712 SPECIAL SENSES 



itself. The deep root passes around the restiform body inward, so that 

 this portion of the bulb is encircled by the two roots. Passing from the 

 superior and lateral portion of the bulb, the trunk of the nerve is applied 

 to the superior and anterior surface of the facial. It then passes around 

 the middle peduncle of the cerebellum and receives a process from the 

 arachnoid membrane, which envelops it in a common sheath with the 

 facial. It finally penetrates the internal auditory meatus. In its course 

 it receives filaments from the restiform body and probably from the pons 

 Varolii. Within the meatus the nerve divides into an anterior and a 

 posterior branch, the anterior being distributed to the cochlea, and the 

 posterior, to the vestibule and semicircular canals. The distribution of 

 these branches will be described in connection with the anatomy of the 

 internal ear. 



The auditory nerves are grayish in color and their consistence is 

 soft, thus differing from the ordinary cerebro-spinal nerves and resem- 

 bling in a certain degree the other nerves of special sense. On the 

 external, or superficial root, is a small ganglioform enlargement, contain- 

 ing fusiform nerve-cells. The filaments of the trunk of the nerve con- 

 sist of very large axis-cylinders, surrounded with a medullary sheath but 

 having no tubular membrane. In the course of these fibres, are found 

 small, nucleated, ganglionic enlargements. 



General Properties of the Auditory Nerves. There can be no doubt, 

 as regards the eighth, that it is the only nerve capable of receiving and 

 conveying to the brain the special impressions produced by waves of 

 sound; but it is an important question to determine whether this nerve 

 be endowed also with general sensibility. Analogy with most of the 

 other nerves of special sense would indicate that the auditory nerves are 

 insensible to ordinary impressions ; and this view has been sustained by 

 direct experiments. In experiments made by passing electric currents 

 through the ears, some physiologists have thought that auditory sensa- 

 tions were produced; but it is probable that the sensations observed 

 were due to clonic spasm of the stapedius muscle and not to impressions 

 of sound produced by the action of the stimulus on the auditory nerves. 

 In cases of complete facial paralysis from otitis, in which paralysis of 

 the auditory nerve could be excluded, it has not been possible to produce 

 subjective auditory sensations, even by powerful faradization by means 

 of a catheter passed through the Eustachian tube into the tympanic 

 cavity or by the external meatus. In addition there are well-established 

 clinical observations which sustain the theory of muscular contraction 

 and are opposed to the idea of impressions of sound produced by direct 

 stimulation of the auditory nerves. The results, then, as regards stimu- 

 lation of the auditory nerves, have been negative. Were it practicable to 



