An analytic investigation of the cavitation hypothesis of brain injury is performed by designing a mathematical model of the skull and brain subjected to an impact load. The skull is characterized as a thin, homogeneous, isotropic, elastic spherical shell, and the brain is assumed to be an ideal acoustic fluid. Using extensional shell theory, the skull-brain system is described by three coupled, simultaneous, linear partial differential equations with variable coefficients. The equations are solved by finite difference techniques. Results demonstrate that two prime focal points of reduced pressure occur within the fluid shortly after the onset of impact. These are located at the impact pole and at the counter pole or “contrecoup” site.