1. By the number of the mucous glands, which may be seen by a magnifying glass to consist partly of a prodigious number of the minutest follicles, not collected in groups, but equally scattered throughout; and partly of glands of a larger dimension, disposed in groups at particular parts of the canal.
2. By the increase in the number and size of the villi, of which there are about four thousand to the surface of a square inch. Like those of the stomach, the villi of the small intestine are composed of arteries, veins, nerves, and mucous ducts; but to the villi of the small intestine, in length about one-fourth of a line, there is added a new vessel, the absorbent of the chyle, the lacteal (figs. 175 and 176), so named from the milk-like chylous fluid which it contains.
3. By the great extension of the mucous coat obtained by the disposition of the membrane into the folds called valvulæ conniventes (fig. [CLXXIII].). These folds, which rarely extend through the whole circle of the intestine, are often joined by communicating folds (fig. [CLXXIII].). The folds are broadest in the middle, and narrowest at the extremities (fig. [CLXXIII].). In general, they are about a line and a half broad. One edge of the fold is loose, but the other is fixed to the intestine (fig. [CLXXIII].). The office of these folds is, first, without increasing space, to extend surface for the distribution of the villi; and, secondly, to retard the flow of the chyle, by opposing to its descent valves so constructed and disposed as, without arresting its progress, to moderate and regulate its course, in order that time may be allowed for its absorption.
Internal view of a portion of the jejunum, showing the arrangement of the mucous membrane into valvulæ conniventes.
Fig. CLXXIV.—View of the Outer Coats of the Small Intestine.
1. Peritoneal coat reflected off. 2. Muscular Coat consisting of—a. longitudinal fibres. b. Circular fibres.
681. The onward flow of the chyle through the course of the small intestines is effected by the action of the double layer of muscular fibres, the circular and the longitudinal fasciculi which compose its muscular coat (fig. [CLXXIV].). The disposition of the muscular fibres of the alimentary canal in general, and of this part of it in particular, deserves special notice. The ordinary arrangement and action of muscular fibres would not have produced in this case the kind and degree of motion required. The muscular fibres that compose the ventricles of the heart are so accumulated and disposed, that their contraction originates, and communicates energetic impulse. The muscles of the arm are so accumulated and disposed that their contraction originates the like energetic impulse. Muscles so accumulated in the alimentary canal would have produced motion, indeed, but motion not only not accomplishing the end in view, but directly defeating it. In order to obtain the kind and degree of motion in this case required, the firm and thick muscle is attenuated into minute, delicate, and thready fibres, not concentrated in a bulky mass, so as to obtain by their accumulation a great degree of force; but spread out in such a manner as to form a thin and almost transparent coat. The tender fibres composing this delicate coat, by their contraction, produce two alternate, gentle, almost constant motions, called the peristaltic, from its resemblance to the motion of the earth-worm, and the antiperistaltic. By the peristaltic action motion is begun at once in several parts of the canal. Whenever the chyle is applied in a certain quantity to any part of the intestines, that part contracts, and makes a firm point, towards which the portions both above and below are drawn, by means of the longitudinal fibres which shorten the canal, and at the same time dilate the under part. By the antiperistaltic action, which is the exact reverse of the former, the chyle is turned over and over, and exposed to the orifices of the lacteal vessels; while, by the motion of the chyle forwards and backwards, and backwards and forwards, produced by these two actions constantly alternating with each other, its slow, gentle, but ultimately progressive course is secured.