47 o TEXT-BOOK OF PHYSIOLOGY. 



THE ISTHMUS OF THE ENCEPHALON. 



The isthmus of the encephalon comprises that portion of the 

 central nerve system connecting the cerebrum above, the cerebellum 

 behind, and the medulla below. Its ventral surface presents below 

 an enlargement, convex from side to side, the pons Varolii. On 

 each side the fibers of which the pons consists converge to form a 

 compact bundle, the middle peduncle, which enters the correspond- 

 ing half of the cerebellum. Above the pons, this surface presents 

 two large columns of white matter which, diverging somewhat 

 from below upward, enter the base of the cerebrum and are known 

 as the crura cerebri. Embracing the crura above are two large 

 bands of white matter, the optic tracts (Fig. 211). 



The dorsal surface presents below two diverging columns of white 

 matter, the inferior peduncles; above, two converging columns, the 

 superior peduncles of the cerebellum (Fig. 212). At the extreme 

 upper part of this surface there are four small grayish eminences, 

 the corpora quadrigemina. From the disposition of the white matter 

 on the dorsal surface of the isthmus and medulla, there is formed a 

 lozenge-shaped space, the fourth ventricle. This space is merely an 

 expansion of the central cavity of the cord, the result of the changed 

 relations of the white and gray matter in this region of the central 

 nerve system. Above, this ventricle communicates by a narrow canal, 

 the aqueduct of Sylvius, with the third ventricle. The floor of the 

 fourth ventricle is covered with a layer of epithelium resting on the 

 ependyma continuous with that lining the central canal of the cord. 

 Beneath this is a layer of gray matter. 



The pons Varolii comprises in a general way that portion of the 

 central nerve system situated between the medulla oblongata and 

 the crura cerebri. The ventral surface is convex from side to side; 

 the lateral surface, owing to the convergence of the fibers of which 

 it is composed, is contracted to form the middle peduncle of the 

 cerebellum; the posterior surface is flat and forms the upper half of 

 the floor of the fourth ventricle. The pons consists of white fibers 

 and gray matter supported by connective tissue and neuroglia. Trans- 

 verse sections of the pons show that it is divided into an anterior or 

 ventral, and a posterior or dorsal portion, the latter being usually 

 termed the tegmentum. 



The ventral portion consists for the most part of white fibers, ar- 

 ranged longitudinally and transversely (Fig. 213). The longitudinal 

 fibers are largely continuations of the pyramidal tracts, or the fibers 

 composing the anterior pyramid of the medulla. In the lower part of 

 the pons these fibers are compactly arranged, but at higher levels they 

 are separated into a number of bundles by the interlacing of the trans- 

 verse fibers. The transverse fibers are divided into a superficial and 



