AUDITORY NERVE. 699 



VIII, AUDITORY NERVE. 



Two roots serve for the origin of the auditory nerve, an anterior median root 

 with coarse fibers, and a posterior lateral root with fine fibers. The vestibular 

 nerve arises from the former, the cochlear nerve from the latter. The two are 

 entirely distinct in the sheep and the horse. Each vestibular and cochlear nerve 

 arises from a peripheral ganglion (the vestibular ganglion in the internal auditory 

 canal and the spiral ganglion in the cochlea), constituted like the spinal ganglia, 

 and at the same time the trophic center for the fibers. Into each gan- 

 glion-cell there enters a cellulipetal dendrite passing from the sensory epithelium 

 in the labyrinth, while on the other hand each cell sends to the medulla oblongata 

 a cellulipetal neurite to the nuclei of origin of the auditory nerve, with whose 

 cells it comes in contact by means of terminal filaments and collaterals. The 

 vestibular nerve is essentially connected with gray matter that is in relation with 

 the cerebellum and probably subserves the purpose of maintaining the equilibrium. 

 From the origin of the cochlear fibers the main portion passes on the opposite 

 side to the posterior quadrigeminate body and the internal geniculate body and 

 further (particularly through the lower fillet, the upper olive and the trapezoid 

 body) to the temporal lobe of the cerebrum, in which the psycho-auditory cortical 

 center is situated. After extirpation of the temporal lobe its fibers through the 

 corona radiata atrophy into the internal capsule, as well as fibers in the posterior 

 quadrigeminate body and the internal geniculate body. The striae acusticas rep- 

 resent a central path for the lateral auditory root. They form a secondary pro- 

 jection-system of the auditory nerve, decussating somewhat like a chiasm. The 

 nuclei of origin of both auditory nerves are connected in the brain by commissural 

 fibers. In the internal auditory canal root-fibers pass from the intermediate 

 portion into the auditory nerve. 



The auditory nerve has a double function : in the first place it is the 

 nerve of hearing. Every irritation at its origin, in its course or in its 

 terminal distribution causes auditory impressions ; every injury, in accord- 

 ance with its intensity, impairment of hearing to the point of deafness; 

 also destruction of the labyrinths, the end-organs of the auditory 

 nerves, causes complete deafness. 



As animals after removal of both cochleae still react to coarse sounds, the 

 ampullas must serve for the perception of the sounds, and the cochlea for the 

 appreciation of the remaining auditory qualities. After extirpation of the laby- 

 rinth the auditory nerve undergoes atrophy in an upward direction. 



Entirely distinct from the auditory is the function of the nerve that 

 is localized exclusively in the semicircular canals, namely that govering 

 the necessary movements for the maintenance of the bodily equilibrium, 

 through stimulation of the peripheral distribution in the ampullae. 



Of especial importance is the behavior of the auditory nerve in response to 

 the galvanic current. If an electrode is placed in a healthy person upon the 

 tragus on each side, it will be found that upon the anodal side with closure of 

 the current silence occurs, on opening the current a sense of sound, while the 

 opposite takes place upon the kathodal side (Brenner's normal formula). If one 

 electrode is placed on the tragus and the other is held in the hand, the same result 

 is observed, except that the sound upon the unarmed ear is much feebler. The 

 sound agrees exactly with the resonance fundamental tone of the sound-conducting 

 apparatus of the ear itself. 



The appearance of this sound is to be explained in the following manner: In 

 the middle ear there exists a permanent blood-murmur, to which the system of 

 cavities of the middle ear resonates with its fundamental tone. In consequence 

 of habituation this tone is, as a rule, not noted, but it appears at once if the auditory 

 nerve is placed in a condition of increased irritation, namely (in the sense of 

 electrotonus) on kathodal closure and anodal opening. 



According to Gradenigo, Pollak, and Gartner, the auditory nerve in healthy 

 persons does not react at all to currents of moderate strength. Only in the pres- 

 ence of hyperemic and irritative states of the auditory apparatus does a reaction 



