Bone is a specialized form of dense connective tissue composed of calcium salts embedded in a matrix of collagenous fibers. Whether a bullet perforates bone is dependent on a number of factors: the velocity of the bullet at impact, its construction (lead, full metal-jacketed, partial metal-jacketed), the weight of the bullet, the angle of interaction between the bone and the bullet, the type of bone (long, flat), its thickness, and its surface configuration.

A minimum velocity of 200 ft/sec is often cited as the minimum velocity required by a bullet to effect penetration of bone.16 This figure is suspect. A review of the original paper reveals that this determination was made using the long bones (femur and humerus) of cattle, with the outer layer of compact bone sawed away, thus, exposing the softer spongy layer.17 Steel spheres were used as missiles.

A limited number of tests by the author using fresh human bone and 9mm Parabellum ammunition loaded with 125 gr. roundnose lead bullets has resulted in some additional data on this subject. With flat bone (cranial vault), 4- 6-mm thick, bullet penetration (depressed fractures) began at about 250 ft/s with perforation the rule at 290 to 300 ft/s. With bone 7-9 mm thick, perforation began at approximately 350 ft/ sec. At 10 mm of thickness, no perforation occurred even with velocities up to 460 ft/s (three tests at 400 ft/s plus). In eight tests using femurs, there was no perforation until 552 and 559 ft/s. Because of the limited nature of this study, these figures should be used with caution.

Once penetration of bone has been effected, the bullet's remaining velocity operates to effect deeper penetration in direct proportion to the square of the velocity and the sectional density of the bullet. As the bullet penetrates, it fragments the bone, creating a temporary cavity. The fragments are initially propelled laterally, toward the periphery of the cavity, as well as forward in the direction of the bullet. As undulation of the cavity occurs, some fragments return to the center. Bony fragments, moving outward and forward with the bullet, act as secondary missiles, causing additional injury.

The direction in which a bullet was traveling when it perforates a bone can be determined by the appearance of the wound in the bone. When a bullet perforates bone, it bevels out the bone in the direction in which it is traveling (Figure 4.38 A). The entrance has a round to oval, sharp-edged, "punched-out" appearance (Figure 4.38B). The opposite surface of the bone, i.e., the exit side, is excavated in a cone-like manner (Figure 4.38C). This

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