370 DIGESTIVE SYSTEM OF THE HORSE 



80 to 90 cm.). Its parietal border is attached along a line extending from the 

 ventral surface of the left kidney to the sacral promontory; it is continuous in 

 front with the root of the great mesentery, and behind with the mesorectum. 



There are two taeniae and two rows of sacculations. Of the tsenise, one is free, 

 the other concealed by the mesentery. When the bowel is hardened in situ its 

 lumen between the pouches is reduced to a narrow slit. 



The Rectum 



The rectum (Intestinum rectum) is the terminal part of the bowel; it extends 

 from the pelvic inlet to the anus.^ Its length is about one foot (ca. 30 cm.). Its 

 direction may l^e straight or oblique. The first or peritoneal part of the rectum is 

 like the small colon, and is attached by a continuation of the colic mesentery termed 

 the mesorectum. The second or retroperitoneal part forms a flask-shaped dilata- 

 tion termed the ampulla recti ; it is attached to the surrounding structures by con- 

 nective tissue and muscular bands. 



The first part of the rectum is related to the small colon and the pelvic flexure 

 of the great colon. It is frequently deflected to the left by the latter. The second 

 part of the rectum is related dorsally and laterally to the pelvic wall. Ventrally 

 the relations differ in the two sexes. In the male they are the bladder, the ter- 

 minal parts of the vasa deferentia, the vesiculae seminalcs, the prostate, the bulbo- 

 urethral (Cowper's) glands, and the urethra. In the female they are the uterus, 

 vagina, and vulva." 



Structure of the Large Intestine. — The serous coat covers the difl'erent parts 

 in varying degrees. It does not cover (a) the opposed surfaces of the caecum 

 and colon which are between the layers of the cseco-colic fold and mesocolon; 

 (6) the areas of attachment of the caecum and colon to the pancreas, right kidney, 

 and sublumbar region; (c) the second part of the rectum. 



The muscular coat consists of longitudinal and circular fibers. The bulk of 

 the former is in the bands already described. Some of the circular fibers pass 

 from one part of the colon to another, where they are attached to each other, 

 forming the fibrse transversse coli. The muscular coat of the second part of the 

 rectum presents special features. The longitudinal layer of fibers is very thick 

 and consists of large bundles, loosely united. A large band, the recto-coccygeus, 

 is detached from it on either side, and passes upward and backward to be inserted 

 into the fourth and fifth coccygeal vertebrae. 



The submucous tissue is abundant in the wall of the rectum, so that the mucous 

 membrane is loosely attached to the muscular coat, and forms numerous folds 

 when the bowel is empty. 



The mucous membrane of the large intestine is thicker and darker in color 

 than that of the small intestine. It forms large crescentic or semilunar folds cor- 

 responding to the external constrictions. It has no villi, Brunner's glands, or 

 Peyer's patches. The intestinal glands (of Lieberkiihn) are large and numerous. 

 Solitary glands are also numerous, especially at the apex of the caecum and in the 

 left dorsal part of the colon. 



Blood-supply. — Greater and lesser mesenteric and internal pudic arteries. 

 The veins go to the portal vein. 



Nerve-supply. — Mesenteric plexus of the sympathetic nerves. 



' There is no natural line of demarcation between the small eolon and rectum: the plane 

 of the pelvic inlet is selected for convenience of description. 



- The anterior part of the rec^tum is very variable in position and relations. It is not often 

 median, but may be deflected either to right or left. Most often it is pushed to the left by the 

 pelvic flexure of the colon. In other cases — especially when empty — it may lie against the right 

 wall, and the space to the left is occupied by the small colon. The amount covered by peritoneum 

 dorsally and laterally is very variable, and appears to be in inverse proportion to the fullness of 

 the bowel. 



