in frames 2 to 4. Thesebulges later collapse (frames 5 to 14) and then appear again (frame 15) assmall, wrinkled projections
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which later merge with the general violent,twisting movements of the abdomen. A similar type of swelling, indicative of alarge temporary cavity within, results from a shot through a rubber tube filledwith water (fig. 85). The abdomen behaves like this model liquid system.
The large temporary cavity within the abdomenis revealed in the microsecond roentgenogram of figure 86, triggered just as thecavity is beginning to collapse, as indicated by the slight indentation on eachside. In this figure and in figure 87, the intestine has been made radiopaque bybarium 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 theshot. The increased diameter of the intestine is readily apparent in the centermicrosecond roentgenogram, probably because of the flattening against theabdominal walls. Note that the barium sulfate has leaked out into the bodycavity after the shot, indicating extensive perforation and damage to theintestine, a point corroborated by autopsy.
Microsecond roentgenograms, taken at a timewhen the second protuberances of frame 15 (fig. 84) have appeared, show nosecond internal cavity. The collapse of the initial temporary cavity seems to becomplete. Since entrance and exit holes in the skin are small and a markedsplash of material
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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 isfilled mostly with water vapor, and consequently complete collapse will occur,with only a few small gas pockets undergoing pulsation. In this respect, a shotinto the abdomen differs from a shot into a tank of water where the partiallyair filled temporary cavity (fig. 62) undergoes a series of marked pulsations.If a steel fragment instead of a sphere is shot through the abdomen, irregulartemporary cavities appear (fig. 88).
During a shot through the thorax, very littlemovement is evident (fig. 89). The lack of movement is connected in part withthe air-filled lungs, which do not fulfill conditions for cavity formation, andin part to the strong rib-reinforced walls of the thorax. In roentgenograms(fig. 90) giving views before, during, and after the shot, no clearly visiblecavity is apparent. Because of the large amount of air in the lungs and thedifficulty of distinguishing cavity from air, a clear-cut temporary cavity ishardly to be expected. It is apparent, however, that the heart has beendisplaced upward and to the right as a result of the shot, so that some type oftemporary cavity is presumably formed.
The pressures which accompany a high-velocitymissile moving through tissue are enormous (pp. 211-223). Therefore, it is notsurprising to find that a steel sphere fired into the head can produce atemporary cavity in brain tissue, despite the apparent strength of the craniumwhich must resist the pressure. The cavity formed by a missile in the brain ofan intact cranium is of finite size, partly because brain tissue is forcedthrough regions of less resistance (such as the frontal sinuses and the variousforamina of the skull) and partly because of the stretching of the craniumitself. When the energy delivered is very great, skull bones are actually tornapart along suture lines.
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The temporary cavity within the skull isapparent in the microsecond roentgenogram of figure 91, a dog's headperforated by a 1/8-inch steel sphere moving 4,000 f.p.s. Figure 92is a similar microsecond roentgenogram of the head of a cat showing viewsbefore, during, and after the shot. A cavity similar to that in the dog's headis apparent in the microsecond roentgenogram of the cat.
The explosive effect of a high-velocitymissile within the cranium increases with increased energy. With very highvelocities, there is complete shattering of the skull, usually along suturelines. This effect is illustrated in figure 93. Movement of brain tissue duringexpansion of the temporary cavity pushes the bone apart.
To demonstrate the necessity of a liquidmedium for the development of these pressure effects, the brain of a cat wasremoved through the foramen magnum and the air-filled head was then shot with a 1/8-inchsteel sphere moving 3,800 feet per second. A photograph of the cleaned skull ofthis cat is reproduced in figure 94. It will be noted that no shattering hasoccurred, the only damage being rather neat entrance and exit holes. Without aliquid medium, the high pressure necessary to blow skull bones apart cannot bebuilt up.
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MOVEMENTS FOLLOWING COLLAPSE OF THEEXPLOSIVE CAVITY
In the preceding pages, the explosive cavity insoft tissue, with its volume many times greater than the volume of materialswept out by the missile, was clearly demonstrated. It was reasonable to supposethat when the cavity collapsed such violent motion would not immediately stop.Investigation of the movement in soft tissue after the cavity has collapsedbears out this conjecture. The motion continues for a considerable length oftime, long after the missile has passed by. Once again, it is instructive toexamine 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 iscompressed to its minimum volume, it springs open again and the process isrepeated. The cavity thus undergoes a series of pulsations. For a 1/8-inchsteel sphere traveling with an impact velocity of 3,000 f.p.s., the first fewpulsations have a period of about 8 milliseconds. The period is greatest for thespheres of greater energy. The period in seconds for all spheres was found toequal the product of 9.85 X 10-6 and the cube root
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FIGURE 87.-Roentgenograms ofabdomen of a cat. The alimentary tract has been made radiopaque with bariumsulfate. A. Roentgenogram (No. 186) made before the shot. B.Microsecond roentgenogram (No. 186) showing the large temporary cavity formedafter the passage of a 4/32-inch steel spherewith an impact velocity of 3,200 feet per second. C. Roentgenogram (No. 186)made immediately after the shot. Note distribution of opaque material ascompared with that shown in A.
