ISTHMUS OF THE ENCEPHALON 553 



The posterior column is composed largely of fibers continuous with those 

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

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

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

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

 ,re apparently continued into the restiform body. Owing to the divergence 

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

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

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

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

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

 :olumn end in brush-like expansions (see Fig. 233). 



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

 Cradle nuclei axons emerge which pass forward and upward through the 

 *ray matter and decussate with corresponding fibers coming from the op- 

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

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

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

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

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

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

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

 racts 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 

 uerve 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 wh] 

 matter, the optic tracts (Fig. 235). 



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. 236). At the extreme upper part of this surface 

 there are four small grayish eminences, the corpora quadngemma. 

 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 T. 

 space is an expansion of the central cavity of the cord, the result of 

 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- 

 tinuous with that lining the central canal of the cord. Beneath this 



f en's varoHi comprises in a general way that portion of the central 



