[AS.] The brain is the principal nerve centre, and occupies the whole cavity of the head. It is carefully enclosed by membranes, its upper part beirig called the cerebrum, and its lower part the cerebellum. The interior mass of the brain is composed of white matter, but the entire surface has a thin gray covering. The surface of the cerebrum is made up of irregular rounded ridges or convolutions. The greater the number and depth of the convolutions, the greater the amount of brain surface, and the greater the amount of gray matter which covers the surface. It is supposed that the gray matter increases with study or thought. A deep indentation extending from front to back divides the brain into parts, so that in reality the brain is double, corresponding to the pairs of the external portions of the body. The two halves are connected by a central mass of fibres. From the nervous mass within the skull twelve pairs of cranial nerves extend to different parts of the head and face. From the brain the spinal cord extends downward through the spinal column to the lower extremity of the body. In every action which comes from thought, the mind, through the brain, with its outgoing nerves, directs the first steps, and the brain is regarded as the organ of intellect and the seat of the soul.