The Mechanics of Brain Injuries 81 



a blow on the forehead. In both of these cases, the damage is 

 clearly symmetrical with respect to the midplane ; but it is also 

 approximately symmetrical with respect to the midplane when- 

 ever the head is hit at any point whatsoever by a blow whose 

 direction is exactly perpendicular to the midplane. Such a blow 

 causes a rotation about an axis lying in the midplane or parallel 

 to it. 



If the distribution in any region is sufficiently great, every- 

 thing in that region that can be injured will be injured — blood 

 vessels will be torn, axons torn, synapses disrupted, etc. The 

 injury due to lesser amounts of distortion will depend on the 

 degree of distortion, on the nature of the distorted region, and 

 on the directions of the shear strains relative to fiber directions. 

 But, in general, it must take less distortion to produce a quickly 

 reversible effect in a cell body or axon than it takes to produce 

 an actual tear in them or in a blood vessel. The small distor- 

 tions in a peripheral nerve produced by a falling drop of mercury 

 or a jet of air are known to excite it without causing injury.^ 

 It is reasonable to suppose that there is some similar sort of effect 

 in the brain, and thus, that blows so small that they produce no 

 anatomical injury nevertheless momentarily upset the existing 

 activity in the brain. Possibly momentary amnesia or the splash 

 of light which often accompanies a blow are due to this effect. 

 The shear strains which arise as a result of squeezing a peripheral 

 nerve can cause it to fail to conduct impulses, and if the strains 

 have not been too severe the nerve will recover spontaneously 

 after some minutes, even in the absence of a blood supply. Once 

 again, one would expect a similar effect in the brain. Amnesia 

 lasting only a few minutes might be the result of such a mech- 

 anism. 



Although the whole brain is distorted by rotation, some parts 

 are much more distorted than others. Thus, so far as the physics 

 of the problem is concerned, loss of consciousness might be due 

 to a diffuse neuronal injury, or to injury to a particular region, 

 or both, or sometimes one, sometimes the other, 



