832 THE NERVES. 



muscle : notwithstanding their intimate relations, it does not detach the smallest 

 filament to it. 



Its anastomoses with the various branches of the trigeminus and pneumo- 

 gastric nerves, endow some of its branches with great sensibility. 



8. Eighth Paie, or Auditory Nerves (Figs. 424, 455, 456). 



This is the nerve of hearing ; it has a very simple disposition, which we will 

 sum up in a few words. 



Origin. — The auditory nerve {portio mollis) proceeds from the medulla 

 oblongata by two roots — an anterior or lateral, and a posterior. The latter 

 (Fig. 425, 20) commences on the floor of the fourth ventricle by some con- 

 vergent strise (limce transverse^ stricB medullares), as is admitted in the majority 

 of treatises on human anatomy — though we have never been able to discover 

 these strise in the domesticated animals ; it is afterwards directed outwards, 

 round the posterior cerebellar peduncle, and unites with the anterior root on 

 the side of the medulla oblongata. The latter root (Fig. 456, g), consists of 

 a single fasciculus joined with that of the facial, and escapes from between the 

 fibres of the restiform body. The nucleus of the auditory nerve has been dis- 

 covered by Schroeder Van der Kolk, a little below that of the facial nerve. 



Course and Termination. — These two roots immediately unite into a single 

 soft cord, situated behind that of the seventh pair, with which it is directed 

 outwards to reach the internal auditory meatus. There it divides into two 

 branches — an anterior and posterior — the fasciculi of which traverse the fora- 

 mina at the bottom of the meatus : the former to gain the axis of the cochlea 

 (the cochlear branch), and the latter the semicircular canals {vestibular branch). 

 The description of these two branches will be deferred until we come to study 

 the sense of hearing. 



9. Ninth Pair, or Glosso-Pharyngeal Nerves (Figs. 456, 3 ; 459, 10). 



The glosso-pharjTigeal is a mixed nerve, which conveys general sensation, 

 with gustative sensibility, to the posterior third of the tongue, and excites 

 contraction of the pharyngeal muscles. 



Origin. — This nerve originates on the side of the medulla oblongata, behind 

 the eighth pair, by eight or ten fine roots, some of which are implanted in the 

 restiform body ; while the others — the smallest number— escape, like the fila- 

 ments of the facial nerve, from the interstice between that body and the lateral 

 column of the medulla oblongata.^ The roots of this nerve pass into two 

 diflPerent nuclei. The sensitive fibres reach a nucleus (Fig. 455, pn) situated near 

 the floor of the fourth ventricle, in the prolongation of the superior cornu of the 

 medullary axis ; while the motor fibres enter another nucleus (Fig. 455, s) 

 belonging also to the hypo-glossal, and which lies in the direction of the inferior 

 cornua of the spinal cord. At their exit from the medulla oblongata, these 



' This disposition— which is readily exposed in the Horse— appears to us sufficient to remove 

 all the doubts existing iu the minds of a large number of anatomists, as to tiie nature of the 

 glosso-pharyngeal nerve. It evidently possesses at its origin, as motor filaments, those arising 

 from the same part as the facial nerve, and as sensitive filaments those from the restiform 

 body. Besides, we may object to the opinion whicii would also attribute the motor property 

 of the glosso-pharyngeal nerve to the anastomosing branches passing between it and the 

 seventh pair, on the ground that tliese anastomoses are far from being constant, and that in 

 Bome species they are always totally absent. 



