* 



THE EIGHTH OR AUDITORY NERVE. 745 



Sterno-mastoid muscle and mastoid process until the upper border of the posterior belly of the 

 Digastric is found. The nerve is parallel to this on about a level of the middle of the mastoid 

 process. When found, the nerve must be stretched by passing a blunt hook beneath it and 

 pulling it forward and outward. Too great force must not be used, for fear of permanent injury 

 to the nerve. 



Eighth Nerve. 



The Eighth or Auditory Nerve (portio mollis) is the special nerve of the sense 

 of hearing, being distributed exclusively to the internal ear. 



Origin of the Eighth Nerve. The eighth nerve consists of two sets of fibres, 

 which, although differing in their central connections, are both concerned in the 

 transmission of afferent impulses from the internal ear to the medulla and pons, 

 and from there, by means of new fibres which arise from collections of gray 

 matter in these structures, to the cerebrum and cerebellum. One set of fibres 

 forms the vestibular root of the nerve, and arises from the cells in the ganglion 

 of Scarpa ; the other set constitutes the cochlear root, and takes origin from the 

 cells in the ganglion spirale or ganglion of Corti. At its connection with the 

 brain the eighth nerve occupies the groove between the pons and medulla, where it 

 is situated between the restiform body, which is behind, and the seventh nerve, 

 which is in front. 



Vestibular or Ventral Root. The fibres of this root enter the medulla to the 

 inner side of those of the cochlear root, and pass between the restiform body, 

 which is to their outer side, and the inferior root of the fifth, which lies to their 

 inner side. They then divide into an ascending and a descending set. The fibres 

 of the latter end by arborizing round the cells of the internal nucleus, which is 

 situated in the trigonum acustici in the floor of the fourth ventricle. The ascend- 

 ing fibres either end in the same manner or in the external nucleus, which is 

 situated to the outer side of the trigonum acustici and farther from the ventricular 

 floor. It is described as consisting of two parts, an inner, the nucleus of Deiters, 

 and an outer, the nucleus of Bechteren. Some of the axons of the cells of the 

 external nucleus, and possibly also of the internal nucleus, are continued upward 

 through the restiform body to the roof nuclei of the opposite side of the cere- 

 bellum, to which also are prolonged other fibres of the vestibular root without 

 undergoing a relay in the nuclei of the medulla. A second set of fibres from the 

 internal and external nuclei end partly in the tegmentum, while the remainder 

 ascend in the posterior longitudinal bundle to arborize around the nuclei of the 

 oculo-motor nerve. 



Cochlear or Dorsal Root. This part of the nerve is placed externally to the 

 vestibular root. Its fibres end in two nuclei, one of which, the accessory nucleus, 

 lies immediately in front of the restiform body ; the other, the tuberculum acusti- 

 cum, somewhat to its outer side. 



The striae acusticse or medullary striae are the axons of the cells of the tuber- 

 culum acusticurn. They pass backward and inward over the restiform body, and 

 across the floor of the fourth ventricle toward the middle line. Here they dip into 

 the substance of the pons, to end around the cells of the superior olive of the same or 

 opposite side. There are, however, other fibres, and these are both direct and 

 crossed, which do not arborize around the tegmental nuclei, but pass into the lateral 

 fillet. The cells of the accessory nucleus give origin to fibres Avhich pass trans- 

 versely in the pons and constitute the trapezium. The description given as to the 

 mode of ending of the striae acusticae is applicable to that of the trapezoid fibres, 

 viz., around the cells of the superior olive or of the trapezoid nucleus (which lies 

 ventral to the olive) of the same or opposite side, while others, crossed or uncrossed, 

 pass directly into the lateral fillet. 



If the further connections of the cochlear nerve of one side, say the left, are 

 considered, it is found that they lie to the outer side of the main sensory tract, the 

 fillet, and are therefore termed the lateral fillet. The fibres comprising the left 

 lateral fillet arise in the superior olive or trapezoid nucleus of the same or opposite 

 side, while others are the uninterrupted fibres already alluded to, and these are 



