If physiologists experience much difficulty in satisfactorily explaining all the phenomena of chymification or stomach-digestion, the reflecting reader will not be surprised to learn that they are still more puzzled to account for those of chylification or intestinal digestion. The organs concerned in the latter are so deep-seated and inaccessible during life, that very few opportunities occur of obtaining accurate information on the subject; and, therefore, in what follows, I shall not enter into disputed or intricate details, but confine myself to such general views as are not contested, and as the reader may easily understand. Fortunately, ignorance of this branch of the inquiry is of less practical importance than if it extended to stomachic digestion also; because such is the harmony between all the parts of the system, that whatever conduces to the perfect accomplishment of the first stage of the process, chymification, is in so far equally conducive to the proper fulfilment of chylification or intestinal digestion.
The simple fact, indeed, of our having no direct control over the process of chylification, and of our being able to modify it only by varying, through the medium of the stomach, the elements out of which chyle is to be formed and the mode in which they shall be digested, is a proof that, practically speaking, it is chiefly the laws or conditions of stomachic digestion which are intended to regulate our conduct; and that, in obeying them, we in reality obey also those of intestinal digestion.
The organs concerned in chylification are the duodenum, the liver, and the pancreas; but in order to avoid repetition, I shall, in describing the first, notice also the remainder of the intestine.
The intestine or intestinal canal, as represented in the subjoined figure, begins at the pyloric orifice of the stomach P, and after many windings and turnings, called convolutions (from the Latin word convolutus, rolled or folded together), terminates in the rectum or straight gut Y, at the external orifice called the anus. Although continuous throughout its whole extent, the intestinal tube is nevertheless divided by anatomists into six portions, to each of which a different name is assigned: the distinction between some of these is more nominal than real, but it still continues to be made on account of its convenience.
The first grand division is into the small and great intestines; the former beginning at the stomach, including all the convolutions marked RSSSS,—and the latter beginning at T, where the small intestine terminates, and including the large gut UUUUXYY, which surrounds, and is partly hidden by, the other bowels.
The small intestines, again, are subdivided into three portions,—the duodenum, the jejunum, and the ileum; and the larger, in like manner, into three portions—the caput cœcum or simply the cœcum, the colon, and the rectum. Of the whole length, the small intestines constitute by much the greater part, and they differ somewhat from the larger in function as well as in magnitude.
In structure, the intestines exhibit a great analogy with the stomach. They consist, in common with it, of three coats or layers of membrane; the outer or peritoneal,—the middle or muscular,—and the internal, mucous, or villous.
The peritoneal coat is the white, firm, smooth, shining, and moist membrane, seen on the outside of the intestine on opening the cavity of the abdomen. It serves both as a support and as a medium of attachment to fix the intestine in its place. By its smooth, soft, and lubricated surface, it admits readily of the change of place among the bowels necessarily produced by respiration, exercise, and even by different degrees of distention of the bowels themselves. Every time we breathe, an undulating motion is communicated to the whole intestines, which facilitates their action, but which could not take place unless they were capable of gliding easily and freely over each other. The peritoneal coat, being strong, extensible, and elastic, is very useful also as a support to the other coats.