- 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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