The fiicial nerve, or the portio dura of the 7th pair, is divisible into three stages. The first stage is the intercranial, from its origin to its exit from the cranial cavity, in associ ation with the polio mollies or auditory nerve (q.v.), at the internal auditory meatus. The second stage is contained in the aqueduct of l'allopias, a bony canal lying in the pet rous portion of the temporal bone. In this stage it anastomizes with other nerves, and thus fibers are introduced into it from the 5th pair and other sources, which make irritation or some of its branches to cause pain. The third stage commences with the emergence of the nerve through the stvlo-mastoid foramen. The nerve now lies in the parotid gland, and after giving oft the posterior auricular, and a few smaller branches, finally divides into the temporal, facial, and cervical branches. This diverg ing distribution of the nervous branches over the face forms the pes anserinvs of the older anatomists. from the supposed resemblance to the expanded foot of a goose. Care ful dissection of this nerve shows that the great majority of its fibers are distributed to muscles; and indeed, if we except the muscles of mastication, which receive their motor power from the 3d division of the 5th pair, this may be regarded as the general motor nerve of the face. "The muscles which are supplied by the facial nerve are chiefly ',hose upon which the aspect of the countenance and the balance of the features depend. The power of closing the eyelids depends upon this nerve, as it alone supplies the orb cularis palpebmrum; and likewise that of frowning, from its influence upon the corrucator supercilii. Anatomy indicates that this nerve is the motor nerve of the superficial muscles of the face and ear, and of the deep-seated muscles within the ear. 'fits cruclusion is abundantly confirmed by comparative anatomy. For wherever the superficial muscles of the face are well developed, and the play of the features is active, this nerve is large. In monkeys it is especially so. That extremely mobile instru ment, the elephant's trunk, is provided with a large branch of the facial as its motor nerve, In birds, on the other hand, it is very small."—Todd and Bowman, Physiologi cal Anatomy and Physiology of Man, vol. ii. p. 107.
Before sir Charles Bell commenced his experiments on the functions of the nerves, it was believed that the facial was the nerve of sensibility of the face, and it was on sev eral occasions divided with the view of relieving tic douloureux, of which it was sup posed to be the seat. But the operation, of course, yielded no relief, and always inflicted a permanent injury, since it was succeeded by paralysis of the facial muscles, with total loss of control over the features and over t..e closing of the eye, on the side on which the operation was performed.
The treatment of facial palsy, which is often, especially if it arises from cold, a very temporary affection, although a very alarming one to the patient and his friends, is described in the article PARALYSPS.
The leypoglosSal. nerve (derived forom the Greek words hlipo,. under, and glotta, the tongue) escapes from the cavity of the skull by the anterior condyloid foramen, and passes outward and forward around the pharynx to the interior surface of the tongue, where it breaks up into its terminal branches, which supply the muscular structure of that organ with motor power. This nerve communicates with the puennumgastric nerve,
with the sympathetic (by branches derived from the superior cervical ganglion), and with the cervical plexus, soon after its emergence from the cranium; and subsequently, as it curves round the occipital artery, it gives off the long anastomosing brunch known as the descendens noni.
Experiments on living animals, comparative anatomy, and pathological investigations alike indicate that this is the motor nerve of the tongue. In cases of paralysis of this nerve the power of articulation is much injured or totally destroyed; and this is often one of the first symptoms which lead the physician to mippreliend serious cerebral lesion.
We now proceed to the consideration of the compound nerves, beginning with the trifacial or fifth lierVe. This nerve, as was first pointed out by sir Charles Bell, presents a remarkable resemblance to the spinal nerves in its mode of origin; for it arises by two roots, one large and the other small, and on its larger root, as on time posterior and larger root of the spinal nerves, is a distinct ganglion; the two roots being quite distinct until after the formation of the ganglion, when the lesser one coalesces with the lowest branch, which emerges from the ganglion to form the inferior maxillary nerve. This ganglion, which is known as the gasserian ganglion, and which is formed upon the larger root of the nerve, lies upon the upper surface of the petrous portion of the temporal bone, and is of a somewhat triangular form, with its base directed forward and outward. From this base there proceed three nerves—viz., the ophthalmic, on the inside; the supe rior maxillary, in the middle; and the inferior maxillary, externally. The first two of these nerves consist exclusively of fibers from the ganglionic root, while the third—the inferior maxillary—is composed of fibers from both roots, and is, therefore, a compound nerve. From the mode of distribution, as well as from that of origin, it is inferred that the eplithalmic and superior maxillary are purely sensory, while the inferior maxillary is a motor and sensory nerve. Experiments on living animals confirm the inference, that have been drawn on anatomical grounds. Division of the ophthalmic or of the superior maxillary nerve induces loss of sensibility, without ally serious impairment of muscular power; but when the inferior maxillary nerve, on either side, is divided, the power of mastication is destroyed on that side, and the sensibility of the tongue and of the lower part of the face on that side is lost.
The lingual or gustatory branch of the inferior maxillary is distributed to the mucous membrane and papilhe at the fore part and sides of the tongue, where it acts both as a nerve of common sensibility and of taste. (The consideration of the respective parts which this nerve and the glossopharynireal play in the sense of taste, is considered in the articles TONGUE and SENSE OF TASTE. ) The trifacial nerve is the seat of the affection known as tic-douloureux, and described in the article NEURALGIA. It is is the,dental branches of this nerve that toothache is situated; and in the process of teething in young children, the irritation of these branches, consequent upon tlie pressure of the teeth, often gives rise to convulsions, by being con veyed to the medulla oblongata, and exciting motor nerves by reflex action.