5 66 TEXT-BOOK OF PHYSIOLOGY 



The dorsal funiculus is composed largely of fibers continuous with those 

 of the dorsal funiculus of the cord. The subdivision of this column into a 

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

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

 lus 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 collec- 

 tions of gray matter are found, known respectively as the nucleus cuneatus 

 and nucleus gracilis (see Fig. 233). Around the cells of these nuclei 

 many of the fibers of the posterior column end in brush-like expansions. 



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 dorsal 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 mesial fillet is also augmented by the addition of the ascending 

 fibers from the spinal cord and known in this organ as the spinal lemniscus 

 or fillet. 



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. 232). 



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. 233). 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- 

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

 layer of gray matter. 



