THE AUDITORY NERVE. 



1257 



Although the auditory nerve as a whole may be conveniently followed from the 

 brain to the ear, as has been done in the preceding sketch, it is evident since its 

 fibres are sensory and therefore afferent, that they are the processes (axones) of 



Fig. 1070. 



nerve-cells situated somewhere along the course 

 of the nerve. It is necessary, consequently, to 

 seek the real origin of these fibres in the ganglia 

 occurring on the divisions of the nerve. In 

 recognition of the functional differences of the 

 two roots of the eighth nerve, it is desirable to 

 trace separately the pathway followed by the 

 impulses conveyed by each of these components. 



Peripheral, Central and Cortical Connections of 

 the Cochlear Nerve. — The true cochlear fibres arise 

 within the internal ear (cochlea) as axones of the 

 cells of the spiral ganglion or ganglion of Corti (g. 

 spirale) ( Fig. 1071). This structure consists of a 

 series of bipolar neurones which occupies the spiral 

 canal in the base of the lamina spiralis. The dendritic 

 processes of these cells begin as fine fibrils which lie 

 in close relation with the neuroepithelial cells compris- 

 ing the inner and outer hair-cells of the organ of Corti. 

 Leaving the hair-cells as nonmedullated fibres, they 

 traverse the foramina nervosa of the labium tympani- 

 cum, at which point they becoine medullated. They 

 then interlace to form an elaborate flat felt-work that 

 lies between the layers of the lamina spiralis and soon 

 assembles to form bundles which pass to the cells of 

 the ganglion spirale, each fibre probably joining its 

 individual cell. Leaving the ganglion, the axones of 

 its cells enter the bony canals within the modiolus, 

 from which they emerge at the tractus spiralis forami- 

 nosus and are collected into a single bundle, the coch- 

 lear nerve proper. The latter, however, soon receives 

 two accessions, one of which consists of fibres from 

 the saccule and the other from the posterior semi- 

 circular canal. From what has been said, it is evident 

 that these accessions are parts of the vestibular nerve 

 and, beyond their temporary companionship, have nothing to do with the cochlear root. 



On reaching the medulla, the cochlear fibres come into relation with their nucleus of recep- 

 tion, which includes two superficial aggregations of nerve-cells that collectively constitute the 

 acoustic nucleus (nucleus acusticus). The latter consists ot two parts (Fig. 932) of which one, 

 the ventral cochlear nucleus, also called the accessory acoustic nucleus (nucleus accessorius ), lies 

 ventral to the inferior cerebellar pedimcle, and the other, the lateral cochlear nucleus, or tuber- 

 culum acusticum, rests upon the dorso-lateral surface of the peduncle and occupies the extreme 

 outer part of the triangular acoustic area seen in the lateral angle of the floor of the fourth ven- 

 tricle (page 1097 ). The greater number of cochlear fibres end in arborizations around the stel- 

 late cells of the ventral ganglion, while others terminate in relation with the more elongated, 

 fusiform cells of the lateral nucleus. From the neurones of these subdivisions of the reception 

 nucleus, the auditory pathway is continued as two chief tracts, the axones of the cells of the 

 ventral nucleus passing for the most part ventral to the restiform body and the spinal root of the 

 trigeminus to form the corpus irapezoides, while those from the lateral nucleus sweep around the 

 outer surface of the restiform body and then medially beneath the ependyma of the floor of the 

 fourth ventricle, where they show with varying degrees of distinctness as the acoustic strics 

 (Fig. 918). 



The corpus trapezoides, the conspicuous transverse tract that separates the tegmental from 

 the ventral region of the pons in its superior part, is formed chiefly by the axones of the cells 

 within the ventral cochlear nucleus, supplemented by a limited number of fibres that spring 

 from the lateral nucleus. In addition it contains axones from the large cells found within the 

 trapezoid body, on each side of the mid-line, that constitute the nucleus trapezoideus. In close 

 relation with the dorsal surface of the corpus trapezoides, within the superior olive and on 

 either side of the median raphe, lies the superior olivary nucleus (nucleus olivaris superior), a 

 collection of nerve-cells around which many of the cochlear fibres, chiefly from the opposite 

 but also from the same side, end and from which the tract of the lateral fillet principally takes 



Reconstruclion of left membranous lat)y. 

 rimh of human embryo of ten weeks (30 mm.), 

 lateral aspect ; vestibular nerve and ganglion 

 are red ; cochlear nerve is blue ; vestibular 

 rami are seen passing- to ampullae of semi- 

 circular canals aiul 'o maculfe of utricle and 

 saccule. X 20. (Sli rrto .) 



