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. [Illustration] The peritoneal coat, after forming the outer covering of the intestine, represented by the dotted line round the circle I in the figure on next page, is continued from it in the form of a double membrane (represented by the two dotted lines) towards the spine S, to which it is first firmly attached by cellular substance; after which the folds again separate, each being continued or _reflected_, as it is called, over the whole inner substance of the cavity of the abdomen in the course shewn by the dotted line, the figure itself representing a transverse section of the abdomen. By this arrangement two important objects are attained. _First_, the abdominal peritoneum AP forms a soft lubricated surface, corresponding to that of the bowels themselves; and, _secondly_, a firm point of attachment for the bowels is secured, by which they may be supported in their proper places, and at the same time admit of some change of position. The floating portion of the peritoneum M, by which the attachment is effected, is called the _mesentery_ (from μεσος, _mesos_, the middle, and εντερον, _enteron_, intestine). But the intestinal canal being so much longer than the portion of the spine, to which the mesentery is attached, the latter is necessarily disposed in folds converging towards the spine, something like the folds of a fan converging towards its narrow end. In this way, the mesentery, besides serving as a support to the gut, serves also to receive and afford protection to the numerous vessels, nerves, and