THE AUDITORY NERVE. 



1259 



FIG. 1072. 



Floor of IV. ventricle 



Superior cerebellar 

 peduncle 



Inferior cerebellar 

 peduncle 



Lateral vestibular 

 (Deiters') nucleus 

 Median vestibular 

 nucleus 

 Cochlear fibres 



Dorsal cochlear nucleus 



Descending vestibular 

 root 



4. Neurones of the inferior colliculus and of the median geniculate body, whose axones 

 pass, as the auditory radiation, to the auditory cortical area within the temporal lobe of the 

 cerebrum. Although the exact extent of the auditory area is still uncertain, the most important 

 part of this centre includes the superior temporal and the subjacent part of the middle temporal 

 convolution. 



The cochlear fibres that do not undergo decussation ascend through the lateral fillet of the 

 same side and eventually establish cortical relations with the corresponding hemisphere ; from 

 th.e preceding account, however, it is manifest that the auditory area is connected chiefly with 

 the cochlea of the opposite side. 



Peripheral, Central and Cortical Connections of the Vestibular Nerve. The fibres of the 

 vestibular portion of the auditory nerve are the axones of the bipolar nerve-cells situated within 

 the small vestibular ganglion (g. vestibulare) or Scarpa's gang/ion, which lies at the bottom 

 of the internal auditory canal. The dendrites of these cells constitute the five branches of dis- 

 tribution of the vestibular nerve and pass through the various openings in the inner wall of the 

 bony labyrinth, in the manner above 

 described (page 1256), to reach the 

 specialized areas, the macula acusticce, 

 within the saccule, the utricle and the 

 ampullae of the semicircular canals, where 

 the nerve-filaments end, really begin, 

 in intimate relation with the neuro- 

 epithelium. While the centrally directed 

 axones of the neurones supplying the 

 utricle and the superior and external 

 semicircular canals become consolidated 

 to form the vestibular nerve of descriptive 

 anatomy, those from the saccule and the 

 posterior semicircular canal join the coch- 

 lear fibres and with these course within 

 the cochlear nerve until the latter and 

 the vestibular nerve unite to form the 

 common auditory trunk. Where the 

 common trunk separates into the two 

 roots, the vestibular fibres leave the 

 cochlear and permanently assume their 

 natural companionship with the remain- 

 ing fibres of the vestibular root. 



The vestibular fibres enter the 

 brain-stem at a slightly higher level than 

 does the cochlear root, lying mesial to the 

 latter and the ventral cochlear nucleus, 

 and pass dorsally within the pons between 

 the inferior cerebellar peduncle and the 

 spinal trigeminal root. On reaching a level dorsal to the latter, the vestibular fibres divide 

 into short upward and longer downward coursing branches, which, after condensing into an 

 ascending and a descending root respectively, end in arborizations around the cells of the 

 vestibular nucleus of reception. The exact extent and constitution of this nucleus, which under- 

 lies the area acustica in the floor of the fourth ventricle (page 1097), are uncertain, since the 

 neurones directly related to the vestibular fibres contribute only a part of those contained within 

 a large diffuse complex of cells and fibres, many of whose constituents probably have only an 

 indirect connection with the vestibular nerve. When reconstructed, as has been successfully 

 done by Sabin, this complex has the form shown in Fig. 1072 and comprises two general parts, 

 (a) an extended irregularly triangular mass of cells lying for the most part mesial to the tract 

 formed by the ascending and descending branches of the vestibular fibres, and (d) a smaller 

 mass of cells which lies above the larger one and partly to the inner and partly to the outer side 

 of the tract of the vestibular fibres. The apex of the large triangular mass approaches the 

 mid-line and its superior and inferior basal angles are prolonged upward and downward along 

 the vestibular tract. 



When examined microscopically the large mass is found to include three subdivisions : (a) 

 a tapering caudally directed nucleus which continues the inferior angle along the descending 

 vestibular root, (k) an extended triangular nucleus that includes the greater part of the large 

 mass and (c] an irregular pyramidal nucleus that prolongs upward the superior angle. The first 

 of these subdivisions (a) is known as the spinal vestibular nucleus (nuc. spinalis n. vestibularis), 

 the second () as the median vestibular nucleus (nuc. mediali.-s n. vestibularis), also as the chief 

 nucleus or the triangular nucleus and the third (r) as the superior vestibular nucleus or the 



Nucleus cuneatus 



Closed part of medulla 



Vestibular nuclei as shown in reconstruction by Dr. Florence 

 R. Sabin. 



