The fascicular and filamentous disposition which has been described, is not, however, presented by the encephalic portion of the nerve through its entire extent, but only in that part of it which is superficial to the brain ; nor is it acquired by it until after it has emerged one or two lines from the substance of the organ, and then it does not assume it through-. out at once, but at first superficially and later internally. The appearance of distinct fila ments and fasciculi in one part and their ab sence in the other appears owing to the exist ence of neurilema in the former, for in one as in the other the nervous matter appears to be arranged in longitudinal tracts, which pre sent in one case the form of expansions, and in the other are divided by the neurilema into separate curds ; and again the occurrence of the filamentous disposition earlier upon the surface than internally, is attributed to the superficial substance of the nerve being pro vided with neurilema sooner than the inter nal ; hence the length of the substance of the nerve without neurilema is greater internally than externally, and when the nerve has been pulled away from its attachment to the brain, the rupture occurring at the point at which the neurilema commences, the part which is left projects in the middle, and presents a conical eminence of white matter: this, as Cloquet justly remarks, is but an incidental appearance, and not entitled to be considered, as it was by Bichat,* a real tubercle, from which the nerve arose. In neither packet are the fasciculi laid simply in apposition ; in both, but more remarkably in the larger, they are connected by frequent interchanges of filaments, and that to such a degree that the nerve when opened out appears to form an inextricable plexus, in which it is not improbable that every filament of it is connected directly or indirectly with all the others; this plexiform arrangement diminishes as the nerve approaches the gan glion, before reaching which the fasciculi be come more distinct.
The fifth nerve is attached to the surface of the brain on either side of the pons Varolii, at a distance of three-fourths of an inch from its middle line. It is attached to the middle crus of the cerebellum, on its anterior inferior surface, about one-fourth of an inch from its superior, and half an inch from its inferior margin.
The place of the attachment of the nerve to the exterior of the brain varies greatly in dif ferent classes of animals ; in man, it is, as has been mentioned, the emus cerebelli on either side of the pons; in the other orders of the Maminalia it is either, as in the human sub ject, the crus cerebelli, or, when the pons is less developed than in man, the nerve is at tached behind that part between it and the trapezium of the medulla oblongata; in the other three classes of vertebrate animals, in which the pons and trapezium are both want ing, the nerve is uniformly attached to the la teral parts of the spinal bulb. This contrast is equally curious and important; it affords us a natural analysis, which will throw much light on the next step in our inquiry, viz. the origin of the nerve, or its ultimate connexion with the brain. It furnishes also, as has been sug gested by Gall and Spurzheim,t an explana tion of the complication which exists in the human being, in whom the great developement and the situation of the pons render it neces sary that the nerve should traverse it, in order to reach the surface of the brain.
At the attachment of the nerve to the crus cerehelli in the human subject, the non-gan glionic portion or lesser packet is situate above and to the inner side of the greater. At that
place it is separated or separable into two parts, while the greater continues undivided, and hence the nerve is described as having three roots, one for the greater and two fur the lesser packet. The existence of two roots for the lesser packet had been announced by Santorini,/ but they have been more parti cularly and accurately described by PaHotta." They arc distinguished by the latter into supe rior and inferior, being attached to the erns cerebelli, one above and behind the other, and they are frequently separated from each other at their attachment by an interval of one or two lines or more. In such case the superior root is superior and parallel to the inner side of the greater packet, while the inferior is in ternal to it, and, it may be, on a level with its inferior surface; hence, in such instances, the greater packet corresponds to the interval be tween the roots of the lesser, and the inferior root of the lesser, in its course from the brain, is placed at first along the inner side of the greater packet, while the superior descends internal to the greater packet, and joins the inferior beneath it to constitute the lesser packet. This is not, however, uniformly the relation of the roots of the nerve at their at tachment to the crus, for the distance at which they are placed from each other varies very much; in some instances the roots of the lesser packet are perfectly distinct and separated by the interval mentioned, the inferior being either in immediate contact with the greater packet, and even entering the erns through the same aperture, or being separated from it by an interval varying, according to J. F. Neckel, from a quarter of a line to a line; while in others the roots of the lesser packet are not manifestly distinct, but the fascieuli of which they consist are attached to the crus in an un interrupted series reaching, from the attachment of the greater packet, to within a line or less of the posterior face of the eras, and separated the one from the other by trifling intervals; in the latter case the lesser packet is, for the most part, altogether superior to the greater at their attachment. But even in this the lesser is still distinguishable into two sets of fasciculi, which take different routes through the substance of the crus, one traversing it nearer to its ante rior, the other to its posterior surface. It has been already stated that the lesser packet of the nerve is characterized by the absence of a ganglion; it also has no connexion with the ganglion of the larger packet, but passes it without entering into it, and then becomes attached to one of the trunks proceeding from it ; it is further maintained to be distributed ultimately into those branches which are given by the third division of the fifth to the muscles of mastication. Panetta-I- concluded from these circumstances that it was a nerve distinct from the remainder of the fifth ; and observing that the superior root was principally consumed in the temporal muscle, and the inferior in the buccinator, forming the long huccal nerve, he tailed the former the " crotaphitic," and the latter the " buccinator" nerves. The distri bution of the lesser packet to the muscles of mastication has been confirmed by Mayo! from the dissection of the nerve in the ass. Ile differs, however, from l'alletta with regard to its distribution to the buccinator, which he denies: this point will come under considera tion again. It has been proposed by Eschricht* to denominate it the masticatory nerve.