290 MH'Hosrnnr A\AToMY <)!< THE ORGANS. 



The fibres of the trigeminal pass directly to a surface origin 

 in the lateral line. 



The sensory nuclei (Fig. 216) represent the dorsal horn, and 

 lie for the most part in the dorsal part of the medulla and pons. 

 The type of a sensory nerve in this region is to divide into a long 

 descending and a short ascending tract, each of which is accom- 

 panied by a nucleus. The long descending tract of the glosso- 

 pharyngeal and vagus is the tractus solitarius, which lies in the 

 border of the central gray matter of the lower half of the 

 medulla. It has its own nucleus. Parallel and just internal 

 to it is another long nucleus, the ala cinerea, belonging to the 

 same nerves. 



The tracts of the vestibular and trigeminal nerves are par- 

 allel, the former lying just dorsal to the latter (Fig. 216). 

 The descending tract of the vestibular nerve is in the medulla, 

 while the ascending tract enters the pons. The cells which 

 accompany these two tracts have received three names : those 

 opposite the descending fibres are called the median nucleus ; 

 those opposite the ascending fibres, the superior nucleus ; while 

 a small group of cells in the angle of the two tracts makes the 

 lateral nucleus. The lateral nucleus places the nerve in com- 

 munication with the cerebellum. The trigeminal tract is long, 

 covering half of the pons and all of the medulla. The descend- 

 ing tract joins with Lissaner's zone in the cord, and its nucleus 

 joins the posterior horn. Lissauer's zone is external to the 

 substantia gelatinosa of Rolando. The ascending fibres end in 

 the main sensory nucleus. 



The cochlear nerve has no descending tract, but a long and 

 complex ascending one. Its fibres end in a nucleus on the 

 lateral surface of the medulla (Fig. 215). Part of the fibres 

 pass inward, dorsal to the inferior peduncle, and decussate in 

 the floor of the fourth ventricle as the striae acusticse ; others 

 pass ventral to the peduncle as a compact bundle, which, as it 

 decussates, forms the trapezoid body. The fibres of the trape- 

 zoid body and the striae acustica3 make the lateral lemniscus, 

 which passes upward and dorsal ward through the pons to the 

 inferior colliculus. Here many fibres end ; others pass to the 



