76 Applied Biophysics 



by Flexner, Clark, and Weed.* It can be shown, however, that 

 in any ordinary sort of accident very Httle blood or other fluid 

 is forced out of the brain, and most of it will return when the 

 blow is over. Therefore, exsanguination is not the cause of 

 immediate loss of consciousness, and the brain during an accident 

 may be assumed to be nearly as incompressible as water. The 

 medical man may, perhaps, be more easily convinced of the 

 unimportance of immediate exsanguination by the observation 

 that it is clearly not responsible for such things as massive 

 hemorrhages into the temporal lobes, and that in slow crushing 

 injuries, where exsanguination is greatest, there is no concus- 

 sion.^ Of course, long after the blow is over, anemia may occur 

 owing to various pathological processes; but this is outside the 

 scope of the present article. 



Comparative Effects of the Forces : Linear and 

 Rotational Acceleration 



To recapitulate, therefore, 4:he forces (a, b, Ci, C2) are im- 

 portant only in so far as they give rise to (a), distortion (or 

 shear strain) or (P2) decrease in pressure sufficient to cause 

 cavitation. 



On these assumptions, bending of the skull (a) produces, 

 owing to distortion, superficial bruising of the brain near the 

 spot hit, combined with injury (usually negligible) where tissue 

 is squeezed out of a foramen or defect; (c) causes distortion 

 injury to brain and blood vessels near the fracture; (ci) can be 

 neglected because, as the brain is nearly uniform macrascopically 

 in density, it causes almost entirely increases of (|3i) or de- 

 creases of (P2) in pressure at every point. ((3i), as explained, 

 is harmless. ((3^) would be injurious only if the pressure fell by, 

 say, 5 X 10'*^ dyne per square centimeter. Now, in the average 

 accident, the pressure fall due to linear acceleration is accom- 

 panied by a shear stress in the brain due to rotational acceleration 

 of about equal order of magnitude when expressed in dyne per 

 square centimeter. But 5 X 10^ dyne per square centimeter of 

 shear stress would cause utter destruction of brain. Therefore, 