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FIGURE 88.-Microsecondroentgenogram (No. 267) of the abdomen of a cat showing the temporary cavityformed by the passage of a small cylinder of steel (11 X 2.5 mm.) weighing 420mg. Its striking velocity was 3,000 f.p.s. Note the irregular shape of thecavity.
of the impact energy in ergs. The periodicity ofthe cavity is clearly illustrated in figure 62, the first minimum appearing inframe 23 and the second in frame 47. The pulsations in water for a 6/32-inchsphere traveling with a velocity of 3,000 f.p.s. have been observed to last atleast one twenty-fifth of a second. The pulsations in water occur because air istrapped within the missile track. As air rushes into the cavity, the cavity issealed off by Bernouilli forces.
In gelatin gel, the cavity also appears topulsate about an air bubble, but in this case the pulsations are directed alongthe track of the missile. A typical pulsation cavity is shown in figure 95. Thecavity closes in from the top and bottom to form two internal nipples, as can beseen in frame 11. Eventually the cavity breaks up in two segments, as shown inframe 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 largerthe energy of the shot, the greater the action on the abdomen. Some concept ofthe violence of this movement can be obtained from inspection of figure 84. Inframes 10 and 13 of figure 84, the abdomen is considerably indented where thebullet perforated. This is also shown in figure 96. Some of the expansivemovement is directly upward toward the thoracic cavity. However, the motion inthe abdomen is
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not like that of the pulsating cavity in the water tank butrather like the distortion waves which are set up in a block of gel when it isgiven a sharp blow. The microsecond roentgenograms show a complete absence of anoscillation bubble, as was seen in water.
The shot into the thigh of a cat also produces a violentaction. 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 surfaceare produced, as in the "bullet-view" moving pictures of figures 97and 98. These waves travel down the thigh with velocities ranging from 4.1 to5.2 meters per second. It is not clear whether this wave was the regularmuscular contraction wave (velocity between 6 and 12 meters per second) orrather a mechanical disturbance.
Unlike the abdomen, the cavity in the thigh pulsates on apartially air filled cavity. When the moving pictures are studied, thesepulsations can be observed and timed. For example, a sphere traveling with avelocity of about 3,000 f.p.s. was observed to start pulsations having a periodof about 3 milliseconds. Microsecond roentgenograms show an air bubble in thethigh at a late stage. Figure 99B is a microsecond roentgenogram taken 3.5milliseconds after the missile passed through the leg. This is at a time whenthe second expansion of the cavity occurs and the entrapped bubble of air isplainly visible.
It is of interest to conjecture on what would happen to partsof the body when struck by a missile, if these parts were not confined by suchstructures as
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skin, abdominal wall, or skull. The disintegration of the tissuewill presumably be greater when it is unconfined. In figure 100 is shown thebare muscles of the thigh as they are struck by a missile. The muscles areextensively separated, and the bullet hole shows clearly, although the path ofthe bullet was in the plane of the picture. In figure 101 is shown a pig spleenwhen struck by a missile. This picture was taken with two mirrors; the one aboveprovides a top view, while the one on the left shows the entrance hole. Thetissue 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 thethigh. Some attention, however, will be given to wounds of the abdomen andthorax. The nature of the damage produced in the thighs of anesthetized dogs andcats by high-velocity missiles is representative of that occurring in muscularand connective tissues. In consideration of such a wound, it is necessary todistin-
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guish between damage to soft tissues, such as muscle andconnective tissues, and damage to the more specialized structures of the thigh,such as the femur, nerves, and larger blood vessels. Only those in the firstcategory will be described here, while damage to the more specialized structureswill be considered later (pp. 200-211).
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Obviously, soft tissues directly in the path of a missile arebadly damaged. These tissues are reduced to a pulp and much of the material isactually thrown out of the thigh during the expansion of the temporary cavity,as discussed previously (pp. 167-180). The loss of this material leaves anexcavation, the permanent cavity.
It has been shown earlier that the expansion of the temporarycavity results in a stretching and tearing of the tissues for a considerabledistance away from the missile track. With the collapse of the temporary cavity,these tissues regain their original positions and, except for darkened areas ofextravasated blood, may have a fairly normal appearance, macroscopically.
A more complete assessment of the exact type of damagesuffered by these soft tissues can be had from a histologic study. In each caseto be described, a considerable volume of tissue adjacent to the wound cavitywas fixed and sectioned at thicknesses ranging from 20 to 50 microns.
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Tissues bordering the wound cavity in the thigh suffer twoprimary types of damage: (1) That affecting the muscle fibers and (2) thataffecting the intermuscular and intramuscular connective tissues and small bloodvessels. Damage to the muscle fibers is manifested by a coagulation and swellingof the fibers
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in a region extending for some distance from the woundcavity. The muscle fibers (fig. 102) in this region are unique in their stainingproperties and often swell to twice the diameter of normal fibers. Swollenfibers are well shown by the photomicrograph in figure 102A. These fibers shouldbe compared with normal undamaged fibers, photographed at the same magnificationand shown in figure 102B. More distal to the wound cavity, "muscleclots" are formed, accompanied by other phenomena of cellulardisorganization. Still further distally, however, the muscle fibers exhibit aremarkably small amount of damage despite the fact that they have been movedconsiderably by the expansion of the temporary cavity. The three regions justmentioned are visible in the photomicrograph in figure 102C. Normal undamagedfibers 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 fromthe permanent wound cavity. Multiple ruptures of the capillaries occur, and themuscle fibers are widely separated by accumulations of extravasated blood. Thisis illustrated by the photomicrograph in figure 103. These areas of hemorrhagemay extend for considerable distances along fascial lines. Histologic sectionsshow that the larger blood vessels, even though they lie close to the woundcavity, are undamaged. Bleeding around the wound appears to be a matter ofcapillary bleeding, unless a larger blood vessel is struck directly.
