The Mechanics of Brain Injuries 



79 



FIG. 4. Relative movement of the brain with respect to the skull when the skull 

 is rotated as in figure 3. In other words, this is a diagram of the lag of the brain 

 behind the skull. The tail end of an arrow marks the starting position of a particle 

 of. brain relative to the skull and the point the final position, e.g., a sulcus moves 

 in relation to the skull from the dotted position A to the full line position B. It is 

 seen that the brain makes the only lagging movement open to an incompressible sub- 

 s.tance in an enclosed space, viz., a whirling movement. The amount of the rotation 

 and rotational acceleration is completely independent of the position of the point o 

 in figure 3. Neither this figure nor figure 3 are quantitatively accurate. 



Distribution of Damage from Rotation 



As rotation is theoretically so important, it is of interest to 

 find the distribution of damage produced by it. This is done 

 easily, though approximately, by making a model of a section of 

 the brain out of gelatin, and giving it a rotational jerk in a cir- 

 cular polariscope which renders the shear strains in the gelatin 

 visible. Figure 5 shows a system of shear strains obtained in 

 this way. The good agreement with the findings at necropsy is 

 to some extent fortuitous, as the following approximations have 

 been made. There are no fissures or sulci in the model. The 

 elasticity of the gelatin is uniform throughout the model, whereas 

 white matter, for example, is stiffer than gray matter. This 

 nonuniformity would tend to cause specially large strains near 

 the junction between white and gray matter.^ There is a two- 

 dimensional strain system in the model, but a three-dimensional 

 strain system in the brain. The brain has different rheological 

 properties from gelatin, which nearly obeys Hook's law. On 

 the other hand, differences in stiffness between gelatin and brain 

 do not matter; in fact exactly the same strain diagram would 



