As regards these folds, we niay point out, that their usual situation corresponds to the most prominent parts of those three curves of the rectum which we have already alluded to. Thus the third answers to that convex mucous surface which marks the transition of the sigmoid flexure of the colon into the rectum ; the second indicates the spot where the bowel reaches the median line of the sacrum ; and the first is nearly op posite to its bend in the hollow of the latter bone and the coccyx. And frequent as their presence undoubtedly is — and important as they therefore are with respect to the surgery of the rectum, — it may still be doubted whether they possess those characteristic anatomical features that would alone entitle them to rank as true permanent folds, like the transverse or falciform septa which isolate the several pouches of the large intestine. For, unlike these, they are not only somewhat irre gular in number and position, but are effitced by complete distention of the tube. And, finally, they appear to contain not a trace of the proper transverse stratum of unstriped fibres. Hence they probably express a mere passive arrangement of the loose mucous membrane; — a relaxation which is perhaps chiefly due to contraction of the powerful longitudinal layer of the muscular coat of the bowel.
In the rectum, the muscular lamina of the mucous membrane resumes its usual thick ness. At the lower part of the bowel, the skin and mucous membrane become con tinuous with each other. But, as might be expected from the great dissimilarity of these structures, there is a distinct line of d eraarcation between the two. Their junction is situate, not exactly at the anus, but at a point from two to four or five lines above this aperture. Here the skin terminates by a wavy margin, having a distant resemblance with that dentate edge, by which the thick white epithelium of the cesophagus adjoins the delicate pink mu cous membrane of the stomach. And the apex of each of these waves usually corre sponds to the starting point of a long,itudinal fold of mucous membrane ; which, after pro ceeding a short distance up the bowel, either becomes indistinct and disappears, or is crossed and effaced by others that take a different direction. It is nearly in this situation that Kohlrausch* describes a thin layer of un stripe(' muscular fibres, lying between the sphincter ani internus and the mucous mem brane. Traced upwards from their intimate union with the latter structure, these fibres are seen to take a longitudinal course; and to end, about one and a half inches above the aperture of the anus, by joining the lay er of circular fibres immediately external to them. Regarding these latter as their origin, it is evident that their action would raise the mu cous membrane, and oppose its prolapse.
Hence they are described as forming a " sus tentator tuniere nnteosee." Below its junction with the mucous membrane, the moist skin possesses its ordinary structure. And around the anus, it is occupied by numerous hair bulbs ; as well as by sebaceous follicles, which pour forth a large quantity of a peculiar odo rous secretion.
The contents of the lam intestine are of two kinds. The first is a mass which, usually of a semifluid consistence, ranges from the state of a thin liquid to that of a hard friable solid. This mass, when evacuated from the rectum, constitutes the faeces, ordure, or ex crement. The second is an elastic or ga seous fluid, which occupies the intestine in very variable amount, and unless its quantity be excessive, is not necessarily or regularly expelled at all.
Feeces. It will be some clue to the compo sition of the fmces if we recollect, that the large intestine so far resembles the small, as to justify our inferring that it continues the various metamorphoses which the contents of the canal begin to undergo in its upper segments. These metamorphoses are due, partly to a spontaneous decomposition of the alimentary substances themselves, partly to changes set up by the various secretions mixed with them. And they are accom panied by processes of absorption and se cretion, which may probably be regarded as in some degree peculiar to this segment of the tube. Of these two processes, that of absorption seems chiefly destined to de prive the intestinal contents of their more watery and soluble parts. While the act of secretion pours forth fluids which, from their proximity to the end of the bowel, may be assumed to be, in great extent, excremen titious. The matters thus excreted may be divided, histologically, into two chief con stituents : — a structureless alkaline fluid which is furnished by the tubes ; and a scaly epithelium, which is a desquamation from the mucous membrane of the rectum.
But it would be wrong to suppose that the whole of the processes which engage the contents of the large intestine can be comprehended in three such acts of meta morphosis, absorption, and secretion as those just alluded to. On the contrary, each of these three exerts its usual complex reaction upon the other two. Thus the soluble re sults of metamorphosis undergo absorption, as do also sonie of the substances secreted. The fluids secreted into the bowel no doubt modify the spontaneous changes which en gage its contents. And, finally, the slow transit of these contents along the intestine is accompanied by the precipitation of in soluble matters from the various secretions of the upper segments of the canal, prior to their expulsion from its lower orifice.