It should be emphasized that these observations are based onmaterials fixed within an hour or so after the shot. No attempt has been made toconduct survival studies or to follow the course of wound healing.
Because of their structural characteristics, it is verydifficult to determine the exact type of damage suffered by the diffuseintermuscular connective tissues. The are elastic, and, as a result, thepermanent cavity formed in them is quite small. Examination of areas around thewound shows that the individual muscles are often widely separated and strippedfrom their surrounding connective tissues. It appears quite likely that a greatdeal of the expansion caused by a missile follows these intermuscular fascialplanes and causes damage in these tissues at considerable distances from thewound cavity.
Because of the heterogeneous nature of the tissues and organsinvolved, wounds of the abdomen are much more difficult to evaluate accurately.If the missile passes through the intestinal mass, regions of the intestinedirectly in the path of the missile are usually completely severed or exhibitlarge tears. A chief factor in causing damage in the abdomen is the rapidlyexpanding temporary cavity which momentarily blows apart the components of theintestinal mass, as illustrated by high-speed motion pictures and microsecondroentgenograms on pages 182 through 185. This cavity may produce large tears inthe mesenteries with damage to such organs as the pancreas and spleen. Breaks inmany of the mesenteric blood vessels occur, causing severe hemorrhage into theperitoneal cavity.
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Perforations of the intestine are often observed at pointsquite distant from the path of the missile. These are undoubtedly due to rapidpressure changes associated with the temporary cavity, acting on gas containedin the intestine. A short period of lowered pressure in the cavity around theintestine causes the intestine to explode at points where these gas pockets arepresent, as explained on pages 211-223.
Damage to thoracic structures was restricted primarily tolung tissue, as in none of the experiments were the heart or great vesselsstruck directly. The wound track in lung tissue was never large, probablybecause of the sponginess and elasticity of this type of tissue. The thorax, onautopsy, usually contained a considerable amount of blood, a result ofhemorrhages of the smaller
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pulmonary vessels. In all the animals studied, the lungs weregreatly 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 toflat bones, such as those which comprise the skull.
The most obvious type of fracture of a long bone is one whichresults from a missile striking the bone directly. In none of the experimentswas 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 microsecondroentgenograms were obtained of thighs in which this was the case.
Figure 104 is a microsecond roentgenogram of the thigh of acat, made immediately after the passage of a 4/32-inchsteel sphere whose impact velocity was 3,000 f.p.s. The sphere struck the femurdirectly. The fact that the bone has been hit has not markedly affected theexpansion of the temporary cavity. In fact, it appears from this roentgenogramthat the femur also "explodes," in a manner very similar to the softtissues around it.
A second case is shown in the microsecond roentgenogram infigure 105 where the femur was struck by a small fragment (originally part of a75 mm. shell). In this case, the fragment was broken into two pieces as a resultof its impact with the bone. One piece has remained in the thigh, the second hasemerged. Figure 106 is a microsecond roentgenogram of a beef rib, madeimmediately after the passage of an 8/32-inchsteel sphere whose impact velocity was 2,800 f.p.s. The behavior of the bone isvery remindful of the manner of formation of the temporary cavity in softtissues.
The question whether bone fragments may be driven out intothe soft tissues and act as secondary missiles is a significant one. The presentobserva-
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ions indicate that fragments fly out into the temporarycavity and, with the collapse of the cavity, are forced back to approximatelytheir former position. Dissection of wounds, where such extensive shattering ofa 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 figure107, which was made shortly after the femur was struck by a 4/32-inchsteel sphere whose impact velocity was 3,000 f.p.s. The sphere was firedparallel to the X-ray beam so as to pass into the plane of the paper. Althoughthe bone is badly shattered, the fragments are closely clumped together and seemto retain a connection with the parent bone, possibly being held there by thefibrous periosteum. They are free to move but actually are not separated fromthe bone.16
A second and less severe type of fracture is that produced bya missile which passes near but does not strike the bone directly. This can betermed an indirect fracture. Roentgenograms of a large number of thighs showthat the femur can be broken even though the missile passed as far as 2 or 3centimeters from the bone. A roentgenogram of this type of fracture is shown infigure 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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It is also clear that the cavity has expanded toward the femurand that the bone is fractured, as if it had received a heavy blow from thedirection of the cavity.
Figure 108 is a roentgenogram of the thigh of a dog madeafter the thigh was struck by an 8/32-inchsteel sphere with an impact velocity of 4,000 f.p.s. The femur has beenfractured although the sphere passed at a considerable distance from it.
A second case is illustrated by the roentgenogram shown infigure 109. In this case, the thigh was struck midway between the femur and thesciatic nerve. The nerve in this case has been made radiopaque by the injectionof iodophenylundecylate. The femur shows a simple fracture. This type offracture should be compared with the marked comminution of that shown in figure107, which resulted from a direct hit on the bone (see also roentgenograms onpp. 173-181).
The incidence of the indirect type of fracture appears to berelated to the
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striking energy of the missile. In the case of 4/32-inchsteel spheres, it was found that no fractures of this type occurred atvelocities 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 velocitiesused, 4,500 to 4,800 f.p.s., in 45 percent of all the cases.
