Each nerve is composed of two portions, which are remarkable for particular characters, and have received distinct names; they differ from each other in size, in anatomical disposi tion, and in function ; one of them, larger than the other, is provided with a ganglion, and dif fers in its distribution ; it also differs in proper ties, being subservient to sensation ; the other is small, has no ganglion, and is destined to volition ; they are hence denominated, the former the larger, the ganglionic or the sentient portion, the latter the smaller, the non-ganglio nic or the voluntary portion.
The distinction of the nerve into two por tions appears to prevail uniformly throughout the animal series. According to Al. Serres, it is to be observed in all the classes of the ver tebrate animals except the Reptiles; but in them, according to him, the laterolfasciculi • are wanting. The latter assertion, however, is incorrect, the distinction being to be observed as satisfactorily in that class as in any other.t Again, the distinction is not equally remarkable in all ; in some it is still more so than in man ; in others it is less; and according to the same authority, it is to be observed among Mam malia the more easily as we pass from Man to the Rodentia. Among the Cetacea it is divi ded throughout into two separate faseiculi.j Each of the two portions of which the nerve consists is a packet containing numerous fas ciculi, which are again divisible into filaments. The fasciculi, of which the packets are com posed, are differently circumstanced in different stages of the course of the nerve; in one part they are bound up so closely together that they cannot without difficulty be separated from each other and disentangled, while in another they are but loosely connected and are easily sepa rated.
The two packets are associated together more or less intimately throughout their course; but inasmuch as they present remarkable varieties in their disposition and mutual relations at dif ferent parts, it may be advantageous to divide the nerve, through its course, into three por tions or stages; one from the ganglion to the connexion of the nerve with the brain, which may be denominated its internal or encephalic portion; a second from the ganglion to its ulti mate distribution, its external or peripheric portion ; and, thirdly, its ganglion. Such a distinction may not be free from objection, but being adopted for the convenience of descrip tion, it possesses at least the recommendation that there exist well-defined points of demar cation, whether there exist or not any difference in the of those several portions. The nerve, in its encephalic portion, is partly within and partly superficial to the substance of the brain. The superficial part is from one-half to three-fourths of an inch in length, of a flattened form, and of very considerable size. It presents a loose fascicular texture, and is enclosed within a prolongation of the arachnoid membrane sent off upon it from the surface of the brain; this prolongation is, as in the case of all those sent upon the vessels or nerves, in their passage from that organ to the parietes of the cranium, a cylindrical sheath, within which the nerve is enclosed ; it is at first remarkably loose, but as the nerve recedes from the brain, the membrane invests it more closely, and is continued upon it as far as the ganglion, from which it is reflected to the surface of the canal in which the nerve is contained. In the last particular the disposition of the membrane is subject to variety, for it is at times continued beneath the ganglion, and partially invests the trunks proceeding from this body before it is reflected to line the canal.
Throughout this part of the nerve the two packets composing it are connected by cellular structure and vessels, and are enclosed within the prolongation of arachnoid membrane just described; but there does not appear to be any interchange of nervous filaments between them, and they are connected so loosely that they can be separated from each other with great facility. They consist each of numerous fasciculi held together, like the packets themselves, so loosely that the latter can be easily opened out and decomposed. The fasciculi of both packets are irregular in size, some large, others small; those of the larger are for the most part some what smaller than those of the lesser, but they are much more numerous, amounting, accord ing to J. F. Meckel,* to thirty or forty; while those of the lesser amount, according to the same authority, only to from nine to fourteen. The fasciculi again are composed of numerous and delicate filaments. The number of the fila ments is very great, but differently estimated by different authorities; according to Meckel those of the greater packet amount to about one hundred, collected into thirty or forty fasciculi ; while, according to Cloquet,t the total number of filaments contained by both packets varies from seventy to one hundred, of which he allots five or six to the smaller, and the remainder to the larger packet. This difference of opinion Meckel explains by sup posing that fasciculi have been taken for fila ments and not decomposed, and this appears very probable, inasmuch as Cloquet takes no account of fasciculi, and in his description of the smaller packet it is manifest that lie has assumed the fasciculi, of which it is composed, to be filaments, for he does not attribute to it a greater number of filaments than it contains of fasciculi. But if Cloquet have underrated the filaments of the larger packet, Aleckel junior has certainly overrated the fasciculi of the smaller one. From his account of the latter, it is to be concluded that it contains from three to fourteen fasciculi, but either of those numbers is too great, as will be seen from an examination of the subject, from which it will appear that they do not exceed the number attributed to them by Cloquet. The ultimate number of filaments, however, would seem to be somewhat uncertain, for it appears to depend very much upon the delicacy with which the separation of them may be effected ; and after all it is not a matter of any great importance. According to Wrisberg. and Sceminerringi the number of fibres contained in the greater packet is always less in the foetus than in the adult. The filaments of the smaller are stated by Cloquet to be larger, softer, and whiter than those of the other; but with regard to the difference of size it is probable that this opinion has arisen also from his having assumed the fasciculi to be filaments, inas much as, when the fasciculi have been decom posed, the filaments seem to be equally fine in both packets ; and for the other points of sup posed difference the author has not been able satisfactorily to observe any in man. In other animals, however,—in some fish at least—a remarkable difference may be observed between the characters of the ganglionic and non-gan glionic portions, the latter of which, in the Cod, is much softer, and of a darker, not whiter, colour than the other.