The brain is enclosed by the cranium, and the lower and hind-part of the brain, called the medulla, is prolonged through the foramen magnum of the occipital bone into the vertebral canal. The upper part of the brain-the cerebrum-is the larger; it is divided longitudinally by a deep cleft into two halves, each of which are divided again into lobes, corresponds ing with, in shape and position, the bones of the cranium - namely, frontal, parietal, temporal, and occipital. These are again subdivided by numerous clefts or sulci. Beneath the occipital lobes of the cerebrum lies the cerebellum, or hind-brain, much smaller than the cerebrum. It also is divided longitudinally into two principal lobes, which are again subdivided by sulci, which, however, are very much shallower than those of the cerebrum.

Fig. 60. - The Brain (Side View).

Fig. 61 . - Base of the Brain.

Looking at the base of the brain, it is seen that two thick cords of white matter issue from the cerebrum. These are called the cerebral peduncles, and joining them together is the pons varolii, which is seen to consist of transverse fibres, the ends of which pass into the cerebellum. Below the pons varolii is the upper end of the bulb or medulla oblongata, which lies on the upper aspect of the basilar process of the occipital bone.

The medulla tapers somewhat sharply, and is continued as the spinal cord.

From each side of the pons varolii and the medulla, issue the larger number of the twelve pairs of cranial nerves, which supply the tissues of the head and neck.

In transverse section the brain is seen to consist of white matter with a covering about quarter of an inch thick of grey matter, both of which are folded into convolutions and sulci. There are also nuclei of grey matter embedded in the white. The cerebellum is of similar structure.

The brain has three membraneous coverings, named, respectively, dura mater, arachnoid mater, and pia mater.

The Dura Mater is a tough membrane which lines the inside of the cranium, and is closely attached to it. It dips down into the great longitudinal fissure, and forms a pocket, or sinus. It also sends a process into the fissure between the cerebrum and cerebellum, called the tentorium cerebelli, in the edges of which are found sinuses. These sinuses are for the collection of venous blood.

The Arachnoid Mater is a more or less areolar structure which connects the pia mater with the dura mater.

The Pia Mater is a very thin epithelium which covers the brain substance, and follows intimately all the convolutions and sulci. In it numerous small bloodvessels ramify.

Blood-Supply Of The Brain

The blood-supply of the brain is very free, and is derived from four large arteries - the internal carotid and vertebral arteries of each side. These enter through the carotid canal in the temporal bone and the foramen magnum respectively. On the lower surface of the medulla they anastomose together to form the circle of Willis, from which branches are derived which supply freely the different parts of the brain.

There are no veins proper in the brain; the venous blood is collected in the blood-spaces or sinuses in the dura mater, which-finally leave the cranium by the jugular foramina as the internal jugular veins.