1170 DIGESTIVE SYSTEM 



size, usually approaching the intestine in width and averaging 5 cm. in length (ranging from 

 1 cm. to 13 cm.). Its attachment to the intestine varies from 15 cm. to 360 cm. (average 

 80 cm.) above the caecum. ' It is usually attached opposite the mesentery. It may end freely, 

 but is occasionally adherent to adjacent intestinal coils or connected with the anterior abdominal 

 waU by a cord or band-like process. 



Other diverticula of variable size and number may occur, usually along the mesenteric 

 border of the intestine. They may be either congenital (probably from the embryonic pockets 

 previously mentioned) or acquired. They occur most frequently in the duodenum (found by 

 Baldwin in 15 of 105 cases) where they are usually associated with the openings of the bile and 

 pancreatic ducts. 



Comparative. — The comparative anatomy of the small intestine wiU be discussed later 

 together with that of the large intestine. 



THE LARGE INTESTINE 



The large intestine [intestinum crassum] is that part of the aUmentary canal 

 which extends between the ileum and the anus. It is divided into the following 

 parts: Caecum, ascending, transverse, descending, and sigmoid colon, and rec- 

 tum. It is so arranged as to surround the small intestine, making a circuit around 

 the abdominal cavity from right to left (fig. 899). The ccecum lies in the right 

 iliac fossa; thence the colon passes vertically upward on the right side (ascending 

 colon) until the liver is reached. Here it forms a more or less rectangular bend 

 (the right colic or hepatic flexure), and then passes transversely across the belly 

 (transverse colon) below the stomach. It then reaches the spleen, where it makes 

 a second sharp bend (the left colic or splenic flexure), and, passing vertically down- 

 ward on the left side (descending colon), reaches the left iliac fossa. At this point 

 it forms the loop of the sigmoid colon, and finally passes through the pelvis as the 

 rectum (fig. 906). The large intestine is much larger in diameter than the small 

 intestine, and is not so much convoluted. Excepting the dilated portion of the 

 rectum, it is wider at the beginning than at the end. It varies in width at different 

 parts from 3 to 8 cm. The length from the root of the appendix or tip of the cae- 

 cum to the point where the meso-colon ends is, in the male, about 140 cm., and 

 in the female about 130 cm. The average -total length, including the rectum, is 

 about 150 cm. (5 ft.). The extremes found are 100 to 200 cm. 



The large intestine, in all parts except the rectum, has a peculiar arrangement 

 of its walls, which gives it a very different appearance froni the small intestine. 

 It is sacculated, and the sacculations [haustra] are produced by the gut having to 

 adapt its length to three shorter muscular bands which run the course of the intes- 

 tine. These bands, which are about 12 mm. wide and 1 mm. thick, aie really the 

 longitudinal fibres of the muscular wall, which are chiefly collected along three 

 lines (fig. 935). One band [taenia mesocolica], corresponding to the attachment of 

 the mesocolon, is posterior on the transverse colon, and postero-median on the 

 ascending and descending colons. A second band [taenia omentalis] is antero- 

 superior on the transverse colon, elsewhere postero-lateral. The third band 

 [taenia libera] is free; it is inferior on the transverse colon, anterior elsewhere. All 

 these bands start on the caecum at the vermiform process, and spread out to form 

 a uniform layer on the rectum. Between the sacculations are semilunar folds 

 [plicae semilunares coli], which involve the entire thickness of the intestinal wall, 

 forming crcscentic ridges of the mucosa which project into the lumen (figs. 932, 

 935). Along the free surface of the colon, especially near the taeniae, are numerous 

 small appendages [appendices epiploicae], which are pouches of peritoneum con- 

 taining fat (fig. 906). 



The caecum. — The caecum [intestinum caecum] is a cul-de-sac forming the first 

 part of the large intestine. It is defined as that part of the colon which is situated 

 below the entrance of the ileum. Its breadth is about 7.5 cm., and its length 

 about 6 cm. (Fig. 932). 



There is usually a more or less well-marked constriction opposite the ileo- 

 caecal orifice marking the boundary between caecum and colon. The caecum itself 

 also frequently presents a constriction dividing it into two sacculations. 



It lies in the right iliac fossa, and is usually situated upon the ilio-psoas muscle, 

 and so placed that its apex or lowest point is just projecting beyond the medial 

 border of that muscle (figs. 899, 906). It is usually entirely enveloped in pc^riton- 

 eum, and is free in the abdominal cavity, but more or less attached in about 10 

 per cent, of all cases. The apex of the caecum usually corresponds to a point a 



