Hard ivory-like growths from the petrous temporal bone sometimes extend into the cavity of the cranium and occasion pressure upon and absorption of the brain substance. These tumours, developing very slowly, afford the neighbouring parts an opportunity to accommodate themselves for a time to the altered state, but sooner or later the pressure they impart to nerves and vessels produces various forms of structural and functional derangement, among which may be mentioned deafness, paralysis of the muscles of the face, loss of motor power, unsteady movements, convulsive fits, followed by apoplexy and death.

Exostoses or bone tumours sometimes occur on the floor of the cranium as the result of a blow on the poll or back of the head, such as would be inflicted by a horse falling backward or striking the head violently against some fixed object. In these cases blindness may follow as a consequence, from pressure on the optic nerves at their bifurcation; or the muscles of the eye may suffer paralysis, and disorders of some of the other nerves issuing from the base of the brain may result.