The place at which the nerve is attached to the surface of the brain in the human subject is to be regarded only as the point at which it enters or emerges from the substance of the organ, inasmuch as it can be, without difficulty, followed to a much deeper part, and the fibres of the eras, which are transverse to those of the nerve, manifestly separate from each other, at the entrance of the nerve, to allow it a passage. The larger packet of the nerve is that whose course into the brain can be most easily traced; this circumstance depends partly upon the greater size of the packet, and partly upon the fact that, for the most part, its tracts are not separated from each other by those of the eras, but traverse that part in a body, the fibres of the crus seeming to be simply laid in apposi tion with it, and connected to it by some deli cate medium; while those of the lesser are, in the greater number of instances, separated from each other, or even interlaced with those of the emus; hence the fibres of the erns may be easily raised, without injury to the nerve, from the larger packet, and its course be displayed, while the lesser cannot be followed but with difficulty. The larger is, however, subject to variety in the latter respect ; in many instances the fasciculi 'of the crus do traverse and divide it, and very frequently near its ultimate attach ment, and this circumstance, when it occurs, renders the pursuit of its course more difficult ; but even here the fasciculus merely traverses it, and its tracts are not permanently separated, but reunite after the fasciculus has passed. The course of the packet may be exposed to a considerable extent even in the recent brain ; but for the satisfactory determination of the point, it is necessary that the brain be prepared by some of the methods recommended for that purpose, of which immersion in strong spirit is by far the best, nor does it require much time, for the substance will be found to separate more easily when it has acquired only a certain degree of firmness, than when hardened to the degree which long immersion produces; the plan which the author has found most success ful has been to commence the dissection early, to return to it frequently, and at each time to pursue it so far and so far only as it was satis factory. The course of the larger packet is also beneath and before that of the lesser, and hence, in the usual mode of dissection,in which the brain is reversed, it presents itself first. Its direction is backward, downward, and in ward, toward the upper extremity of the spinal bulb; in its course the packet first traverses the middle crus of the cerebellum from its an terior toward its posterior surface, and from its superior toward its inferior margin; it pursues this course until it has reached the back of the erns, and descended so low as its inferior mar it is then situate in the angle formed by the three peduncles of the cerebellum at their junction with the hemisphere; behind the middle, beneath the superior and above the in ferior, and before, or in common language, be neath the floor of the fourth ventricle. Thus far the course of the nerve may be ascertained without much difficulty; it is probably the same point to which Santorini had traced it, as described in his ' Observationes Anatomicw; in 1724, and from which Seemmerring has more expressly stated it to be derived, in his work De corporis humani fabrica; pub lished 1798, in which he states " that it ap pears to arise almost from the very floor of the fourth ventricle."* At the point last described the greater packet is attached to the side of the medulla oblongata. The point of attachment is very close to the interior of the fourth ven tricle, being separated from it only by a thin lamina, which is little, if any thing, more than the " epithelium" of Reil : it is situate in the angle formed by the peduncles of the cerebel lum, behind the middle one, by the outer margin of the pons, and posterior to it, and above its lower one : it is also superior to the attachment of the auditory nerve, separated from it by au interval of some lines.
NVe shall, in the next place, direct attention to the course and connection of the lesser packet of the nerve.
In none of the authorities which the author has had an opportunity of consulting, has he found a particular origin assigned to the lesser packet. By most anatomical writers it is over
looked; J. F. Meckel states that it can be traced a certain way into the crus, but he goes no further; Mayo asserts that the lesser portion arises close upon the greater, and, in a sketch of the origins of the nerves given by him in his Physiology, it is represented traversing the crus cerebelli, as a single fasciculus, above and behind the greater, and attached to some part above that from which the greater is re presented to arise: but still the origin is not defined, aed it is manifestly intended to be distinct from that of the greater packet.
The author has succeeded, as it appears to him, satisfactorily in tracing both the roots of the lesser packet to a destination for which he was not prepared; at setting out he expected to have found the origin of the lesser different from that of the greater packet, and to have followed it to a prolongation of the anterior columns of the spinal cord, as has been stated by Harrison ;* it was therefore with surprise that, after a patient dissection, he succeeded in tracing both its roots to the same point, to which the greater packet is attached, behind the middle crus of the cerebellum (seefig. 140, 12); both the roots traverse the crus, as the greater does, the inferior very frequently in company with and internal to the greater packet, or separated from it by a very thin stratum of the substance of the crus, the superior near to the superior surface of that part, and separated from the greater packet by an interposed stratum of two or more lines; the course of the latter is so near to the surface of the crus, that it can frequently be traced for a considerable way by the eye without dissec tion : they present, in their mode of traversing the crus, two remarkable varieties; in some in stances the fascienli, of which they are com posed, are separated from each other and even interlaced with those of the crus, and in such the pursuit of them is intricate and difficult ; in others they pass in two distinct packets, and in these they are more easily followed. As they proceed they approach the greater packet, so that the interval between them and it gradu ally diminishes, and having traversed the crus, they are both attached below and behind it to the same part as the greater packet, and poste rior to it. (Seefig. 140). This view of the con nection of the lesser packet, if confirmed, must lead to interesting results with regard to the rela tions of the two portions of the fifth nerve at least; it will at all events decide the question as yet in dispute, whether they are to be regarded as distinct nerves, or parts of the same ; upon this point further light will be thrown by the disposition of the same part in fish, in which the source of the uncertainty prevailing with regard to the nerve in the higher classes does not exist to the same amount ; inasmuch as the ganglionic and non-ganglionic divisions of the nerve seem for the greater part associated in their distribution.
When the adjoining matter has been care fully cleared away from the part to which the packets of the nerve are attached, that part ap pears to be a longitudinal tract of a yellowish white colour, composed of fibres running in the same direction, and capable of being followed both upward and downward : upward this tract seems continued beneath the superior peduncle of the cerebellum;* downward it descends from behind the pons into the spinal bulb, and after a short course divides into two cords, one fur each column of the spinal marrow (see figs. 140, 141). At the entrance of the tract into the bulb it is situate deep, before the floor of the fourth ventricle and behind the superficial attachment of the two portions of the seventh pair, which must be separated from each other and displaced in order that it may be ex posed : externally the tract corresponds to the peduncles of the cerebellum, and is united in ternally to the cineritious matter of the floor of the ventricle. At the point of attachment the tract presents a somewhat prominent en largement, (figs. 140, 141, 12,) which the au thor will venture to call an eminence, though with hesitation, lest it be considered an ex aggeration, from which the nerve may be held to arise.