At the posterior part of the space, the nerve is immediately beneath the mucous membrane; as it proceeds it descends from, but toward the anterior part again ascends, and is in con tact with the membrane as it becomes attached to the tongue.
Having reached the anterior margin of the hyo-glossus the nerve breaks up into three branches, posterior, middle, and anterior. Of these the posterior is the shortest, and ascends almost directly; the middle runs upward and forward, and the anterior, which is much longer than, and at the same time inferior to the others, almost directly forward, along the under surface of the tongue, between the genio glossus and the stylo-glossus ; the former muscle internal, the latter external to it. In its course beneath the tongue it is accompanied by the ranine artery, which joins it at the anterior margin of the hyo-glossus, and is situate inferior to it, immediately above the mucous membrane.
The lingual nerve does not give off many branches in the first part of its course : soon after its origin it receives the branch of com munication, by which the inferior dental nerve is connected to it. About the same point or presently after it is also joined by the chorda tympani. The uncertainty which has prevailed with regard to the source of this nerve renders a more particular account of it necessary than would otherwise be required. The chorda tym pani—a delicate filament—is given off from the porno dura shortly before that nerve escapes from the aqueduct of Fallopius, behind and be low the tympanum : it passes upward and for ward toward the tympanum, contained in a spe cial canal of the bone, and having reached the back of the chamber it emerges from its posterior wall through a small aperture beneath the base of the pyramid ; it then attaches itself to the outer wall of the tympanum and crosses it toward the anterior, having first received* a delicate filament from the sympathetic, and running forward, upward, and outward. During its course from the posterior to the anterior wall it is situate at first beneath the short crus of the incus, then between the long ems of the incus and the superior part of the handle of the malleus, to which it is connected by the lining membrane of the tympanum. Having ascended above the internal muscle of the malleus it changes its direction and runs down ward, forward, and inward along the superior anterior part of the circumference of the mein brana tympani, until it has reached the anterior wall of the chamber, from which it goes out through the Glaserian fissure, along the tendon of the anterior muscle of the malleus. It is throughout excluded from the interior of the tympanum by the lining membrane, which is connected to it upon that side; it is therefore incorrect to say that it crosses the chamber. After its escape from the tympanum the nerve continues to descend forward and inward in front of the levator palati muscle, and after a course from three-fourths of an inch to an inch long it is attached at a very acute angle to the back of the lingual branch, becomes inclosed in the same sheath with the nerve, and con tinues connected with it altogether until the nerve has reached the posterior extremity of the submaxillary gland : at that point the chorda tympani divides into two parts, one of which is despatched to the submaxillary gan glion, and the other continued along with the lingual branch. By somet it is stated that it separates from the nerve at the ganglion, and is altogether ununited to it; this, however, is incorrect. During its descent in company with the lingual branch there may be observed. upon particular examination of the conjoined trunk, a communication and identification be tween the nervous matter of the two nerves.
Originally the chorda tympani was regarded as either a recurrent filament of the lingual branch of the fifth or a branch of the portio dura : afterwards the opinion was adopted that it was not a branch of the portio dura, but the cranial superficial petrous branch of the Vidian nerve, which, instead of uniting and being iden tified with the portio dura, descended through the aqueduct merely in apposition with it or within the same sheath, separated from it again before the nerve escaped from the aqueduct, and constituted the chorda tympani. This view of the nature of the chord, suggested first, as it would appear, by J. llunter, has been advo cated also by Cloquet and Hirzel, and is at present entertained by many in this country at least ; it has been objected to by Arnold, and another has been advanced by him from obser vations made upon the calf and the human subject. Hunter's account of the connection of the nerves is as follows : " This nerve com posed of portio dura and the branch of the fifth pair sends off, in the adult, the chorda tympani before its exit from the skull, and in the fictus, immediately after. The termination of the branch called chorda tympani I shall not de scribe, yet I am almost certain it is not a branch of the seventh pair of nerves, but the last-described branch from the fifth pair," i.e.
