This is the term applied to that portion of the encephalic mass which unites the cerebrum, cerebellum, and the spinal bulb. It is the point of union of the three great divisions of the nervous centre. It comprises the pons Varolii, the peduncles of the cerebrum and cerebellum, the corpora quadrigemina, and the valve of Vieussens.
At the base of the encephalon there is a convex projection which surrounds the peduncles of the cerebrum and cerebellum like a large ring, and which covers the expansions of the spinal bulb toward these peduncles like a bridge. This is the pons Varolii, or bridge of Varolius. This projection is the centre of convergence or of emergence of the nervous fascicles or bundles which it seems to cover. It is joined to the bulb behind, and in front to the peduncles of the cerebrum, and on the sides to the peduncles of the cerebellum. Its inferior surface, which rests on the basilar apophysis of the occiput, shows fibres running transversely. It is grooved along the median line, and is perfectly symmetrical.
On its upper face there are four mammillar projections; these are the corpora quadrigemina. Behind these, and between the superior peduncles of the cerebellum, stretches a thin layer of nerve substance, which has been called the valve of Vieussens, and which forms part of the boundaries of the fourth ventricle.
This is the term applied to the enlargement of the upper extremity of the spinal cord. Pointing upward and forward, its anterior surface corresponds to the basilar groove of the occiput; posteriorly it rests in a depression of the cerebellum. Although it is within the cranium, the bulb should be studied at the same time as the spinal marrow, of which it forms a part.