It is significant that, of the total number of indirectfractures, 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 wereshot. A probable explanation of this result is that the humerus isarchitecturally better able to stand the high pressures imposed on it by themissile than is the femur. Also, the humerus appears to be better protected bythe surrounding muscle and fascia than is the femur.
The explanation of the indirect type of fracture is found inthe rapidly expanding temporary cavity. As this cavity expands, high pressuresare brought to bear against the rigid bone. The situation is similar to that ofstriking the bone a hard blow with a hammer. Figure 110 illustrates this pointnicely. This figure shows a microsecond roentgenogram of the thigh of a cat madeimmediately after the passage of a 4/32-inchsteel sphere whose impact velocity was 2,800 f.p.s. The temporary cavity isexpanding, and careful examination of the femur shows that a clean fracture linehas appeared in the bone. A second and similar case is shown in figure 111.
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Studies on skull damage were made chiefly with 4/32-inchsteel spheres. Damage to the skull varied from the presence of neat holes, atthe points of entrance and exit, to extensive fractures, sometimes resulting incomplete 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 missilevelocity and probably depends on the striking energy of the spheres. This isillustrated by the series of skulls shown in figure 112. Figure 112Ademonstrates the neat type of hole which ordinarily occurs when the skull is hitby a 4/32-inch steel sphere with an impactvelocity of approximately 1,100 f.p.s. The more extensive damage which occurs athigher velocities is shown in figure 112B, a case where the skull was struckwith a sphere having a velocity of approximately 4,000 f.p.s. The extensivesplitting along sutures and shattering which frequently occurred at the highervelocities is illustrated in figure 112C, a skull struck with a sphere whoseimpact 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 resultsfrom pressure developed within the skull at the time a temporary cavity isformed in the brain immediately after passage of the missile. A complete accountof 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 awound in the soft tissues of the thigh resulted primarily from the rupture ofcapillaries and small blood vessels. It has been a matter of frequentobservation that the larger blood vessels, particularly the arteries, passing inor near the wound cavity were apparently undamaged. These vessels are veryelastic, and the assumption was made that, unless they lay directly in the pathof the missile, they were merely blown aside during the expansion of thetemporary cavity and sprang back to their original positions with its collapse.
The correctness of this assumption is confirmed bymicrosecond X-ray studies (fig. 113). Figure 113A shows a roentgenogram of thethigh of a cat in which the femoral artery and its tributaries have been maderadiopaque 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. Figure113B is a microsecond roentgenogram of the same thigh made immediately after thepassage of a 4/32-inch steel sphere with avelocity of 3,200 f.p.s. The large temporary cavity, resulting from the passageof the missile, is seen in cross section. It is evident that, although thesphere passed at a considerable distance from the vessels, they have been forcedaside and follow the contour of the margin of the cavity. Figure 113C shows thesame thigh immediately
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after the shot. The location of the permanent cavity is welldefined. The blood vessels have moved back to their original position as shownin figure 113A. Subsequent dissection disclosed that both the artery and thevein were undamaged. The magnitude of the blow suffered by these vessels wassuch as to fracture the femur.
Unlike arteries and veins, large nerves, as the sciatic nerveof the cat, are often severely damaged as a result of being displaced by thetemporary cavity. This displacement may cause a stretching and a compression ofthe nerve sufficient to block its ability to conduct impulses, even though thereis no detectable break in the continuity of the nerve.
The sciatic nerve can be made radiopaque by injecting it witheither iodobenzene or iodophenylundecylate (fig. 114). The exact manner in whichthese 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 greatlydisplaced as the temporary cavity expands. Figure 114C is a microsecondroentgenogram of the thigh shown in figure 114B, made immediately after thepassage of a 4/32-inch steel sphere with avelocity of 3,200 f.p.s. The cavity is seen in cross section. The roentgenogramshows that the nerve has been pushed aside
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and follows around the margin of the cavity. Because of theextreme rapidity with which this displacement occurs, the situation iscomparable to striking the nerve a sharp blow. Figure 114D shows this same nerveimmediately after the shot. Subsequent dissection showed no break in thecontinuity of the nerve and nothing to suggest gross anatomic damage to thenerve.
In a number of cases where the nerve had been subjected tocompression and stretching by the expansion of the temporary cavity, conduction,as determined by electrical stimulation, was blocked. In general, it wasnecessary for the missile to pass within 1 centimeter of the nerve in order toblock 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 whichaccounted for the loss of conduction. Figure 115 is a photomicrograph of alongitudinal section
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of an undamaged control sciatic nerve of a cat. Figure 116 isa similar photomicrograph of a nerve in which conduction was blocked. Thisfigure shows that the nerve fibers have been widely separated and that manyfibers are completely severed, with their ends badly frayed. A critical study ofmany of the fibers at very high magnifications indicated that the axis cylindersof many of them were broken, but the myelin sheath and neurilemma showed nosigns of damage. Figure 117 shows a section from another nerve. In this
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FIGURE 115.-Photomicrographof a section of an undamaged control sciatic nerve of a cat.
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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 describedhere, the nerve sheath (epineurium) appeared undamaged.
PRESSURE CHANGES ACCOMPANYING THE PASSAGE OF MISSILES
When a high-velocity missile strikes the body and passes throughsoft 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 themoving missile; (3) relatively slow low-pressure changes connected with thebehavior of the large explosive temporary cavity formed behind the missile.
