The unravelling of the nerves themselves, even as performed in tbe above dissections, requires, perhaps, to be received with con siderable caution ; and that natural separation or simplification which is afforded by their comparative anatomy must be regarded as vastly increasing the value of the results obtained. The chorda tympani affords a good instance of the conflicting results of these dissections, when unaided by this latter method of inquiry. Some have considered, with H. Cloquet and Longet, that, after remain ing a short time in contact with the gustatory or lingual branch of the fifth, the whole of this nerve passes away from it, to form one of the roots of the submaxillary ganglion. Others have described it as only giving a fila ment to this ganglion, and uniting itself by the remainder of its bulk with the branch of the fifth ; while others have failed to detect any direct transition of the chorda tympani into the ganglion, but, on the contrary, have found the two nerves inseparably mixed up below the situation of their visible junction. And more recently it has been traced by Guarini 4' to the lingualis muscle. On general grounds, the first of these notions is liable to much objection, since it seems singular that a nerve so far removed from the facial as the chorda tympani is at the base of the skull, should be involved in such an accidental proximity as this would make it, or should run so closely to the gustatory without any interchange of fibres. Again, the total pas sage of the nerve to the ganglion appears very improbable, when the relative size of the entering and crnerging branches is considered, —that is, on comparing the bulk of the chorda tympani with that of the two or three filaments which join the ganglion, it may be seen that the former is larger than all of them together. This is especially the case in some of the lower animals, as the dog ; in whom the submaxillary ganglion and its roots from the cerebral nerves are so greatly reduced in size as to be scarcely visible to the naked eye, while the chorda tympani continues a compa ratively large branch. But these general objections will not apply to the supposition of a partial connection of the chorda tympani with the submaxillary,ganglion, and the ques tion must accordingly still remain in doubt.
Physiology of the seventh The func tions sustained by the auditory nerve are recognised with sufficient certainty. The anatomy of its distribution, its variations in the different classes of the animal kingdom, and the results of experiment or disease affecting its structure, all unite to indicate it as the nerve which, specially adapted at its periphery in the organ of hearing to receive impressions from the vibrations of the ex ternal air, conveys them to the brain, and by exciting corresponding impressions there, gives rise to the production of a sensation which we term a sound. For the further details of its function, reference is made to the article HEARING.
The facial It has been seen from the preceding description, that the facial nerve is distributed almost exclusively to muscular structures ; and, although these are very numerous, yet they all admit of being reduced to one class, viz. the muscles of the face. A further subdivision would next sepa rate them into several groups, which serve to enlarge or diminish the size of the various apertures by which impressions are admitted to the organs of the special senses, as the eye, ear, nose, and tongue. But these ori fices are also the pathways of food and air, so that the muscles which regulate their size have thus far an influence on the functions of respiration and digestion. In man, they fulfil the further purpose of organs of expres sion ; their various and complicated adjust ments conveying, for the most part, a toler able index of the passions or emotions of the presiding mind.
This prominently muscular distribution of the facial would lead us to expect priori that the nerve was chiefly motor in its func tion ; and, if we turn from its anatomy in the human subject to its varieties of arrangement and appearance in the animal kingdom, this view will be abundantly confirmed. Not
only is its peripheric distribution almost ex clusively muscular, and in connection with the same facial set of muscles, but it also experiences a development which is co-equal with that of these organs, increasing with their augmented development, or disappear ing with their suppression. Thus, in Fishes, the facial scarcely exists as a separate nerve. In Reptiles and Birds, its small size corre sponds with the bony and immobile state of the face. In the Mammalia, it becomes much more considerable, and both the nerve and muscles experience various degrees of aug mentation. Thus, in the monkeys it attains a large size, in accordance with the- number and magnitude of the facial muscles generally ; and the trunk of the elephant, and the mus cular apparatus connected with the blowholes of Cetaceans, are supplied by large branches from this nerve, which here experiences a partial development, to meet the special exi gencies of the case. In man, the nerve at tains the maximum of general development.
Experiment also confirms the testimony afforded by the human and comparative ana tomy of the facial nerve: indeed, the results afforded by this method of enquiry first led Sir Charles Bell* to the discovery of its function. On cutting across the trunk of the nerve, he found that the whole side of the face on which it was divided had com pletely lost the power of movement, while its sensibility remained unimpaired. His experi ments have since been frequently repeated, and invariably with the same results.
The over-excitement of the nerve affords evidence of its motor function, equally with the destruction of its continuity. Thus, gal vanism of the distal extremity of the cut nerve at once sets up convulsive movements in the muscles to which it is distributed.
The paralysis produced by section includes all those muscles which the facial has been previously described to supply ; but the muscles of the jaw, which are furnished with nerves from the inferior maxillary division of the fifth, are still free to execute their con tractions, and hence the movements of the jaw continue. But although these are still carried on, yet the act of mastication as whole is rendered very imperfect ; since this not only requires the apposition and trituration of the teeth upon each other, but also demands accessory though subordinate movements of the neighbouring lips and cheek, and the section of the facial nerve distributed to these parts render these adjuvant movements im possible. In such cases an imperfect masti-' cation may indeed be seen to take place ; but the cheek and lips, having lost their contrac tility, instead of pressing in the food towards the teeth, and submitting it again and again to their action, allow it constantly and gra dually to accumulate in this flaccid and yielding pouch; or permit it to fall out of the anterior opening in the mouth. In this instance, experirnent throws a reflected light upon descriptive anatomy. The buccinator muscle, which forms the greater part of the fleshy parietes of the cheek, was previously mentioned as receiving branches from both the facial and inferior maxillary nerves; and were we to confide altogether in the appear ances seen in dissecting these nerves in the human subjectl-, we might perhaps justifiably regard them as sharing between them the supplying of the muscle. But the paralysis of the buccinator, which is always present in those instances where the facial nerve has suffered division, points distinctly enough to the latter as at least taking the more consi derable and important part of the two; while the failure of the galvanic stimulus to affect the muscle through the buccal nerve, indicates that the filaments of the facial are in all probability the only motor nerves which are distributed to it.