The passage of the nerve over the margin of the petrous bone is marked by an inter ruption in the sharp edge, which the bone presents external to that point, and its site upon its anterior surface, as also that of the ganglion by a corresponding shallow depres sion.
Throughout the course of this portion of the nerve, the relation of the two packets to each other varies ; at the attachment of the nerve to the crus cerebelli, the smaller packet, allowance being made for those varieties pre sented by it in its mode of attachment, is superior and internal to the larger; in the in terval between the emus and the margin of the petrous bone, the smaller packet gradually descends along the inner side of the larger, until it has reached the same level, so that the two packets are placed immediately side by side upon the margin of the bone, the lesser internal to the greater ; but as the nerve pro ceeds into the middle fossa, the smaller, at the same time, passes from within outward beneath the larger, and also beneath the gan glion, toward its outer and posterior extremity; during this course it has no communication with the ganglion, but is quite distinct from it, though inclosed in common in the chamber formed by the dura mater, and connected with it by a dense cellular or fibrous structure; but having thus passed the ganglion, the lesser packet is united to the third trunk proceeding from that body, and with it constitutes the third division of the nerve.
The larger packet, on the contrary, is at tached to the ganglion. It has been before stated that the plexiform arrangement, which it presents, becomes less, as it approaches that body; its fasciculi become more distinct; they separate from each other, so that the width of the packet is greatly increased, and having reached the posterior margin of the ganglion they are received into the channel which it presents; in which they are ranged, in series, from one extremity of the body to the other, overlapped by its edges, and enter abruptly into the substance of the ganglion.
External portion of the nerve.—The external or peripheric portion of the nerve consists of three large trunks or divisions, which are connected, on the one hand by their ramifications, with tho organs to which the nerve is distributed, and on the other, with the ganglion and the brain. They are distributed, generally speaking, to three different regions of the head and face, one to the uppermost, another to the middle or superior maxillary, and the third to the lowest or inferior maxillary regions, and they are denominated, either numerically, first, second, and third, as by the first Meckel ; or, according to the parts to which they are dis tributed, the first the ophthalmic, by Willis; the second the superior maxillary, and the third the inferior maxillary, by Winslow.
These methods of distinction have their several advantages. Could we select names which would give adequate ideas of the distribution of the trunks, the latter would certainly be preferable; but inasmuch as those which have been selected do not at all adequately express that distribution, and are attended, therefore, with the inconvenience of not giving a suffi ciently enlarged idea thereof, it would probably have been better, had the former been from the first adopted and adhered to, for such names could not create any incorrect impression with regard to the distribution of the several divisions of the nerve; in fact, the epithets ophthalmic, superior, and inferior maxillaries ought to be altogether discarded, for, beside the objection to their use already stated, it will be found, upon reference to the anatomy of other animals, that they are by no means dis tinctly appropriate, and that the circumstances upon which they are founded are purely inci dental, associated with the peculiarities of the animal; for the proof of which, see the com parative disposition of the fifth nerve in the several classes.
The three trunks differ from each other in size. The first, the ophthalmic, is the smallest; the second, the superior maxillary, is inter mediate in size; and the third, the inferior maxillary, is by much the largest. They are connected to the anterior convex margin of the ganglion,—the first to its superior internal extremity, the second to its middle, and the third to its inferior external extremity. At their attachment they are wide, flattened, and of a cineritious tint; but as they proceed they become contracted in width, cylindrical or oval in form, and of a white colour. Their texture is fascicular and compact, the fasciculi of which they are composed being bound up closely together, and they differ remarkably in com position, the two first, the ophthalmic and superior maxillary, being derived altogether from the ganglion, and thus being, in anato mical constitution, simple; whereas the third is composed of two parts, one derived from the ganglion, and another formed by the lesser packet of the nerve, which does not join that body, and hence that division is compound.