No doubt the brain in its reaction to the bullet forms as near an approach to a fluid as any solid tissue in the human body, and experimental observation has shown how greatly its presence or absence in the skull affects the degree of comminution on the exit side; hence the fondness for the so-called hydraulic theory that has been always exhibited in the case of these injuries. The localisation of the injury in its highest degree to the neighbourhood of the exit aperture, however, shows that in any case the main wave takes a definite direction in a course corresponding to that of the bullet.
The real importance of the presence of the brain within the skull in increasing the amount of damage at the exit end of the track, is as a medium for the ready transmission of forcible vibrations. That the latter are to some extent conveyed as by a fluid is evidenced by the occasional presence of brain matter and fragments of bone in the aperture of entry, which suggests recoil or splash such as would be expected from a fluid wave.
Experience of the character of the lesions observed after severe concussion by the ordinarily somewhat coarser forms of violence common to civil life, fully explains the severity of the damage to the brain tissue met with in injuries due to bullets of small calibre. Viewing the elaborate arrangements which exist for the preservation of the central nervous system from the moderate vibration incidental to ordinary existence, it is easy to appreciate the harmfulness of such exquisite vibratory force as that transmitted by a bullet of small calibre travelling at a high rate of velocity.
Effect of ricochet in the production of severe forms of injury.—In connection with the lesions above described mention must be made of cases in which the aperture of entry reaches a large size, or a portion of the skull is actually blown away.
Examples of the former class were not uncommon; I will briefly relate one.
(48) A Highlander while lying in the prone position at Rooipoort, was struck by a bullet probably at a distance of about 1,000 yards. A large entry wound in the scalp was produced, while the defect in the skull was coarsely comminuted and was capable of admitting three fingers into a mass of pulped brain. Both brain matter and fragments of bone were found in the external wound, which was situated just anterior to the right parietal eminence. The bullet passed onwards through the base of the skull, crossing the external auditory meatus, fracturing the zygoma and probably the condyle of the mandible, and eventually lodged beneath the masseter muscle. Blood and brain matter escaped from the external auditory meatus.
The patient was brought off the field in a semi-conscious condition, the pupils moderately contracted but equal, the pulse 66, very small and irregular in beat, the respiration quiet and easy, and with paralysis of the left side of the body. The fæces had been passed involuntarily.
The wound was cleansed and bone fragments removed. The patient had to travel in a wagon for the next three days until the column halted. The progress of the case was unsatisfactory, as the wound became infected, and the man eventually died on the 14th day of general septicæmia, but with little evidence of local extension of septic inflammation.
In this instance the head was no doubt struck by a bullet which had previously made ricochet contact with the ground. I saw several such cases.