The Mechanics of Brain Injuries 75 



or separation of sutures, (c) forces resulting from movement 

 of the head as a whole and which would exist even if the skull 

 were undeformable. (c) may be subdivided into (ci) linear 

 acceleration forces, (C2) rotational acceleration forces, (C3) cen- 

 trifugal forces, (C4) Coriolis forces. Of these (ca) and (C4) are 

 clearly negligible. 



Now it is allowable to analyze the deformations of each 

 infinitesimal element due to (a, b, ci, Co) into two and only two 

 types (a) change of shape, or distortion, without change of 

 volume (this is analyzed by physicists into a set of shear strains) 

 and (P) a change in volume without distortion, (a) is extremely 

 liable to injure animate^ or inanimate objects. (P) is of two 

 kinds (Pi) decrease in volume due to increase of hydrostatic 

 pressure and (Pi.) increase in volume due to decrease in hydro- 

 static pressure. Common sense suggests that (Pi) is harmless 

 provided it does not cause prolonged occlusion of blood vessels. 

 Its harmlessness has been verified for peripheral nerves.^ (P2) 

 is also harmless unless the decrease in pressure is sufficient to 

 cause cavitation, i.e., liberation of bubbles of vapor or dissolved 

 gases. 



Changes in Volume 



Unfortunately the terms "increase in volume" and especially 

 ''decrease in volume'' are imprecise. Decrease in volume of a 

 particular region might be brought about by a true hydrostatic 

 pressure acting equally in solid tissues, blood, and tissue fluids, 

 and not allowing anything to pass out of the given region. Under 

 such conditions the ratio of the volume decrease of a cubic 

 centimeter of brain to the pressure increase is the true com- 

 pressibility, and is the same as that of water, 5 X 10'^^ dyne"^ 

 square centimeter. Alternatively, the pressure causing the de- 

 crease in volume might act only on the solid tissue and might 

 allow blood, or blood and certain tissue fluids, to escape from 

 the region considered. Under such conditions one would obtain 

 a pseudocompressibility, whose value would depend on many 

 things. A value of 2 X 10 *" dyne"^ square centimeter was found 



