The great nerve centre of the body, the brain, is the large upper portion of the cerebro-spinal axis, which fills the cavity of the skull. It consists chiefly of soft nerve substance, gray and white. The surface of the brain is covered by delicate -connective tissue plentifully supplied with arteries, which supply the brain with blood. It is called the Pia Mater (soft or pious mother). Over this there is a delicate transparent serous membrane, the Arachnoid (spider's web). Over this there is a third, a tough and fibrous membrane, the Dura Mater (hard mother).

The principal divisions of the brain are the Cerebrum,                          Pons Varolii,

Cerebellum,                       Medulla Oblongata.

The cerebrum or greater brain fills the upper and frontal portion of the cranium, and consists of two hemispheres, right and left, separated above by a deep fissure, but united below it. It is composed of gray matter, which surrounds white fibrous nerve substance. The gray matter is drawn up into a great number of convolutions, or folds, by which its surface is increased; the pia mater dips between these folds, the other two coverings pass over them. The cerebrum is the seat of intelligence, will, emotion, and sensation. When the cerebrum of an animal is removed, it loses the power of voluntary movement and lies in a state of stupor, but it retains the power of performing involuntary actions.

Fig. 22.—Under Surface of Brain, 1, Cerebrum ; 2, cerebellum ; 3, medulla oblongata; 4, pons varolii.

The cerebellum, or lesser brain, lies under the back portion of the cerebrum, and also consists of two hemispheres, each composed of an external layer of gray matter surrounding white fibrous substance. Its convolutions are different to those of the cerebrum, but the pia mater dips between them. The function of the cerebellum seems to be the co-ordination of muscular movements. When the cerebellum of an animal is removed, it retains sensation, and can move voluntary muscles, but it cannot walk, nor can it balance its body. Voluntary movements do not originate in the cerebellum, only the power of co-ordinating these movements.

The pons varolii is a broad band, or bridge, of nerve matter which unites the cerebrum, cerebellum and medulla oblongata. The medulla oblongata lies below the pons ; in this the gray matter is interior, and surrounded by the white.

It contains important nerve centres, which govern the involuntary movements concerned in breathing, swallowing, etc. If it is destroyed, death immediately ensues. Nerve fibres decussate or cross in the medulla. The result of this is that injuries to nerve centres on the right side of the brain will affect the left side of the body, and vice versa.