There are both intrinsic and extrinsic causes of insanity.

The intrinsic are the structural differences in the brain as compared with the normal or usual, whether these are due to imperfection in development or to defective heredity or to the injury of disease; the extrinsic causes are those which come from without and bring the intrinsic into activity.

Syphilis is a frequent cause of insanity, and probably the only cause of the condition known as general paralysis of the insane, acting by means of the injury which it produces in the cortex of the brain.

The abuse of alcohol is another fertile cause, but the changes produced in this are not so obvious as in the case of syphilis.

Tumors of the brain are not infrequently a cause, and the same is true of infections, even those not located in the brain.