Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/124

110 fundibular recess is the extension of the ventricle into the infundibulum. Posteriorly the ventricle opens into the cerebral aqueduct (aqueduct of Sylvius), which leads back through the mesencephalon to the fourth ventricle, the cavity under the cerebellum and in the medulla oblongata. The aqueduct may be seen best in a transverse section through the corpora quadrigemina, where it will be seen as a narrow vertical slit.

The corpora quadrigemina will be seen above the aqueduct. (Compare them with the optic lobes of the dog-fish.) Observe that the superior coUiculi are longer than the inferior. The isthmus is the deep dorsal fissure that separates the corpora quadrigemina from the cerebellum.

In the sectioned cerebellum note the deep fissures (sulci) which separate the prominent folds (gyri). The gray matter of the cerebellum is on the surface of the gyri, and so completely penetrates the white matter that the latter resembles the branches of a tree, and is therefore called the "arbor vitae."

The fourth ventricle is covered dorsally by the anterior, and the posterior medullary velum, both thin membranes underlying the cerebellum. The former extends from the base of the arbor vitae forward to the inferior colliculus, while the latter spreads backward.

The cerebellum is strongly attached by the peduncles to the medulla on each side. Locate these attachments by dissecting away the peripheral tissue on the under side of the cerebellum. The lateral fibrous tract (brachium pontis or middle peduncle), which runs anteroventrally along the lateral surface of the medulla, enters the pons. The median tract (brachium conjunctivum or anterior peduncle) goes forward toward the corpora quadrigemina. The posterior peduncle appears as a ridge passing back to the dorsal side of the medulla. These tracts may be seen more clearly