a. Cutis.—The sub-basement areolar tissue of the tongue exists in sufficient quantity and density to deserve the name of a true chorion or cutis. It is thickest at the upper surface, where it underlays the espe cially towards the median line; its density too is the greatest here, sometimes amounting to almost a cartilaginous hardness, and the pro portion of white fibrous tissue to the yellow is the greatest; it is thinnest on the under surface and edges, where it contains more of the elastic element, especially in the neigh bourhood of the epiglottis ; at the line where the attached and free portions of the tongue meet, it gradually merges off into the loose elastic web that underlays the mucous surface in these situations. Its inner surface receives the insertion of all the intrinsic muscles of the tongue, among which for a short distance it dips, and it sends processes into the folds that attach the tongue to neighbouring parts, as the glosso-epiglottidean and frmnum. It is the medium in which the nerves and vessels destined to the surface break up previous to their ultimate distribution: the vascular rami fications form a plane network, coincident with the surface, from which, at regular intervals, the papillary vessels ascend.
b. The basement membrane is variously modified in the three situations above indi cated; it is either continued plane, projected into papillm, or folded into mucous crypts, from which its further involution constitutes the minute ducts and ultimate follicles of the mucous glands opening into these crypts: its description will be involved in the particular consideration of these structures.
c. Epithelium.—This very nearly approaches in character the cuticle of the skin, which it resembles in being of the scaly variety, in the amount to which it exists, and in its being divisible into two layers, a deep one closely adherent to the basement membrane, consist ing of more recent cells, retaining much of the cellular form (fig. 753., c.), and a superficial one, readily desquamating, the cells of which are older and flattened into scales (fig. 733., d.). It exists in very different quantity in different parts, being most abundant where it invests the papillary structures. The shape of the in dividual cells is very various ; where they are flattened their area is much extended (fig. 753., a.), and they look four or five times as large as the deep-seated ones ; but probably there is no increase in size, their lateral extension resulting from their greater thinness: viewed in profile they appear quite filamentary ; some of them are not flattened but elongated, so that they appear linear in all aspects, and are really so. In spite of all these modifications a small single circular nucleus may generally be seen very plainly occupying a central posi tion in the cell, and unchanged in form and size by any of the changes that the cell under goes. Certainly the epithelium, covering most parts of the tongue, does not contain any pigment ; but I think that covering the filiform papillm in the centre of the dorsum very frequently does ; and for these reason s,— because, first, the growth of the epithelium here is very abundant, and it seems a general rule that pigment should be associated with an abundantly nourished or rapidly developed epidermis, as is seen in hair, in the colour of those spots that are called moles, and the hair thatproceeds from them, which, instead of being invisible, as in neighbouring parts, is dark and rank, and in the change of colour which the cuticle and hair covering the parts of genera tion undergo at the increase of the nutrition of the parts that accompanies the accession of the generative function ; secondly, because we see that the epithelium does undergo great changes of colour, being generally dark est when most abundant; thirdly, because, in two cases that I have seen, in which the only diseased condition was an enormous develop ment of the filiform processes which the epi thelium forms in the centre of the tongue, the colour of the fur was a dark sepia or Vandyke brown, almost black, exactly that of pigment in other situations : we see too, when the epithelium has been rendered opaque by soaking the tongue in alcohol, that all the other papillm are whiter than the fill form occupying the centre of the dorsum, which retain their tawny colour. The epithe
lium of the tongue differs from the dermic cuticle chiefly in its moisture, and the delicacy and softness of its structure.
Papillary structure of the tongue.— The pa pillm of the tongue are generally contrasted with those of the skin, in that while the latter are covered over with an even layer of epi dermis, and therefore not visible to the naked eye, the latter stand out free from the surface of the epithelium, dipping down between them. But viewed by the light of recent researches, this is seen to be only the appa rent difference, the real difference being that while the papillm of the skin are sessile, the papillm of the tongue are arranged in groups on proper pedicles or supports, whose tops and sides they cover, and which elevate them above the general surface. For it has been recently shown by Professors Todd and Bow man #, that the papillte, heretofore considered simple, are really compound organs, and that they are covered by other smaller papilke, whose form and whose method of nervous and vascular supply, show their true analogy to the papillm of the skin. In physiological exactness, and to carry out the analogy insti tuted by these anatomists, we may say that these large papillm, so called, are no more true papillm, than the long processes in the intes tine of the rhinoceros are true villi. Now, these secondary, or true physiological papillm, are covered in by an even layer of epithelium, in just the same way as the papillm of the skin ; hence we see that the distinction gene rally laid down ceases. It would, however, be too great an innovation to reject the name of papillm for those organs that have so long possessed it, and as the value and office of their different parts will be implied in their description, no misconception can arise. I shall, therefore, continue the old nomen clature.