Some characteristics of shock waves in water have alreadybeen considered (pp. 152-158). Attention was also directed to the high-pressureregions around the moving sphere, whose effects are seen in figure 60. Shockwave pressures and cavity pressure changes in the body can be investigated intwo 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 foundin the body. For accurate recording, a calibrated tourmaline piezoelectriccrystal gage was placed in the stomach of the deeply anesthetized animal whichwas then shot through the posterior part of the abdomen. The method is describedon pages 147-152. In order to record shock wave pressures, the amplifier gainwas low and the sweep rapid, calibrated in microseconds. To record pressurechanges around the temporary cavity, the gain was high and the sweep relativelyslow, calibrated in milliseconds.
For visualizing the shock wave pressures, the sparkshadowgram method described on page 150 was used. The tissue, placed on thesurface of a tank of Ringer's solution, was shot with a high-velocity missileand the spark triggered to catch the shock waves as they moved from tissue tosolution; or the tissue was suspended in the solution and the behavior of shockwaves on reflection or transmission recorded.
Shock waves in tissue arise at the impact of the missile withthe skin or other tissue surface. The velocity of shock waves in tissue isapproximately the same as in water, 4,800 f.p.s. The chief difference inbehavior of shock waves in the body, as compared with water, is associated withthe heterogeneity of the tissues. The wave is dispersed on transmission through,or on reflection from, surfaces. Instead of a single clean wave, there appears amass of wavelets with a series of high-pressure peaks. Figure 118 shows a shockwave in water partially reflected and partially transmitted by a slab of gelatingel suspended in the tank. The gelatin is sufficiently homogeneous to give goodreflection. Figure 119A shows waves which have arisen in, and passed out of, amass of thigh muscle suspended at the surface of Ringer's solution. Figure119B shows a shock wave which has originated from a thigh muscle surface. Inboth cases, the dispersion of the wave is apparent.
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Reflection and transmission also occur from a piece of cat'sstomach spread on a frame, as illustrated in figure 120. The behavior of a shockwave at the body wall is illustrated in figure 121, which shows a piece of theabdominal wall (skin and muscle) of a cat stretched on the surface of a tank ofRinger's solution. The tank has then been penetrated by a horizontal shot (toright). The shock
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wave can be seen toward the left and has been reflected fromthe undersurface of the body wall. Note that a light band precedes the dark bandof the shock wave, indicating that, on reflection, a pressure pulse has beenchanged to a tension pulse. Such reversal occurs whenever the acoustic impedance(defined as the density multiplied by the wave velocity) of the reflectingmedium is less than that of the medium in which the wave was moving. At an airsurface, the pressure wave is always reflected as a tension wave.
Reflection from bone, in this case the surface of a humanskull suspended in water, is illustrated in figure 122, while figure 123 depictsa row of beef
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ribs (seen in cross section) tied together so as to representthe skeleton of the thoracic wall. Reflection from each bone is clearlyapparent, as well as the secondary wavelets formed when a shock wave movesthrough the opening between ribs.
Shock waves do not appear to pass into water when a bone ishit directly by a high-velocity missile. Figure 124A is part of a beef femurwhose upper end has been struck. No waves are visible moving from the bone towater, as appear when a bar of steel, shown in figure 124B, is substituted forthe bone.
Tourmaline crystal pressure records of four shock waves inthe abdomen of a cat are reproduced in figure 125. It will be observed thatthese records differ from a shock wave in water in that the descending limb ofthe pressure peak is steep and the shock waves themselves are often multiple. Inall these records, the pressures stop abruptly at a certain point. This is anartifact due to blocking of the piezoelectric amplifiers by the surge of currentthrough the microsecond X-ray apparatus used to record conditions within theabdomen at the time the pressure record is made. Such a microsecondroentgenogram is reproduced in figure 126.
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It will be noted from figure 125 that the first pressurepulse of a series may not be so high as the succeeding pulses. This can beexplained in part by the reflection of shock waves in the abdomen and in part bythe presence of gas pockets in the alimentary tract. Whenever a missileperforates a gas pocket and enters tissue on the opposite side, a new shock wavewill be generated. Since the new wave is nearer the crystal, its peak will behigher 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 somuch depends upon reflection and distribution of gas in any particular case.
The manner in which a series of shock waves could appearwithin the abdomen is illustrated in the spark shadowgram of figure 127, whichshows three loops of a cat's colon, each containing an air pocket, suspendedin Ringer's solution in the form of a triangle. A shot was fired through oneloop of colon and shock wave A-a was formed at the liquid surface. This wavewas reflected from each of the other loops of colon and shock waves B-b and C-cwere formed. When the shot had passed the gas mass and
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hit the further side of the middle piece of colon, anotherlarge shock wave was formed, D-d. If a crystal had been placed at X, it wouldhave recorded a medium, followed by a weak and then a strong shock wave, givinga multiple record somewhat like that of figure 125.
When the crystal is in the stomach and the animal is shotthrough the thigh, about 14 cm. from the crystal, the type of pressure recordshown in figure 128 is obtained. There results a jumble of small pressure peaksabout 5 microseconds apart and of an intensity of about 10 to 20 pounds persquare inch. The pressure record is very similar to what might be expected fromthe 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. Thetiming is in milliseconds. The first peaks mark the shock wave, whose pressureis so great as to rise completely off the record. From measurements of thehigh-speed motion picture of the shot, taken simultaneously with the pressurerecord and reproduced in figure 130, it is found that the second maximumpressure corresponds to the collapse of the temporary cavity. The subatmosphericpressure between the shock wave and the second pressure peak corresponds to themaximum of the temporary cavity, visible as two prominent bulges, as shown inframes 3 and 11.
