Large intestine. — The remaining portion of the alimentary canal forms the large intestine (c aids r, fig. 276.) (intestinum crassunz, Lat.; gros intestin, Fr.; dickes Gedarm, Germ.) :— a name which alludes to the size that is one of its chief characteristics. Beginning at the termi nation of the ileum, in the right iliac fossa, it passes upwards to the under surface of the liver. Here it turns at a right angle, and runs horizontally below the stomach, to the left extremity of this organ. By. a second bend, it here resumes the vertical direction, and then passes downwards towards the left iliac fossa. In this region it undergoes a remark able curvature, which has the shape of the italic letter S. From the lower end of this "signgoid" flexure, it passes obliquely towards the median line ; where it terminates in a straight, short tube, that runs vertically through the pelvis to the outlet of the anus. Hence the entire segment of large intestine has the shape of a horse-shoe ; and forms a large bend, which is concave downwards, and passes ahnost round the confines of the abdomen before ending at the inferior extremity of this cavity. While its general arrangement is such, that the intestinal canal, which diverges from the median line at the lower end of the oesophagus, returns to it shortly before terminating in the posterior or lower segment of the trunk.
An accurate measurement of the length and width of this tube is opposed by the diffi culties already alluded to in the case of the small intestine. My own observations would indicate an average length of from four to six feet, and a mean diameter of about lf to 21 inches:— the two measurements usually vary ing inversely to each other, except in the cases of extreme distention or contraction, when both respectively increase or decrease simul taneously. Hence the large intestine has about a quarter the length, and twice the width, of the small. From such an estimate we may conclude that, while its capacity is almost equal to that of the narrower tube, its active surface is scarcely half as large. And even this great difference is much in creased by the absence of villi and valvtilw conniventes from the interior of tine large in testine.
Like the rest of the canal, the wall of the large intestine is composed of the serous, mus cular, and mucous coats; and of vessels, nerves, and lymphatics, which are distributed to them.
The nature and arrangennent of these tunics vary, however, in the several parts of the tube. And these differences, aided by others which affect its size, shape, and situa tion, subdivide the large intestine into the following segments :—the CCECUM ; the vermi form apperidix ; the colon, in which we dis tinguish an ascending, transverse, and descend ing portion, and a sigmoid flexure; and, finally, therectum. The anatomy of each of these will demand a brief notice.
The ccecunz(c,figs. 276, 277.) (formerly blind gut, Eng.; blind Darm, Germ.) is the first and largest of these segments. Its arrangement may be described as due to the fact, that the small intestine, instead of being sitnply continuous with the large (like the stomach with the duo denum), opens into it at right angles to its axis, and at some distance from its commencement ; so as to leave a blind extremity of the larger tube at the site of their mutual junction. The size of this cul-de-sac generally exceeds that of the remainder of the bowel; it being larger than any other part of the alimentary canal, with the single exception of the stomach. When moderately distended, its diameter ranges from 21 to 31 inches ; and its length is about as much. Its vertical extent is,however, somewhat arbitrary ; since, though defined in part of its circumference by the aperture of the ileum and by the ilio-cmcal valve, it is elsewhere only limited by an imaginary line drawn around the tube at the level of the latter orifice.
The situation of the cmcum, in the left iliac fossa, allows it to vary considerably in size, without undergoing any marked change of its relations. Bound down as it is to the fascia over the iliacus muscle by peritoneum and loose areolar tissue, its enlargement merely causes it to displace such portions of the small intestine as may hitherto have shared the oc cupation of the iliac fossa. After its distention has removed these from its anterior surface, it reaches the anterior wall of the belly in the iliac region ; where its size, shape, and con tents can be more or less recognized during life, by the ordinary- means of physical investi gation.
The above dimensions render it obvious that the shape of the cmcum is somewhat globular. This shape is, however, modified by the arrangement of its muscular layers; which here begin to offer a peculiarity that is main tained throughout the whole of the colon. The uniform external or longitudinal layer present in the small intestine is here contrasted by one which is separated into three flattened bands, that occupy the side of the tube at nearly equal distances from each other. In the cmcurn one of these (and the larger of the three) is ante rior ; one posterior ; and one external. And all three of them become continuous above with the corresponding bands around the as cending colon. Between these slips of muscle, the bowel presents a more or less dilated and projecting external surface ; which is again sub divided by transverse constrictions into subor dinate pouches or sacculi. On laying open the bowel, and removing the mucous membrane from its inner surface, it may be seen that these transverse constrictions are in reality formed by the circular muscular coat ; which gives off projections or incomplete septa, that compli cate the general cavity of the tube, by adding a number of supplementary cells. These cells are arranged in three vertical rows ; which are separated by ridges, that correspond to the external depressions formed by the longitu dinal bands above mentioned. Between these bands, the "haustra," or pouches of the bowel possess a muscular tunic of very inconsider able thickness : the transverse or circular layer being reduced to a thin membranous lamina ; and the longitudinal being, as before stated, altogether absent. The close relatioh of these longitudinal and transverse septa to the length and width of the bowel is well shown by the effect of cutting across or re rnoving its three bands, and then distending the tube by artificial inflation. This obliterates the " falciform folds " or transverse septa ; and thus converts the sacculated intestine into a canal, the length and diameter of which are nearly double of what it formerly pos sessed when retained in its proper shape by its longitudinal bands or " tmnim." The serous covering of the ccecum is chiefly remarkable from the closeness with which it generally attaches the bowel to the fascia over the iliacus muscle. When the tube is but moderately distended, it covers only its ante rior surface. Extreme contraction can, how ever, render it a more complete covering; and may even produce it into a kind of nieso cmcum behind the bowel. While conversely, great distention of the tube reduces the peri toneum to a partial investment ; which occu pies but a third, or even less, of the intestinal surface.