- Two types of impact occur on skull and the sound wave
elicited by the impact. The first at the site where the force is applied,
the second where the advancing segments of the accelerated brain are
brought suddenly to rest by the forces that resist its forward motion.
- Counter coup lesions of the brain were once thought to
be caused by the brain moving within the skull in a straight line with the
force of the blow striking against its opposite side. But, experiments on
mechanics of head injuries have demonstrated these lesions are chiefly due
to local distortions of the skull and the sudden rotation of the head as
result of a blow, which arouse shear strains or slide produced by the
pulling apart of the constituent particles of the brain.
- Shear strains occur and pull apart the constituent
particles of the brain. These are produced in all parts of the brain, but
to a large extent at the base of the frontal lobe and the tip of the
temporal lobe, as the skull gets a good grip on the brain in this region
due to the projecting ridge of the sphenoid bone. Hence, severe and
extensive injuries occur in this region with a blow on the occiput.
- On the other hand, countercoup injuries which are
caused by rotation will not occur if the head is so well fixed that it
cannot rotate at all when it receives a blow. Cerebral contusions and
tears have been reported in autopsy as a result of trauma.
(Ref: Modi’s Medical Jurisprudence, 25th
Edition)
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