If the part selected for examination be the stomach, the same precautions should be ob served, for here this membrane is, if possible, more delicate than below the pylorus, and the epithelial particles are often so large as to bulge the tubes very irregularly, especially towards their blind extremities. It is between these bulges that the basement membrane may be best seen (fig. 276).
Nothing is more difficult than to explore, in a satisfactory manner, the internal structure of the intestinal villi. Their thick epithelial caps and their abundant vascular rete are readily demonstrable, but the arrangement of the lac teals, which undoubtedly exist in them, and of the structure on which the epithelium imme diately rests, has hitherto almost entirely eluded our research. In vertical sections of the recent small intestine of the Car nivora, I have several times seen the direct continuity of the epithelium covering the villi, with that lining the tubes which open at their base; and also a distinct continuity between the inte rior of the villi, where the vessels are spread out, with the vascular intervals be tween the tubes, which con tain the fine capillary web surrounding the tubes, and likewise give passage to the branches to and from the villi. It is then difficult to avoid the belief that the basement membrane of the tubes is continued under the epithelium of the villi, to support it, and form a part of those remarkable projec tions. Nevertheless I have not been able to see it in an isolated and distinct form, and do not therefore assert its positive existence; only I believe that the fact that the injected vessels of a villus, when seen in profile at its margin under a high power, and when the epithelium has been removed, seldom come to the extreme edge, is attributahle to the circumstance of the basement membrane still investing them. This membrane, if it really exist here, adheres intimately to the parts within the villus.
It might seem at first sight a hopeless task to search in so dense and complicated a struc ture as the skin, for the analogue of a mem brane like that I have been describing, which, in no situation where it can be unequivocally brought into view, exceeds the 8000th of an inch in thickness. But as it must exist, if at
all, between the epidermis and the vessels and nerves of the cutis, in a position sufficiently determinate, much of the apparent difficulty is removed. The most favourable situations for its detection are those in which the skin is highly developed, presenting, like the small intestine, villi (termed papilla) on its free sur face. The close resemblance between these papilke and the villi of mucous membranes has been observed by many anatomists. The dis tribution of the vessels within both is essentially the same. Here, then, under the epidermic layer, we might expect to find the basement membrane. I have removed with great care the whole of the epidermis from a thin ver tical section of such a specimen (and it is better to have previously steeped it in solu tion of carbonate of potass), and have then examined the outline of the bare papilla with a power of 300 diameters. This outline is sharply defined, and appears to be formed by a homogeneous membrane, enclosing the vascular and nervous contents. This mem brane I believe to be that which I am now describing, though, as in the case of the intes tinal villi, 1 have never been able to isolate it, and thus unequivocally prove its presence. This is a part of the skin which has never been noticed by anatomists on account of its tenuity, but which is quite distinct from the cuticle, and the great eat mass of that complicated struc ture to which the terms cutis ' and dermis ' are applied.
A very strong reason for believing this mem brane to be present in the skin, is the fact of its existence in those minute organs, so profusely scattered under the cutaneous surface, the se baceous and sudoriferous glands. In fig. 277 I have represented it in a portion of one of the latter, taken from the axilla, where they are very large. These glands are nothing more than involutions of the external tegument, and correspond closely with the labial and allied glands connected with the ordinary mucous membranes. It is impossible to suppose that a structure attaining so marked a developement in those parts, should be wanting in the general superficies, with which they are, at numberless points, directly continuous.