The trunks rest partly against the outer side of the cavernous sinus and in part upon the base of the cranium in its middle fossa, and they are enclosed in offsets from the fibrous chamber, in which the ganglion is contained. Their relative position corresponds to the posi tion of the ganglion; the first is superior and internal to the other two, the second is inferior and external to the first, and the third is exter nal, posterior, and inferior to both the others. They go off from the ganglion at different inclinations, the first forward and slightly upward, the second directly forward, and the third almost directly downward ; hence the first and second form a very acute angle with each other, while that between the second and third is much greater.
First or ophthalmic diviston.—This division is distributed to the eye and its appendages, to the nostril, and to the forehead. It is the smallest of the three trunks proceeding from the ganglion, and is situate superior and inter nal to the other two. It is about three-fourths of an inch long from the ganglion to its division into hranches, and is contained thus far within the cranium. Its course is forward, upward, and slightly outward toward the upper part of the foramen lacerum of the orbit. It is laid against the outer side of the cavernous sinus, in company with the third and fourth nerves, and is contained in the external wall of the sinus, being separated from the interior of that chamber by a thin septum, which is a prolon gation of the inferior internal wall of the canal in which the nerve and ganglion are contained. The septum is dense, but at the same time so thin and transparent that the nerve can be seen through it from the side of the sinus, while the lamina of the dura miter, by which it is sepa rated from the interior of the cranium, is so thick and opaque, that the course of the nerve is altogether concealed from that side. At its outset the nerve is beneath, and external to the third and fourth nerves, and external and some what superior to the sixth, which is within the sinus; but ascending as it proceeds, it gains, about the middle of the sinus, the same level with the third, placed still at its outer side, and inferior to the fourth, and then terminates by dividing into branches.
Presently after its origin from the ganglion the nerve is joined by one or more very fine filaments from the sympathetic : this is ex pressly denied by the first Meckel, but he was certainly mistaken ; they are very faithfully represented by Arnold. In order to display
them the sixth nerve may be separated carefully from the carotid artery in the cavernous sinus, after which it will be found that branches of the sympathetic ascend upon the artery internal to that nerve, and distinct from those which are connected with it. having surmounted it they branch off, some upon the artery as it passes to the brain, others to other destinations, and of the latter some incline outward above the sixth nerve and are connected to the first division of the fifth: they are short and very delicate.
The first division of the fifth gives off no branch from its outset to its final division, except an extraordinary filament described by Arnold, and denominated by him the recurrent branch of the first division of the fifth. It arises from the upper side of the trunk immediately after it leaves the ganglion, runs backward above this body at a very acute angle, enters the struc ture of the tentorium cerebelli, and divides be tween its laminae into several very delicate fila ments.
The branches into which the first division of the fifth ultimately divides are either two or three; according to the elder Meckel and the greater number of authorities they are three; according to others they are sometimes three, but are more frequently only two. The three branches are the frontal, the nasal, and the lachrymal. When the branches are but two, they are, according to J. F. Meckel, the nasal and the frontal, the latter in such case giving off that, which in the other mode of distribution is the third, the lachrymal. The elder Meckel attributes the difference of opinion which prevails with re gard to this point to the fact that the lachrymal nerve frequently has a second root derived from the frontal, which in such cases has been assumed to be the origin of the nerve. The names which have been applied to those branches have been taken either from their destination or from their relative course; thus the frontal, so called from its distribution to the forehead, is also called the superior or middle branch ; the nasal, so called because finally distributed to the nostril, the internal or inferior, and the lachrymal, which derives its name from the lachrymal gland, the external. The three branches differ in size; the frontal is considerably larger than either of the others, the nasal is second, and the lachrymal is much the smallest. They all three traverse the orbit, but they pursue different routes, and have, at entering, very different relations.