546 THE NERVOUS SYSTEM 



the dorsal root ganglia of the spinal nerves. Thus, the nerve cells 

 of the petrosal ganglion send one process peripheralward, while the 

 other enters the medulla through the sensory portion of the glosso- 

 pharyngeal nerve roots. These fibres pass toward the dorsal grey 

 matter and divide into a long ascending and a short descending 

 branch. The former enters the tractus solitarius and ends about 

 the adjacent cells of its nucleus in the manner already described 

 for the vagus nerve. The latter, with some collaterals from the 

 descending branches, passes to the triangular or chief nucleus of 

 the glossopharyngeus just above but continuous with the similar 

 nucleus of the vagus, in the floor of the fourth ventricle. From 

 both of these nuclei new neurones of the second order continue the 

 path cerebralward, decussating as internal arcuate fibres to enter 

 the mesial fillet of the opposite side. 



The glossopharyngeal roots are peculiar as compared with those 

 of the vagus in that the root fibres of the former form finer bundles 

 in their passage through the lateral columns to reach their sensory 

 nuclei ; also in the fact that the glossopharyngeus distributes the 

 larger part of its fibres to the tractus solitarius, while the vagus 

 sends the larger portion to the triangular or chief sensory nucleus. 



The intermediate nerve of Wrisberg has its peripheral neurone 

 cells in the geniculate ganglion of the facial nerve. Their central 

 processes on entering the medulla pass to the tractus solitarius to 

 terminate after the same manner as the similar fibres of the glosso- 

 pharyngeus. Its fibres thus form the most cephalic portion of the 

 solitary tract, the more caudad portions of the bundle being formed 

 from fibres of the glossopharyngeus and vagus, respectively. 



The eighth or auditory nerve (Figs. 384, 385, and 386), cen- 

 tripetal in direction, consists of two distinct divisions which differ 

 in origin, distribution, and function. These are the pars cochlearis 

 (cochlear nerve, true auditory nerve) and the pars vestibularis 

 (vestibular nerve). 



The peripheral neurones of the vestibular nerve arise from the 

 nerve cells of the ganglion vestibulare (ganglion of Scarpa) in the 

 internal auditory meatus. They enter the medulla oblongata at 

 the lower margin of the pons and in the same vertical plane as the 

 ninth and tenth cranial nerves. On approaching the grey matter 

 in the floor of the fourth ventricle these neuraxes divide into a 

 short ascending and a long descending branch. 



The terminals of these fibres, together with those of their col- 

 laterals, end in one of several cell groups. 1. A large number end 



