This portion of the nervous system (d, fig. 174) occupies a position intermediate between the spinal cord and the pons (o, fig. 174), and is continuous with both. It forms a kind of capital to the cord, and possesses a highly intricate- structure. It is the seat of origin of some of the most important nerves in the body, particularly of those which confer sensibility upon the face and head, which perceive sounds, and which are instrumental in carrying-on the function of breathing, and as these last are essential to life, destruction of the medulla oblongata is immediately fatal.

Under Surface of Horse's Brain.

Fig. 174. - Under Surface of Horse's Brain.

A, Temporal Lobe. B, Crura Cerebri. c, Pons Varolii. D, Medulla Oblongata. E, Cerebellum.

F, F, Convolutions of Cerebrum. I, Olfactory Nerve or 1st Nerve. II, Optic Nerve or 2nd Nerve. Ill, Motores Occulorum or 3rd Nerve. iv, Pathetici or 4th Nerve. v, Trifacial or 5th Nerve. VI, Abducent Nerve. VII, Facial or 7th Nerve. VIII, Auditory or 8th Nerve. IX, Glossopharyngeal or 9th Nerve. X, Pneumogastric or 10th Nerve. XI, Spinal Accessory or 11th Nerve. XII, Hypoglossal or 12th Nerve.

The Pons (c, fig. 174) is a broad and thick band of transverse fibres running across from one hemisphere of the cerebellum to the other. It is traversed by the continuation of the columns of the spinal cord. These emerge from the front border of the pons, and form the diverging crura cerebri (b, fig. 174), the fibres of which radiate outwards to the cortex of the brain. Effusions of blood affecting these columns cause paralysis.

Above the pons are the four eminences named the corpora quadri-gemina, which are intimately connected with the function of sight, and are seen in section just above N in fig. 176.

We now reach the great ganglionic masses situated at the base of the brain, shown in fig. 175.

These are large, and composed of gray substance, and therefore contain many cells, in which it is probable many of the nerve-fibres in the crura cerebri end, whilst on the other hand they are brought into relation with the cortex of the brain by radiating fibres. The hinder pair are the optic thalami, and are closely connected with the optic tracts. The front pair are the corpora striata, so termed because the gray substance is traversed by bands of white fibres. These great ganglia are connected by transverse fibres forming the gray and white commissures, and the rest of their opposed surfaces form the lateral boundaries of the third ventricle.