110 ANATOMY OF THE RAT 



fundihular 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 aque- 

 duct. (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 mat- 

 ter 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 coUiculus, 

 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 {hrachium pontis 

 or middle peduncle), which runs anteroventrally along the 

 lateral surface of the medulla, enters the pons. The median 

 tract {hrachium 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 



