Concerning the influence of the facial nerve on hearing,little is known at present. Longet, in quoting the above case of M. Roux, in which comparatively faint sounds were painfully distinct, has given a very pro bable and ingenious explanation of the fact, by pointing the derivation of the nerve to the tensor tympani from the otic ganglion, which is itself associated with the geniculate gan glion and facial nerve. Regarding this muscle as the regulator of the acoustic drum, and the tension of this as the means of moderating excessive stimulus, just as the iris does in the eye, he shows the probability that the paralysis of the tensor in this manner de prives the ear of an important protection, and increases the loudness of the sound received.
It has thus been deduced that the facial is chiefly a nerve of motion; or, in other words, that by its central and peripheral organization it is adapted to determine the contraction of the facial inuscles. It has next to be con sidered whether it is exclusively motor, or whether, on the contrary, it contains a certain proportion of nervous filaments, the office of which is the production of sensations.
The highly sensitive integument which forms the surface of the face, evidently re ceives its nervous supply solely from the different divisions of the fifth ; and the anatomy of the distribution of these branches is confirmed by comparing the results which are obtained by artificial section of the facial and fifth nerves. In the case of the divided portio dura, it was previously mentioned, that while motion is lost, sensibility is unaffected ; while in the common instance of thn divided fifth, mobility remains, but the sensibility of this surface completely vanishes, and no ex. pression of pain can be obtained even by cauterising large portions of the integuments.
The facial is thus excluded from all share in the tactile sensibility of this surface ; yet it by no means follows that the nerve itself is wholly insensible. On the contrary, the ex periments of most physiologists from the time of Bell agree in verifying the fact of its sensi bility; as shown by- the expressions of pain which are called forth on mechanically irri tating the nerve in the living animal. Thus, pinching the trunk of the facial, or any of its larger branches, or the act of section itself, have been constantly found to be accompanied by the most unequivocal indications of suffer ing.
From the evidence above stated, it is mani fest that the sensory filaments which we must suppose the trunk of the facial to con tain, are not distributed to the cutaneous surface of the face. But although the skin is
the chief organ of common sensation, it is by no means the only seat of the function : a variable but necessary share is possessed by the whole body, and accomplishes the general purpose of protection, perhaps also confers the muscular sense. Thus, by means of sensa tion, the injury of any particular part deter mines the occurren-ce of pain which is referred to that situation ; and in this manner atten tion is directed to the seat of injury, and its duration or increase is prevented by a voluntary act. And it is probable that the sensitive branches which accompany the portio dura are of this kind ; branches which, although very different in function, travel with the motor nerve, because they experience a distribution in its immediate neighbourhood. Indeed it is perhaps not unlikely that some of the sensory filaments which are included in the facial may bear a protective relation to this important nerve itself, possibly by a virtual distribution among its fibres : — a notion which would thus far approximate to the supposed " nervi nervorum " of the old authors.
But although the sensibility of the facial nerve is well ascertained, the origin or imme diate cause of this endowment is still a matter of considerable dispute. The numerous views adopted by different authors offer many slighter modifications, but they are all re ducible to two chief theories. One of these considers that the facial nerve is insensible at its origin from the brain ; and that whatever amount of sensibility it subsequently exhibits is due to foreign filaments, which come from the acknowledged sensitive nerves of the fifth and pneumogastric ; and which, joining the portio dura in different parts of its course, accompany it beyond these points included in its substance. The other regards the facial nerve as arising by two roots, whereof the larger is motor, the smaller sensitive ; and that the sensibility of the nerve as a whole is the result of its double constitution, and is effected by its own sensory filaments.
Each of these theories has received the sanction of distinguished anatomists. Thus, amongst many others, the first has obtained the support of Magendie, Cruveilhier, Eschricht, Lund, &c.; while the latter numbers amongst its advocates, Arnold, Bischoff, Goedechens, Barthold, and, more lately, Morganti.