816 SPECIAL SENSES. 



ficial root, sometimes called the posterior root, can be seen usually without preparation. 

 This consists of from five to seven grayish filaments, which decussate in the median line, 

 and pass outward, winding from the fourth ventricle around the restiform body. The 

 deep root consists of numerous distinct filaments, arising from the gray matter of the 

 fourth ventricle, two or three of which pass to the median line to decussate with corre- 

 sponding filaments from the opposite side. This root passes around the restiform body 

 inward, so that this portion of the medulla is encircled, as it were, by the two roots. 

 Passing from the superior and lateral portion of the medulla oblongata, 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 mem- 

 brane, which envelops it in a common sheath with the facial. It then penetrates the 

 internal auditory meatus. In its course, it receives filaments from the restiform body, 

 and possibly from the pons Varolii. Within the meatus, the nerve divides into an ante- 

 rior and a posterior branch, the anterior being distributed to the cochlea, and the poste- 

 rior, to the vestibule and semicircular canals. The distribution of these branches will be 

 fully described in connection with the anatomy of the internal ear. 



The color of the auditory nerves is grayish, and their consistence is soft, thus differing 

 from the ordinary cerebro-spinal nerves, and resembling, to a certain extent, the other 

 nerves of special sense. On the external, or superficial root, is a small ganglioform en- 

 largement, containing fusiform nerve-cells. According to the latest researches, the fila- 

 ments of the trunk of this nerve consist of very large axis-cylinders, surrounded by 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 

 portio mollis of the seventh, 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 interesting 

 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 is sustained by direct experi- 

 ments, made many years ago. 



The phenomena observed during the passage of galvanic currents through the audi- 

 tory nerves have, of late years, been the subject of much discussion. The old experiment 

 of Yolta, which was almost immediately confirmed by Hitter, is sufficiently familiar and 

 is often quoted as showing that galvanic stimulation of these nerves produces a sensation 

 of sound ; but the facts ascertained leave room for doubt with regard to the precise mode 

 of action of the current. A careful study of recent observations upon this point renders 

 the question even more obscure ; but, from a purely physiological point of view, we have 

 only to do with the effects of stimulating the auditory nerves in health. Leaving the 

 therapeutic and diagnostic uses of galvanism out of the question, we find that there is 

 considerable uncertainty with regard to the fact of direct stimulation of the auditory 

 nerves, in the recent experiments with the galvanic current. Brenner observed strong 

 sensations of sound with one of the poles of a battery in the auditory passage filled with 

 water and the other connected with different parts of the body. When the cathode was 

 placed in the ear, the sound was heard at the making of the current. With the anode in 

 the ear, there was no sound at the making of the current or during its passage, but a 

 slight sound was heard at the breaking of the current. These phenomena closely resem- 

 ble those produced by the galvanic current applied to ordinary motor nerves, in so far 

 as the action seemed to be most vigorous at the making of the circuit, with the direct 

 current, and at the breaking of the circuit, with the inverse current; for, when the 

 cathode is placed in the ear, the current is direct, following the course of the nerve from 

 the centre to the periphery, and vice versa. Without following out the discussion of this 

 question in detail, it seems only necessary to study the very clear and satisfactory experi- 



