In the Turtle the nerve can be traced in like manner from the point of attachment down ward into the lateral column ; and in Fish the attachment is in all essentials similar : the com parative smallness of the bulb and the direc tion which the nerve takes in its course out ward, make it resemble the spinal nerves more than in the other classes; but its encephalic connection is strictly the same, namely, te the lateral column of the bulb beneath the floor of the ventricle. In the Cod, after the removal of the floor of the ventricle from the back of the nerve, the latter may be followed for some way into the column, though neither to the same extent nor so satisfactorily as in the bird or the Turtle; and in the Ray, while the two inferior fasciculi of the nerve—for in this fish it consists originally of three—are connected in the usual mode to the lateral column, the superior is attached to a convolution formed by the floor, in consequence of a greater developement of its margin. In the Cod the convolution adverted to does not exist, but the floor of the ventricle cannot be raised from the nerve without destroy ing a connection of some kind between them. In the latter fish the fifth nerve is attached before and rather superior to the auditory nerve, and the two nerves are quite distinct as far as the point of attachment, but there they are in immediate apposition and appear to have the same source. In the Ray it is different; in it. the auditory seems merely a branch of the fifth (fig. 145, 7) given off from its posterior ganglionic fasciculus about three lines from its attachment to the spinal bulb, and before the formation of its ganglion.
After the preceding details it must seem extraordinary if the nerve in the higher mals differed, in its ultimate connection with the brain, so very much from that in the ferior, as it is represented by some to do.
Yet it is asserted by M. Serres,* who has founded his opinion upon the observations which he has made upon the successive dc velopement of the brain and nerves in the embryo of vertebrate animals, that in the Mammalia the nerve is implanted upon the trapezium. Such is the form of expression by which he intends, as the author understands, the ultimate connection of the nerve with the brain. Now, in the first place, we have al ready seen where that connection is in those animals in which the trapezium does not exist, and it appears to the author reasonable to con clude that similar nerves have similar or ana logous attachments in the several classes of animals, however the parts with which they are connected may be complicated or ob scured by superadded structures. In the second place the trapezium can be regarded only as a superadded structure, and is not among those parts from which nerves are likely to arise, being itself but a commissure : and, thirdly, the situation and connections of the part to which the nerve is attached, are altogether in compatible with the opinion that it is the tra pezium, inasmuch as the latter is situate be fore the cords, which ascend from the anterior columns of the spinal cord to the crura cerebri, while the structure with which the nerve is connected is posterior to them. For these reasons the author concludes that M. Serres has mistaken the place of the nerve's attach ment in the Mammalia.
In conclusion, the representation of the ori gin of the nerve, which appears to the writer to be the most remote of all from the real one, is that given by Swan, in his plates of the nerves lately published, in which the fifth is re flected into the auditory nerve : such a con nection is merely artificial and does not really exist ; it can be produced only by stopping short in the pursuit of the fifth nerve, and mould ing it into the anterior root of the auditory, which is in contact with it.
This view of its encephalic attachment has probably originated in the intimate connection known to exist between the two nerves in in ferior animals. The complication of the cere bral connection of the nerve in the higher animals may be now better understood. In those, in which the pons and trapezium do not exist, the nerve emerges directly from the spinal bulb, in a manner similar to the ad joining nerves ; but in those, in which the bodies alluded to are present, inasmuch as the attachment of the nerve is behind them, it can reach the surface only by either passing be tween them, or traversing their substance.
Hence, if the nerve simply traverse them, it ought not to receive any accession of fibres from them, and such, according to the writer's experience, is the case. As it emerges from the pons, the lesser packet receives an epithe hum from its surface; but he has not been able to detect any fibres originating within the substance of that part.
The structural arrangement, which the ence phalic portion of the nerve presents within the brain, is different from that, for which it is remarkable, while superficial to it. Exter nally it is, as has been stated, of a fascicular texture; but, within, that appearance is not to be observed : there the larger portion is a white, soft, homogeneous, flattened cord, the delicacy of which, in the natural state, forbids the separation of it into distinct parts; but when sufficiently hardened, it may be divided' into numerous thin strata, and these again into delicate fibrils. That such an arrangement is a natural, and not an artificial appearance, is manifest from the circumstance, that the sepa ration into fibrils can be effected only in one direction, the length of the nerve, and that they break off when it is attempted in the other. The nerve retains those characters as far as its attachment behind the crux, hut there they cease ; the pure white colour suddenly disappears ; the point of attachment and the cords descending from it present a cineritious tint ; and they are not absolutely distinct from the surrounding substance, as the nerve had previously been, but immersed in it ; they are, however, still manifestly composed of fila ments, which may be rent either toward or from the point of attachment ; and after im mersion in spirit they become nearly white. The course of the nerve, from its attachment to the surface of the brain, is forward and out ward toward the internal anterior extremity of the petrous portion of the temporal bone ; it next passes over the superior margin of that portion, and descends upon its anterior surface into the middle fossa of the base of the cra nium, where it reaches the Gasserian ganglion. During its short course, from its attachment to the brain, to the ganglion, it is at first contained within the proper cerebral cavity, by the side of the pons Varolii, and beneath the internal anterior angle of the tentorium cerebelli ; in the second place, in the middle fossa, it is not within the cerebral cavity of the cranium, but beneath it, separated fro n it by a lamina. of dura mater; it is there contained in a canal' or chamber, formed by a separation of the. dura mater into two layers, between which nerve and its ganglion are inclosed, one be neath them attached to the bone, another-above; separating them from the brain. This chamber is situate immediately external to, and lower than the cavernous sinus, but separated from it by the inferior lamina of the dura mater just described, which ascends from the bone to join the superior, and in so doing forms a. septum between the two chambers ; it is about three-fourths of an inch long, reaching from the superior margin of the petrous bone to the anterior margin of the depression upon its. anterior surface, in which the ganglion rests. In front this chamber is wide, containing at that part the ganglion, and•sends fibrous offsets upon the nervous trunks proceeding from it ; poste riorly it is narrow, and presents an oval aperture, about one-third of an inch long, situate ex ternal and inferior to the posterior clinoid pro cess of the sphenoid bone beneath the attach ment of the tentorium cerebelli to that proces, and also beneath the superior petrous sinus : by this aperture the chamber communicates with the cerebral cavity and the nerve enters. The chamber is lined by the arachnord mem brane, as far as the posterior margin of the gan glion, but along this the membrane is reflected from the interior of the chamber to the nerve, and returns upon it the cranium: hence the nerve is free within the chamber, while the dura mater is attached to the surfaces of the ganglion, and so closely that it requires care to separate it from them. The cham ber presents a remarkable variety in its con struction in some animals: in the horse, for instance, its parietes are not simply fibrous, as in man, but, frequently at least, in great part osseous, being at the same time lined by the membrane.