The mucous membrane of the ccecum differs in no essential respect from that of the re mainder of the large intestine, the structure of which is continued up to the very edge of the valve which severs it from the ileum.
The cmcum has three apertures:— one, a large opening by which its cavity is directly continuous with the colon ; a second, which communicates with the small intestine, and is guarded by a double valve ; and a third, which opens into the slender vermiform appendix.
Ileo-ccecal valve. —The opening into the ileum is situated at the upper border of the ccecum ; on its left side, and a little poste riorly. The structures which bound and define this opening are collectively termed the deo ccecal or the deo-colic valve :— although these names ought in strictness to be limited to those separate portions of the entire intestinal valve which their etymology would indicate.
The arrangement of the intestinal tunics in this valve is best seen by inflating and dry ing that part of the intestine, which includes, together with the last inch or two of ileum, the cmcum, and the commencement of the colon. On cutting out a piece of such a dried prepara tion, so as to gain a view of its interior, we see the valve as represented in the accom panying figure (fig. 275.) The small in testine, generally inclining slightly upwards as well as backwards, passes towards the cmcum, at what is thus a rather acute angle. In stead, however, of opening into the bottom of one of the sacculi of the cmcum, it selects for its entry the exact site of the deepest and most projecting of those transverse constric tions which project into the cavity of the large intestine. This constriction occupies the inner side of the bowel ; and is, as it were, split up by the entering ileum into two larninm ; — an upper and a lower, an ileo-colic and an ileo-cmcal (e,f,fig. 275.) While, at the same time, the hitherto cylindrical calibre of the small intestine is gradually reduced to a hori zontal slit or fissure, as it enters this fold.
Such a description at once explains the form of the valve: — how each of its seg ments, for example, constitutes a crescentic membrane, the plane of which meets that of its fellow at an acute angle, and the free edge of which is directed outwards ;— and how both end anteriorly and posteriorly in a commissure or fold, that gradually decreases in depth as it passes either forwards or back wards round the intestine.
As regards the details of its construction, each segment of the valve is chiefly formed by the prolongation of a corresponding por tion of the circular muscular fibres of the ileum, together with a few proper to the large intestine. The assistance afforded to these by the attachment of the peritoneum and the longitudinal fibres of the ileum, to the fixed margin of each segment, is well shown by the effect of dividing the latter structures. For after such an injury, mode rate traction draws out the valve into a sur face, which is directly continuous with the lower end of the small intestine ; and at the same time converts its horizontal slit into a large elliptical aperture. The difference be tween the ileo-cmcal and ileo-colic portions consists chiefly in the fact, that the plane of the former is more oblique, and its margin more concave, than that of the latter.
The mechanism of this valve may be easily deduced from its structure. In all states short of actual distention, the passive contraction of its muscular walls no doubt insures their contact, and shuts off the cavity of the ileum from that of the cmcum. While any approach towards a more active dilatation of the large intestine— whether of the cmcum, or colon —at once brings about a close apposition of the two portions of the valve. And what ever aid may be given to this mutual apposi tion of the surfaces of the valve by its own active inuscular contraction during life, no thing is more certain than that its closure is essentially- independent of any such vital pro cess. For the gradual and equable distention of the cmcurn with liquid in the dead subject can also produce this result.
Nor is it difficult to understand how such a closure is effected. The passage of the con tents of the large intestine, over either plane of the valve, presses it against the opposite one, so as at once to close its orifice. Be sides this, the free margins of the valve form segments of a larger circle than its attached ones. Hence they are disproportionately tightened by the same distending force. In this way, the double curve of each lamina is soon reduced to a stratht line, that brings it into exact apposition with its antagonist. So that, within all ordinary limits," the greater the dilating force, the more closely are the two lips of the valve applied to each other.