Now this difference in the arrangement of the epithelium on the different papillm in dicates, I think, a very important physiolo gical distinction ; in one case we see the sen tient papillm covered by a thin layer of a fine epithelium, thinner over them than in the in tervals between them ; in the other case we see each secondary papilla the seat of a rapid generation of epithelium that clothes the whole compound organ with a dense impene trable brush of hairs : this latter arrangement seems as inconsistent with the possession of sensibility as the former seems adapted to it, and the difference would suggest to me the division of the papillm into "sentient" and "protective ;" among the former I would class the circumvallate and the fungiform, among the latter the filiform and, with a certain qua lification, the conical : but I shall return to this presently in speaking of the functions of these papillm. The great difference between the filiform and conical forms is in the amount of the epithelium : in the filiform it is such as has just been described ; in the conical, the hairs are very short and thick, and terminate in an even plane almost as soon as they be come separate, so that they have not at all a filamentary character. This epithelium is being constantly generated and as constantly thrown off, as is shown by the variation in the quantity offer on the tongue from day to day. It is in these papillae that the separation into two layers is best seen (fig. 757. ef), and the appearances of this separation are such as to make me think that the method of desqua mation is not by the shedding of individual scales, but by the throwing off of the upper layer, the under layer taking its place, itself to become divided into a deep-seated and su perficial portion, and to be in turn thrown off as it grows older. The vascular supply of these papillm is very abundant, as might be expected from the rapid nutrition that is taking place from their surface. The pigmentary character of the epithelium, at least under some circum stances, has been already referred to.
The simple papillm are scattered beneath the apparently non-papillary surface behind the circumvallate, beneath the edges of the tongue, and on the under surface of its free portion, also among the conical and filiform ; in the former situation they resemble the secondary papillm of the circumvallate and fungiform, in the epithelium being continued smoothly over them ; in the latter, those of the filiform, each being the base of a hair like process. The situation of these pa pillm, their number and arrangement, is very well shown by injection, when the individual loops of capillaries may be seen passing up from the sub-mucous plexus at regular inter vals, one to each papilla. The method in which the conical papillm of the upper sur face pass into the simple of the under, is this : —the ridges on the side of the tongue, to which reference has already been made, con sist of fused conical papillae arranged in linear series vertical to the edge, and, like them, crowned with secondary papillm ; as they pass down the edge towards the under surface they become shallower, so that by the time they have reached that surface, the secondary pa instead of being elevated, are sessile, and immediately subtended by the cutis. This is the true nature of those vertical ridges on the edge of the tongue described by Bichat, Scemmering, and many other anatomists.
Structure of papilk.—The nature of the true papillary structure, i. e. the contents of the projection of basement membrane, seems to be partly fibrous, partly granular : cer tainly something like yellow fibrous tissue can be detected, particularly on the addition of acetic acid ; but those specimens that I have examined have appeared to be more granular than fibrous, and in some cases, where there has been a fortuitous rupture of the papilla, there has been an abundant escape of a finely granular material : the exact na ture of this granular material I have not been able to ascertain ; it is not fatty. The me
thod of termination of vessels is sufficiently conspicuous, and has been already indicated. With regard to the termination of nerves I must confess, that, after long-continued and careful search, I have been unable to find any thing I can construe into a looped termina tion. I have traced the nerve for some little way into the simple papilla, and then, its out line becoming less and less definite, it has ceased to be visible, apparently from the loss of the strong refraction of the while substance of Schwann. If I might hazard an opinion, I should say that the axis was probably de nuded of this investment, and, thus laid bare, underwent some specific peripheral modifi cation. In a paper by Dr. Waller, recently published in the" Philosophical Transactions of the Royal Society," a method of nervous termination by open mouths is suggested : con sidering the solid nature of the nerve-axis, this appears to be an opinion that requires either some modification, or the confirmation of re peated and varied observations, before it can be received as certain. At present our means of examining the termination of nerves, either in the organs of sense or elsewhere, are im perfect, and until they are less so we can hope to make but little advance in this, at present, obscure subject.
Functions of the different papillce.—Three offices are to be discharged by the papillm they are to exercise the sense of taste, the sense of touch, and to form a suitable cloth ing and protection to the tongue. The struc ture of the circumvallate and fungiform pa pillm, their shape, their position, the nature of their epithelium, all point them out as the organs that are to discharge the first-men tioned function, and their nervous supply, which may be demonstrated with facility, makes that probability a certainty. To detail the evidence on which this conclusion is based would merely be to repeat what the reader will find given at length under the 'articles FIFTH NERVE, EIGIITII NERVE, and TASTE, to which I therefore refer him. By what pa pillm,then, is the sense of touch exercised? I think by the conical, and by them in contradis tinction to their non-gustatory congeners, the fi/ybraz. My reasons are, first, that the sense of touch is only possessed in any perfection by the papillm at the tip of the tongue.* Now these are conical and of the form re moved from the filiform type, that is, their epithelium is the least abundant of all the non gustatory papillm. Secondly, because the struc ture of the filiform papillm is the most incompat ible that could be devised for the possession of a sense of touch in any perfection. And, thirdly, because we do find, in fact, that the centre of the dorsum of the tongue, where the filiform papillm exist in the greatest abundance, is the least sensitive part of any. Therefore, unless we assign the sense of touch to the gustatory papillm, which the lowness of its condition at the back of the tongue, where the gustatory papilla: are so abundantly developed, seems to negative, we are driven to the conical as the only ones that can possess it. The filiform papillm remain for the third office—that of clothing and protecting the exposed dorsum of the tongue ; their pilose investment ad mirably adapts them for this, and at the same time it imparts to the surface a certain pre hensile power, enabling it to take hold of and move readily what is placed on it, while the backward direction of the hairs adds yet more a special facility for transmitting food towards the pharynx.