After the large temporary cavity collapses, microsecondroentgenograms show no second expansion, such as occurs after a shot into a tankof water. Although the motion picture of the cat's abdomen does showindications of new wrinkled bulges on each side of the abdomen, these secondbulges merge with the subsequent distortion of the abdomen. The two smallpressure oscillations in the pressure record appear to have no counterpart inthe external movements of the abdomen, visible in the motion picture. Thepressure record is, in fact, quite flat during the long period of wavelikeabdominal movements.
In the respect just mentioned, a shot into the body differsfrom a shot into water in a tank, where pulsations of the gas making up thetemporary
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cavity is a striking phenomenon and the pressure changes duringthese pulsations are found to agree exactly with the expansion (decreasedpressure) and contraction (increased pressure) of the cavity, illustrated infigure 60.
It is very probable that the opening made by the shot in thebody wall closes almost immediately, so that little air can rush in behind themissile. In an animal, therefore, the initial large temporary cavity may beconsidered as almost entirely filled with water vapor. When the cavitycollapses, only small pockets of gas are left, comparable in volume and scarcelydistinguishable from the gas pockets already present in the intestine. In thepressure record of figure 129, the pulsation of these gas pockets is representedby the small pressure oscillations, spaced 2 to 3 milliseconds apart. They arequite comparable to the pulsation observed in small submerged balloons when asphere is shot into a tank of water.
Small balloons, filled with air and suspended in a tank ofwater, are instructive for visualizing pressure changes around the temporarycavity resulting from a shot into the tank (fig. 131). As can be seen, theballoons are at first contracted by the high pressure of the shock wave, butvery quickly they expand to a large size, as a result of the decreased pressureduring expansion of the cavity. In addition to the expansion and contraction ofthe balloons, synchro-
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nized with the volume changes of the cavity, they alsopulsate with their own period (about 500 a second), and in this respect theyserve as models for the behavior of small gas pockets in the body.
The importance of gas pockets in tissues in relation topressure changes has been emphasized in the foregoing discussion. That these gaspockets are important in wounding can be determined by suspending in Ringer'ssolution small masses of tissue, with and without gas pockets, and then shootinginto the solution near the tissue masses. Excised hearts of frogs have been usedto investigate the mechanism of wounding by this method.
When isolated frog hearts containing no gas are fixed inposition in a tank of Ringer's solution, it has been determined that damagefrom a shot into the solution occurs only when the hearts are rapidly stretchedon their moorings by the expansion of the temporary cavity. They suffer nodamage from shock waves beyond the boundary of the temporary cavity. Thearrangement of such an experiment is illustrated in figure 132. Only the heartsengulfed by the cavity, or greatly stretched by it, were damaged.
In order to eliminate the cavity formation, a piece ofarmorplate was placed on the water surface and struck by a very high velocitymissile. By this method, shock waves of great intensity can be produced, butonly a minute cavity forms underneath the armorplate. Water movement is therebyreduced 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 hasfirst been injected into a heart, as in A and F of figure 133, the suddenexpansion of this gas from negative pressures in the water around the minutecavity was found to cause damage. The A heart (near the small cavity) wasseriously injured, while the F heart (farther away) suffered no severe damage.
These, and other similar experiments, indicate that it is thesubatmospheric pressure around the temporary cavity, recorded in the crystalrecords of figures 60 and 129, that causes the damage and that this damageresults from the expansion of gas pockets rather than from the high pressuresconnected with the shock wave. Damage by gas expansion may be spoken of assecondary damage, whereas damage from expansion of the temporary cavity itselfis primary damage. In both cases, the destructive effects are due to severetearing of tissue.
A striking demonstration of gas effects is illustrated infigure 134 which shows a loop of cat intestine, with an air bubble within theright end, suspended
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in a tank of Ringer's solution. When a shot is firedthrough the ring of intestine, the gas bubble at the right can be seen to firstcontract and then expand markedly. When the intestine was later examined, themucosa and submucosa were found to have been perforated in the gas-containingregion, although the muscularis layer was intact. Such effects are exactlycomparable to damage to the human body from underwater blast. This damage isrestricted to gas pockets in the alimentary canal, leading to intestinalperforation, or to gas in the lungs, where severe hemorrhage occurs. Althoughsecondary damage from gas is important in rifle shots, it never equals theprimary damage which results from the expansion and tearing caused by theformation of the temporary cavity.
RETARDATION OF MISSILES BY SOFT TISSUE AND TISSUELIKESUBSTANCES
The slowing down or retardation of a missile as it traversestissue is an important factor in determining how and where the missile deliversits energy to the tissue. In order to understand the mechanism of wounding, itis essential to know the law of force which retards the missile. Here, thestudies of retardation in water and in 20 percent gelatin gel are very helpful.It has
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been found that the retardation, dV/dt, isproportional to the square of the missile's velocity, V. This isusually 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 bythe inertia of the water and gel which was originally in the missile's pathand which is forced aside. Since the inertia depends only on the density, it isto be expected that soft tissues, gelatin, and water will behave in nearly thesame manner. This proves to be the case-all three offering a resistance to themissile which is proportional to the square of the missile's velocity.