the Vidian, " for I think I have been able to separate this branch from the portio dura, and have found it lead to the chorda tympani ; per haps is continued into it; but this is a point very difficult to determine, as the portio dura is a compact nerve, and not so fasciculated as some others are."* According to Arnold, nei ther of the previous opinions is correct ; but the petrous nerve anastomoses with filaments of the facial nerve, principally the external, with which it forms a gangliform swelling at the place at which the nerve receives it ; and the branch which forms the corda tympani arises from the gangliform swelling of the facial nerve, and holds in an intimate manner to the petrous nerve ; however it is not to be consi dered a continuation of the latter : it is united, during its course, to the facial nerve by several filaments, and consequently the chorda tympani ought to be regarded neither as a branch of the facial nerve nor as a continuation of the petrous nerve, but as one composed of both.-f Cru veilhiert. maintains that the chorda tympani is not a prolongation of the Vidian nerve, but he assigns no reason for his opinion. The ques tion at issue probably cannot be decided from the human subject : the impediment opposed to its satisfactory determination by the density of the facial nerve, as admitted by Hunter, and by the manner in which the facial and the Vidian nerves are in it blended together at their junction, will hardly permit the point being accurately ascertained ; but the same diffi culty does not exist in other animals, and if the disposition of the Vidian nerve at its junction with the facial be examined, in the horse e. g., no doubt will remain that, 1. the Vidian nerve certainly does not run simply in apposition with the facial nerve, and, 2. the chorda tympani is certainly not a mere conti nuation of the Vidian nerve. In the horse the facial nerve is much less dense, and more easily analyzed than in man, and at the point of junc tion with the Vidian its filaments are so free and so loosely connected, that little more is re quired than to open the packet without violence in order to display satisfactorily the disposition of the Vidian at its junction with the facial : the Vidian passes into the Interior of the packet, crossing its fasciculi nearly at right angles, hut rather in a reflex direction, and then spreads out and breaks up into a number of very delicate fila ments with which cineritious matter is inter mixed, and thus a ganglionic structure is pro duced, which is in some instances more mani fest than in others, and is at the same time connected with fasciculi of the facial nerve. The filaments into which the Vidian separates can be followed in both directions, some re trograde, and some along with the fitcial : the former appear to pass partly to the auditory nerve, as stated by Arnold, and partly to the facial between the point at which the Vidian joins it and the brain : they can be rent from the one into the other, and indeed look more like filaments from the facial to the Vidian than from the latter to the former. The latter fila ments of the Vidian are dispersed among the fasciculi of the facial, with which they become united, and can be followed by means of a careful dissection for some distance : their number the writer is not prepared to state : the fascieulus of the facial from which the chorda tympani more particularly arises, appears deci dedly to receive one or it may be more. Fur ther, the chorda tympani does not arise by a single root, but is formed by two or three de rived from different parts of the facial. The opinion that the chorda tympani is a continu ation of the Vidian nerve appears, therefore, to the writer altogether unfounded, and while he admits that the conclusion of Arnold may proba bly be well-founded,with regard to its compound nature, he yet must dissent from the opinion that the branch which forms it arises immediately from the gangliform swelling of the facial • the faseiculus, from which its principal root pro ceeds, existing distinctly upon both sides of, and consequently not arising from the swelling, however it may receive an accession from, or be affected by its connection with this part. The author cannot refrain from regarding the chord as a branch of the facial nerve in the same sense with any other branch arising within the limits of the influence of the Vidian nerve. Magendie maintains that the chord is a continuation of the Vidian, because the section of the fifth nerve itself deprives the ear of all sensibility, but whatever part the chord may play in the sensi bility of the ear, and it is doubtful that it plays any, the result of the experiment will be easily explained by the doctrine of Eschricht, that the facial nerve owes its sensibility to the fifth nerve, the division of which must in such case influence through the Vidian nerve any branch of the facial arising within the range of its in fluence.