A major function of the cerebrospinal fluid is to cushion the brain within its solid vault. The brain and the cerebrospinal fluid have about the same specific gravity (with only about a 4 percent difference), so the brain simply floats in the fluid. Therefore, a blow to the head, if it is not too intense, moves the entire brain simultaneously with the skull, causing no one portion of the brain to be momentarily contorted by the blow.
Contrecoup. When a blow to the head is extremely severe, it may not damage the brain on the side of the head where the blow is struck but is likely to damage the oppo site side. This phenomenon is known as “contrecoup,” and the reason for this effect is the following: When the blow is struck, the fluid on the struck side is so incompressible that as the skull moves, the fluid pushes the brain at the same time in unison with the skull. On the side opposite to the area that is struck, the sudden movement of the whole skull causes the skull to pull away from the brain momentarily because of the brain’s inertia, creating for a split second a vacuum space in the cranial vault in the area opposite to the blow. Then, when the skull is no longer being accelerated by the blow, the vacuum suddenly col lapses and the brain strikes the inner surface of the skull.
The poles and the inferior surfaces of the frontal and temporal lobes, where the brain comes into contact with bony protuberances in the base of the skull, are often the sites of injury and contusions (bruises) after a severe blow to the head, such as that experienced by a boxer. If the contusion occurs on the same side as the impact injury, it is a coup injury; if it occurs on the opposite side, the contusion is a contrecoup injury.
Coup and contrecoup injuries can also be caused by rapid acceleration or deceleration alone in the absence of physical impact due to a blow to the head. In these instances the brain may bounce off the wall of the skull, causing a coup injury, and then also bounce off the opposite side, causing a contrecoup contusion. Such injuries are thought to occur, for example, in “shaken baby syndrome” or sometimes in vehicular accidents.
