1. The frontal nerve appears in the human subject, both from its size and its direction, to be the continuation of the original trunk. In other animals, however, it is otherwise: in them the predominance of the frontal nerve diminishes along with that of the superior region of the face, until in some it ceases to exist as a pri mary branch of the first division of the fifth, and its place is supplied by a secondary branch of another, while the nasal branch increases in the same proportion, and seems ultimately to constitute itself the first division of the fifth' The frontal nerve passes upward and forward toward the highest part of the foramen lacerum. of the orbit, and enters that region through it. It then continues its course through the orbit to the superciliary foramen and escapes through it to the forehead. During this course it is placed, before it has entered the orbit, at the outer side of the third nerve; it then rises above the third and crosses over it to its inner side. In doing so it is accompanied by the fourth nerve, to which it is external and in ferior; it enters the orbit in company with the fourth and nearly on the same level, but still external to and somewhat beneath it. In entering, it passes above the origin of the superior rectus muscle, and all the other parts transmitted through the foramen lacerum, with the exception of the fourth nerve. At the en trance of the frontal nerve into the orbit and during its course from its origin thereto it is closely attached to the fourth nerve, but pre sently after separates from it, the fourth in clining inward, is continued forward to the superciliary foramen, lying upon the superior surface of the superior rectus and levator palpe brx muscles, being through its whole course within the orbit immediately beneath its roof. having reached the foramen it passes through it, and changing its direction, ascends round the superciliary arch, upon the forehead, be neath the orbicularis palpebrarum and frontalis muscles, and is thenceforth called by some the external frontal nerve in contradistinction to a branch from itself, the su pra-trochlear, or internal frontal. In its mode of escape from the orbit the frontal nerve is subject to some variety, consequent in part upon the mode in which the superciliary foramen is formed, that being in some instances altogether osseous, in others osseous only at its superior part and completed by ligament below; in this case the nerve escapes through an osseous notch, and not a foramen. In other instances, again, when the nerve divides previous to its escape it is some times transmitted through two apertures.
The distribution of the frontal nerve, as well as that of most of the secondary branches, is subject to varieties, which the author has en deavoured to embrace in the following account. In the first place the frontal, at its entrance into the orbit, anastotnoses with the fourth nerve. Next it gives off, some time after its entrance and previous to its division, a long and slender branch, which runs forward and inward toward the trochlea of the superior oblique. Then it divides into two branches, a larger one, the continuation of the nerve, which escapes through the superciliary foramen, and a smaller, the supra-trochlear or internal frontal. The latter passes forward and at the same time inward toward the trochlea of the oblique muscle, escapes from the orbit internal to the continued trunk of the frontal nerve, and ascending upon the forehead beneath the corrugator supercilii, orbicularis, and frontons muscles, it has received the name of internal frontal, in contradistinction to the continued trunk, which is at the same time called external frontal. The point at which the frontal divides
is variable; for the most part the division takes place about midway in the orbit. In some instances it occurs before the nerve has reached that point, and in others, again, not until it has approached nearer to the anterior margin of the orbit. The distance of the division from the margin of the orbit appears to modify the course of the internal branch : when it is far back, the nerve escapes from the orbit above the trochlea, and hence the name supra-tro chlear, given to it by Meckel ; and when near the margin it escapes external to the trochlea, be tween it and the superciliary foramen ; while in the latter case a branch of the nerve is transmitted above the trochlea, in the usual course of the nerve itself. Nor is the size of the two branches into which the frontal divides equal or uni form ; for the most part the external branch is the larger, but in some instances the two are of equal size. In its course forward the supra trochlear nerve gives off first, occasionally a delicate branch, which frequently arises from the frontal itself prior to its division, the course and destination of which have been already described. Next it gives off, in some instances before, in others not till atter it has escaped from the orbit, a branch which passes inward toward the internal canthus, and, uniting with either the infra-trochlear itself or a branch of it, concurs in forming a small plexus, from which filaments are distributed to the structures of the upper eyelid, toward its internal part, and to the eyebrow. Ilaving escaped from the orbit, the supra-trochlear nerve divides into two sets of branches, denominated palpebral ; the first descend into the superior eyelid, and are distributed to the structures of that part; the filaments communicating exter nally with those of the frontal, and internally with those of the infra-trochlear. The frontal branches ascend round the superciliary arch, beneath the orbicularis palpebrarum and the corrugator supercilii muscles, upon the fore head, and these are disposed of in a manner similar to that in which the branches of the proper or external frontal are. Some are dis tributed to the orbicularis, corrugator, and fron talis muscles; other, long branches, ascend beneath the frontalis, traverse it, and become subcutaneous, and are distributed to the inte guments of the scalp upon the forehead. Of these the external unites with the internal branch of the external frontal, and forms with it a common branch, which has the same destination as the others.
The external larger branch of the frontal, called, in contrast with the last, the external frontal nerve, also divides into two sets of branches, palpebral and frontal.
The nerve in some instances emerges from the orbit a single trunk, in others it divides be fore it escapes from that region, for the most part into two branches, which are transmitted sometimes through the same, at others through distinct apertures, and from which the several ramifications arise, they themselves becoming ultimately the long frontal branches.