The abnormal conditions of the nervous system may be subdivided into those of the brain, those of the spinal cord, and those of the nerves. To the description of the two former I shall prefix an account of the disorders of their investing membranes.

The membranes of the brain become involved both in faulty development, and in structural diseases of the cranium on the one side, and of the brain on the other. They are liable also to many idiopathic diseases.