Mucous membrane.— Having thus briefly described the peritoneal and muscular coats of the small intestine, we may next proceed to consider its mucous membranet—the struc ture on which its various functions essentially depend.
This tunic everywhere consists of the ordi nary elements of a mucous membrane :— namely, a basement membrane, an epithelium, and a layer or areolar tissue that contains an admixture of the muscular fibre-cells. But, instead of forming a simple, flat expanse, it undergoes numerous modifications; which, un der the names of valvulce conniventes, intestinal tubes, villi, agminate follicles, solitary follicles, and racenzose glands, will especially claim our notice.
Valvula. conniventes. —Almost all the small intestine is complicated by the presence of transverse folds of mucous membrane ; which project from its inner surface into its cavity. These projections, which were known to many of the earlier anatomists, were named by Kerkring the valvul conniventes ;—apparently from his thinking that they delayed the intes tinal contents, but still as it were, connived at their passage. They begin in the second portion of the duodenum, and only cease in the lower fifth or sixth of the small intes tine. They are best shown by moderate dis tention of the tube with alcohol; which slowly hardens them, so that they retain their shape, even after a portion of the wall of the bowel has been removed to display its interior.— Extreme distention greatly diminishes their size, but never effaces them altogether. And such a permanent character sufficiently distin guishes these folds from those temporary creasings which are seen generally throughout the stomach and intestine, and which are sometimes spoken of as preceding them in the first part of the duodenum. At first they are very small and scattered, rise little above the general mucous surface, extend but a short distance across the tube, and break up at their extremities into still more minute creases, which often pass obliquely to join those next them, above and below. In the lower patt of the duodenum, they gradually acquire a number and size, which are retained throughout the whole of the jejunum. But from the beginnity, of the ileum, they again diminish; first infrequency, and latterly in length and depth. And in the lower third of this segment, they generally disappear alto gether.
Each of these folds consists of a duplica ture of mucous membrane, enclosing a process of the loose areolar tissue which everywhere separates the mucous from the muscular coat.
Opposite the attached border of the valvula, this layer is somewhat thicker ; but does not appear to contain more than its ordinary small quantity of fibre-cells. The process which it gives off' to each of the folds con tains vessels, nerves, and lacteals. The relation of this tissue to thevalvula is well shown by- the result of its inflation ; which produces a kind of artificial emphysema, that completely oblite rates the whole projection. When the cavity of the intestine is forcibly distended, the valvulce are placed vertically to the general surface. But in the ordinary state of the bowel, they are easily moved by any ex ternal force ; so that their free margin is generally directed obliquely upwards or down wards. Their direction is nearly trans‘erse to the axis of the tube. Their variable extent around the wall of the bowel forms one-half, two-thirds, or even three-fourths of a circle. Their greatest projection occupies the middle of their length, where they are often from one-fourth to half an inch deep. But towards either extremity, they gradually sink into the general mucous surface. In doing this, the valves usually swerve a little from their hither to transverse and parallel course; so that each joins by one or both ends with the fold imme diately before or behind it. And sometimes a bifurcation of the tapering fold unites its extremity to two of its neighbours.
The office of these permanent folds has been a matter of comiderable speculation. It is evident that they increase the extent of the mucous surface to at least twice or thrice what it would be in a simple hollow cylinder of equal size. It is equally obvious, that their transverse position is peculiarly calculated to render this enlarged surface an effective one. For they are at right angles to the direction of peristalsis, and therefore to the general course of the intestinal contents. Such an arrangement of the mucous membrane, taken in connection with the great mobility of these folds, tnust not only insure a thorough ad mixture of the various constituents of the chy me, but, by delaying its direct passage onwards, bring every portion of it into con tact with the greatest possible extent of the active intestinal surface.