The retardation coefficient of a 1/8-inchsteel sphere in water, gelatin gel, and muscle has been measured and is asfollows:
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 waterand only 28 percent more retarding than gelatin gel.
These retardation coefficients for missiles were calculatedfrom the loss of velocity which a sphere experienced in going through the thighof a deeply anesthetized cat. The length of the missile track was also used inthe calculation. The retardation coefficients in water and gelatin gel werecalculated from the position-time relationship as measured from the high-speedmotion pictures. The retardation coefficient, a,is equal to (r/ 2) (A / M) CD where A is the projection area, M themass, r the density of the medium, and CDthe drag coefficient. The measured values of CD for these threesubstances were: Tissue, 0.45; 20 percent gelatin gel at 24? C., 0.35; andwater, 0.30.
A sphere or nontumbling fragment loses its energy rapidly intraversing soft tissues and waterlike substances. The energy, E, fallsoff exponentially with penetration distance, s, as follows: E=Eoe-2as.A 1/8-inch steel sphere loses half itsimpact energy after penetrating 2.22 cm. of muscle and nine-tenths of its energyafter penetrating 8.3 centimeters. A 1/16-inchsphere will lose these same percentages of energy in just half these distances,while a ?-inch sphere will require twice the distance.
In the case of high-velocity missiles, certaincharacteristics of the explosive or temporary cavity are related to the energydissipated by the missile. It is possible, therefore, to determine how and wherethe missile lost its energy simply by inspecting a microsecond roentgenogram ofthe temporary cavity. It turns out that the diameter of the temporary cavity, D(measured perpendicular to the missile path) is proportional to the squareroot of the space rate of energy change or D=(8 k aE/π), where k is aconstant having an experimentally measured value of 8.92 X 10-7cm. 3/erg for water and 0.80 X 10-7cm.3/erg for living muscle of a cat thigh.
This decrease in energy is clearly observable in a cavityproduced 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 beseen to be wide near entrance and narrow near exit.
The rate at which energy is lost and the cavity diameter alsoincrease with the ratio A/M, which is the ratio of projection area tomass of the missile. This signifies that a missile of large projection area andsmall mass will lose energy rapidly and will produce a wide, but short cavity.When two spheres of different masses having the same projection area andvelocity 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) wherethe dissipation of energy by an aluminum (left) and a steel (right) sphere iscontrasted.
When a fragment is shot, tumbling of the fragment changes theprojection 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 acat was formed by a tumbling cylindrical fragment (a section of a wire nail).
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The retardation suffered by small steel spheres when traversinghuman skin was also measured. This was done by mounting several layers of skinin the path of a small steel sphere and measuring the velocity before and afterimpact. Figure 138 shows the skin pocket mounted in the middle of a shock wavevelocity recorder. The inclination of the lines of dashes gives the before andafter velocity of the missile. The missile is traveling from right to left.
For equivalent thicknesses, the retardation coefficient forskin was 40 percent larger than that of muscle. The velocity lost by a 1/8-inchsteel sphere when perforating 8 cm. of skin was found to be 0.182(Vo-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. Forother missiles, the relationship just cited may be extrapolated to give 0.30 AM-1(Vo-170)s+170.The effect which skin exerts on certain missiles has been calculated from thisformula, and the results are as follows:
1/16-inch sphere: | ||
| Impact velocity | f.p.s. | 3,000 |
| Velocity lost | f.p.s. | 370 |
| Velocity loss | percent | 12.3 |
| Energy loss | ...do... | 24.6 |
?-inch sphere: | ||
| Impact velocity | f.p.s. | 3,000 |
| Velocity lost | f.p.s. | 235 |
| Velocity loss | percent | 7.8 |
| Energy loss | ...do... | 15.6 |
.30-06 bullet: | ||
| Impact velocity | f.p.s. | 3,000 |
| Velocity lost | f.p.s. | 175 |
| Velocity loss | percent | 5.8 |
| Energy loss | ...do... | 11.6 |
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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 amissile can penetrate different types of tissues becomes a highly important one.Various soft tissues, but more particularly bone, often overlay and serve as aprotective layer to important structures underneath. This section presents datawhich have been secured regarding the problem of penetration.
The distance which a missile travels into soft tissue beforebeing brought to rest depends not only on its impact velocity, Vo,but also on its projection area, A, its mass, M, and its shapefactor, F. Such an inference can be drawn from studies on penetration ina tissuelike substance as 20 percent gelatin gel. That the law of penetrationfor tissue should be the same as the law for gelatin follows from theobservation that they both obey the same retardation law.
The penetration, P, into 20 percent gelatin gel at24? C. by steel spheres is given by P=a-11n(Vo/74)=5.72 A-11n(Vo/74)=59.5R ln(Vo/74)where A
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is the projection area in cm.2,a the retardation coefficient in cm.-1,M the mass of the sphere in grams, Vothe impact velocity in meters per second, and R the radius ofthe sphere in centimeters. The penetration into gelatin gel of eight aluminumspheres having the same velocity but different radii, R, is shown infigure 139 where it is evident that spheres of larger radii penetrate a greaterdistance.
The penetration of small spheres into living soft tissue wasdetermined by shooting into the thighs of deeply anesthetized dogs. It wasassumed that the penetration law was the same as for gelatin gel and that onlythe constants which appear in the formula needed to be ascertained. Thepenetration formula for soft tissue was found to be:
P=a - 1 1n(Vo/84)=4.45A-1M1n(Vo/84=46.3R 1n(Vo/84).
