When the mucous coat of the duodenum is examined with the naked eye, the first part of its course presents a tolerably smooth appearance, gradually, however, becoming irregular in transverse folds, which are so numerous, marked, and regular in the jejunum and ileum as to have obtained from the earliest times the name of valvulæ conniventes. They are most strongly marked in the jejunum, and gradually disappear toward the lower part of the ileum, the inner surface of the large intestines being still smoother than any part of the small, although large pouches or cells are formed in the colon by a peculiar arrangement of the muscular coat. These valves never extend completely round the inside of the intestine, and rarely more than half or two-thirds, although they sometimes bifurcate. They have a velvety appearance, which has obtained for this coat the name of villous as well as that of mucous.
Valvulæ conniventes are peculiar to man; none exist in the ourang-outang or chimpanzee. In the frog there are valvular folds, appearing, at first sight, like the valvulæ conniventes of the human subject; but, on a careful examination, they are found to be mere elevations, without villi. In the tortoise there are similar folds, running however in a longitudinal or opposite direction. In the rhinoceros the mucous membrane is raised up into villiform processes, somewhat like the valvulæ conniventes, or large villi; but they are not villi, as each process is covered with other projections which really are villi. A valvula connivens consists of two layers of mucous membrane and sub-mucous tissue, but the muscular coat is not continued into it.
373. When examined microscopically, the velvety appearance is found to consist of innumerable small processes which have been called villi, each villus being composed principally of a very thin, transparent basement or germinal membrane, forming a sheath or case, inclosing within it an artery, a vein, a capillary plexus, and an absorbent vessel termed lacteal. A nerve has not been discovered, although it is presumed to exist. These villi are longest in the duodenum, and gradually diminish in number and in size from 1/25 to 1/50 of an inch. Between these villi or projections, holes or openings are observable, termed the follicles of Lieberkühn, who first described them; they resemble inverted villi, being in some instances as deep as the villi are long. Unlike the villi, they are found throughout the intestines. The villi in every part in common with all mucous membranes are covered, and the follicles are lined by epithelium, which in this instance is the columnar, situated on the basement membrane, each column being attached by its pointed extremity. A layer of this epithelium extends between the villi, down to the lower part of each follicle, each column being, generally speaking, shorter and rounder than when covering the villi.
The office of the epithelium of the villi has been stated to be protective, that of the follicles to be secretive. A villus, when duly magnified, is seen to have a bulbous extremity without an opening, and to be covered by epithelium when the intestine is in a state of quiescence, uncalled upon for any purpose of digestion. When digestion commences, the epithelium, according to the researches of Mr. Goodsir, is separated and thrown off. As the chyme begins to pass along the small intestine, an increased quantity of blood circulates in the capillaries of the gut. In consequence of this increased flow of blood, or from some other cause, the internal surface of the gut throws off the epithelium of both villi and follicles, which is intermixed with the chyme in the cavity of the gut. The cast-off epithelium, forming 19/20ths of the covering of the villus, is of two kinds, that which covers the villi, and which from the duty it performs may be termed protective, and that which lines the follicles and may be termed secretive, each column having a nucleus situated at some part of it, and bulging out that part.
The villi being now turgid with blood, erected and naked, and covered by the chyme mingled with the cast-off epithelia, commence their functions. The summit of the villus becomes at first somewhat flattened and crowded under the basement membrane with a number of newly-formed and perfectly spherical vesicles, varying from 1000 to less than 2000 of an inch in size. Toward the body of the villus or the inner edge of the vesicular mass, minute granular or oily particles are situated in great numbers, and gradually pass into the granular texture of the substance of the villus. As the process advances lacteal vessels are shown passing up from the root of the villus, subdividing and looping as they approach the spherical mass, which in this stage has become more distinctly vesicular, although no distinct communication can be detected between them. The blood-vessels and capillaries shown in injected preparations are now seen colored red with their own blood, and running up to the basement membrane, looping with each other immediately beneath it, and ending in one or more venous trunks. The vesicles, quite distended and grouped in masses, push forward the membrane, and give to it by these inequalities an appearance resembling that of a mulberry.
The minute vesicles above noticed fulfill the important office of absorption, by drawing into their cavities through their walls, by a process called endosmosis, that portion of the chyme necessary to form chyle; when filled with it they burst or dissolve, their contents being thus discharged into the texture or substance of the villus, fit to be taken up by the granular vesicles interspersed among the terminal loops of the lacteals, and communicating with their trunks, running up from the root of the villus in their center. Absorption is thus shown to be effected by closed vesicles, and not by vessels opening on the surface of the villus.
The débris and the contents of the dissolved chyle cells, etc. pass into the looped net-work of lacteals, as in other lymphatics. When the gut contains no more chyme, the flow of blood to the mucous membrane diminishes, the development of new vesicles ceases, the lacteals empty themselves, the villi become flaccid, and the cast-off epithelium is reproduced, apparently from the nuclei in the basement membrane, in the intervals of digestion, showing that this function should only be induced at regular periods, the presumed special use of the epithelium being to prevent, in a measure, the absorption of any effete or other matters which might exert a deleterious influence oh the system, the epithelium of the follicles now secreting a mucus which may be considered protective.
In the large intestines there are no villi, but the whole surface is covered with follicles which must be capable of absorbing as well as of secreting, as it is ascertained that persons can be nourished and kept alive for many weeks by nutritious enemata which do not pass into the small intestines.
374. On examining the mucous membrane of the stomach, its follicular structure is immediately seen, the follicles resembling much in appearance those of the intestine; but in the stomach minute tubes are found opening into the bottom of each follicle, fulfilling in all probability a different office, the follicles being lined by columnar epithelium, the tubes by spheroidal or glandular epithelium; it is therefore presumed that the gastric juice is secreted by the tubes, the mucus by the follicles. The tubes differ in the middle and lower parts of the stomach, by being longer or more deeply seated, and more numerous as they approach the pylorus, showing in all probability a difference of function between the upper and middle, and the pyloric or lower extremity of the organ.
The intestines are supplied with glands, not apparently for the purposes of absorption, but of secretion; these require attention. They are the duodenal of Brunner, the agminated of our countryman, Nehemiah Grew, and of Peyer, and the solitary, which are found in the lower part of the small and in the whole course of the large intestines.