The division between the small and great intestines is indicated, not only by marked differences in their diameter and external appearance, but also by an internal valve placed between them, the object of which is to prevent the contents of the colon from following a retrograde course and returning to the ileum. It is also worthy of notice, that the colon is not a gradually enlarged continuation of the ileum. On the contrary, the latter enters the side of the colon almost at right angles to its course, at a little distance from its commencement. The small portion of the colon which thus lies at one side of the entrance of the ileum, and which has of course no opening at its extremity, is thence called the _caput cœcum coli_, or _blind head_ of the colon, or simply the _cœcum_. Its position is at T in the figure on p. 155, but it is hidden by the folds of the ileum. The _colon_ (from χοιλον, _coilon_, hollow) or great gut UUU in the same figure, constitutes not more than one-fifth in length of the intestinal canal. It begins at the lower part of the right side of the belly, at T in the cut on page 155; rises upwards on the same side towards the liver; crosses over to the left side in contact with the stomach; descends along the left side of the abdomen; makes a turn at UWX like an italic _s_, (and hence called _sigmoid_ flexure), while lying on the left haunch-bone; and lastly, terminates in the _rectum_ or _straight gut_ YY at the _anus_. Being fixed by local attachments, the colon remains always in the same situation, and thus describes a figure not unlike a square, in the centre of which lie the whole of the smaller intestines. In the cut referred to, the left portion is hidden behind them. The diameter of the colon is about double that of the small intestines. In structure it is analogous to them, having three coats; but the _valvulæ conniventes_ or folds of the mucous coat, are no longer to be seen, and with them all traces of chyle and chyle-bearing vessels also disappear. The colon serves more as a reservoir for waste or excrementitious matter than as a vital organ. Absorption is carried on from its inner surface, but through the medium of lymphatic absorbents and minute venous ramifications, and not of lacteals. Hence not only food, but medicine, are frequently administered by being injected into the rectum, and life has been saved in this way when nothing could be given by the mouth.