ISTHMUS OF THE ENCEPHALON. 521 



anterior pyramid and the restiform body is a small oval mass, the olivary 

 body, composed of both white and gray matter. 



The posterior column is composed largely of fibers continuous with those 

 of the posterior column of the cord. The subdivision of this column into a 

 postero-external (Burdach) and a postero-internal (Goll) is more marked 

 in the medulla than in the cord. The former is here known as ihefuniculus 

 cuneatus, the latter as the funiculus gracilis. These two strands of fibers 

 are apparently continued into the restiform body. Owing to the divergence 

 of the restiform bodies a V-shaped space is formed, the floor of which is 

 covered with epithelium resting on the ependyma. At the upper extremity 

 of the funiculus cuneatus and funiculus gracilis, two collections of gray 

 matter are found, known respectively as the nucleus cuneatus and nucleus 

 gracilis. Around the cells of these nuclei many of the fibers of the posterior 

 column end in brush-like expansions, (see Fig. 236). 



The Fillet or Lemniscus. From the ventral surface of the cuneate and 

 gracile nuclei axons emerge which pass forward and upward through the 

 gray matter and decussate with corresponding fibers coming from the op- 

 posite nuclei. They then assume a position just posterior to the pyramids 

 and between the olivary bodies. These fibers thus form a new tract, termed 

 the fillet or lemniscus. As this tract ascends toward the cerebrum it receives 

 additional axons from the sensor end-nuclei of all the afferent cranial 

 nerves of the opposite side with the exception of the auditory. From the 

 end-nuclei of the auditory nerve, new axons ascend as a distinct tract situated 

 near the lateral aspect of the pons. From their position, these two separate 

 tracts have been termed the mesial and lateral fillets respectively. 



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 corresponding half of the cerebellum. Above the pons, 

 this surface presents two large columns of white matter which, diverge some- 

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

 crura cerebri. Embracing the crura above are two large bands of white 

 matter, the optic tracts (Fig. 238). 



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. 239). 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. The 

 space is 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 con- 