Larger spheres having the same velocity undergo the greatestpenetration. This is shown in the roentgenogram of figure 140 where several2/32- and 3/32-
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inch steel spheres are shown embedded in the thigh of a dog.The spheres had nearly the same velocity, but the lighter 2/32-inchspheres succeeded in going only about two-thirds the distance of the 3/32-inchspheres. For spheres having exactly the same velocity, the penetration distanceis inversely proportional to their radius. For spheres having the same radius,the penetration varies as the log of the impact velocity. This is illustrated infigure 141, where two 4/32-inch spheres havingimpact 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 isnecessary to distinguish between a tumbling and a nontumbling missile. In atumbling missile or fragment, the projection area may undergo considerablechange in magnitude during flight, and the penetration of the same shapedfragment may vary considerably for different shots. For nontumbling fragments insoft tissue, it is supposed that the formula for penetration would be P=6.67FMA-11n(Vo/Vt),where F is a shape factor (one for a sphere) and Vtis a constant, which probably does not differ much from the one for asphere having an equivalent projection area. In figure 142 is shown a fragmentwhich has traveled broad-
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side and has been stopped after traversing only 6.1 cm. of acat's abdomen. A sphere of the same mass, or the same fragment traveling endon, would have passed entirely through the abdomen without any difficulty. It isapparent that missiles other than spheres or spin stabilized bullets will have aconsiderable range of penetration distances, depending on their behavior duringflight.
Spongy bone opposes the motion of a spherical missile with aforce which acts in a different way from the one for soft tissues. In softtissues, the force is proportional to the square of the velocity, while in bonethe force is independent of the velocity. When the end of a beef femur was cutand spherical missiles shot into the spongy bone, it was found that thepenetration was given by P=8.15-5R2(Vo-200)2, where R isthe radius of the sphere in inches, Vo theimpact velocity in feet per second, and P the penetration in inches. Thepenetration is greatest for large spheres and increases with the square of theradius. The soft spongy bone of the femur stops missiles more readily than softtissue. A 4/32-inch steel sphere traveling witha velocity of 2,000 f.p.s. in tissue will penetrate 23.3 cm., while the samesphere in bone will travel only 2.65 cm. before being stopped. It may be assumedthat the penetration into bony tissues harder than those found in the femur willbe correspondingly smaller. The spongy bone of the femur was used in thesetests, because it afforded a large mass of fairly uniform bony material. Figure143 shows three 4/32-inch spheres that havepenetrated different distances into the end of a beef femur. The bone was sawedalong a plane parallel to the axis to give a flat plane of entry. The sphere ofhighest velocity penetrated the greatest distance.
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CASUALTIES IN RELATION TO MISSILE MASS AND VELOCITY
An investigation was made to determine the mass and velocity relationship fora missile which is just capable of producing a casualty. The type of casualtychosen was that which would result when a certain vulnerable region in the bodywas pierced by the missile.
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The total projection area of an erect man and the projectionarea of the vulnerable region in the body was measured by using anatomicdrawings of body sections. The vulnerable regions included the organs, cavities,canals, and those nerves and blood vessels which have a diameter greater than0.25 centimeter. The total projection area from the anterior aspect was 5.3square feet, and the vulnerable projection area was 43 percent of this. Handsand 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. Theaverage thickness of bone and soft tissue on the front and back of the body was0.6 cm. and 3.3 cm., respectively. The vulnerable region was found to be betterprotected by soft tissue and bone from missiles coming from the rear than fromthose coming from directly in front.
The data on velocity losses in living cat muscles and infresh human skin were used, in conjunction with penetration measurements onspongy beef bone, to calculate the minimum energy required to perforate theprotective layer and pierce the vulnerable region. The calculation was made for 1/16-,1/8-, and 1/4-inchsteel spheres. These perforation energies for the 1/8-inchsphere varied from 2 to 216 ft.-lb. and depended on the composition andthickness of the protective layer immediately above the region being considered.
The probability that a hit by a given missile will result ina casualty was determined from the ratio of vulnerable projection area to totalprojection area, where the vulnerable projection area is a projection of thosevulnerable regions which the missile is capable of piercing. This probabilityfor any one missile was observed to rise rapidly with the missile's energy andvelocity as soon as the threshold energy and velocity were attained. Afterpassing an optimum energy, the probability of wounding increased at a smallerrate until a maximum was reached. This optimum energy was chosen as an index ofthe 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-inchsteel spheres, when calculated for missiles striking a man from directly infront or directly behind, was 15 foot-pounds. The average probability ofwounding which this optimum energy gave was 60 percent of the maximum possibleprobability or was 0.25 in absolute units.
It was pointed out that the relationship between the mass andvelocity of all missiles which produce a casualty of a given type depends on twofactors: (1) The severity of the wound which causes this casualty, and (2) theprobability that a hit on the body will produce such a wound. In the presentanalysis, it has been assumed that there is a large group of wounds which havethe same severity and the probability of the occurrence of such wounds has beenevaluated; the resulting relationship between the mass and velocity which wasevaluated was too complex to present in any other way except pictorially.
The mass-velocity data showed that the energy necessary towound a man increases as the mass of the missile is increased. This is true forthe optimum energies and for those energies which give probabilities of woundingequal to 25, 50, and 75 percent of the maximum probability. This increase inenergy with mass is shown to be generally true for any analysis in whichpenetration plays the predominant role.
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BIBLIOGRAPHY
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