354 DIGESTIVE SYSTEM OF THE HORSE 



are normally separated only l^y the thin film of serous fluid (secreted by the mem- 

 brane) which acts as a lubricant. 



The free surface of the membrane has a glistening appearance and is very 

 smooth. This is due to the fact that this surface is formed by a layer of flat en- 

 dothelial cells, and is moistened by the peritoneal fluid. Friction is thus reduced 

 to a minimum during the movements of the viscera. The outer surface of the peri- 

 toneum is related to the subperitoneal tissue, which attaches it to the abdominal 

 wall or the viscera. 



In order to understand the general disposition of the peritoneum, we may 

 imagine the abdominal cavity to be empty and lined by a simple layer of perito- 

 neum, termed the parietal layer (Lamina parietalis). We may regard the organs as 

 beginning to develoj) in the subperitoneal tissue, enlarging, and migrating into the 

 cavity to a varying extent. In doing so they carry the peritoneum before them, 

 producing introversion of the simple sac, and forming folds which connect them 

 with the wall or with each other. The viscera thus receive a more or less complete 

 covering of peritoneum, termed the visceral layer (Lamina visceralis). The con- 

 necting folds are termed omenta, mesenteries, ligaments, etc. They contain a 

 varying quantity of connective tissue, fat and lymph glands, and furnish a path for 

 the vessels and nerves of the viscera. Some contain unstriped muscular tissue. An 

 omentum is a fold which passes from the stomach to other viscera. There are three 

 of these, namely: (1) the small or gastro-hepatic omentum (Omentum minus), which 

 passes from the lesser curvature of the stomach to the liver; (2) the gastro-splenic 

 omentum (Ligamentum gastrolienale), which extends from the greater curvature 

 of the stomach to the spleen; (3) the great omentum (Omentum majus), which 

 passes from the greater curvature of the stomach and from the spleen to the terminal 

 part of the great colon and the origin of the small colon. It does not pass directly 

 from one organ to the other, ])ut forms an extensive loose sac (Figs. 278, 279). 

 A mesentery (Mesenterium) is a fold which attaches the intestine to the dorsal 

 wall of the abdomen. There are two mesenteries, namely: (1) the great mesentery 

 which connects the greater part of the small intestine with the dorsal abdominal 

 wall; (2) the colic mesentery, which attaches the small colon to the dorsal abdom- 

 inal wall. Ligaments are folds which pass between viscera other than parts of the 

 digestive tube, or connect them with the abdominal wall. The term is also applied 

 to folds which attach parts of the digestive tract to the abdominal wall, but do not 

 contain their blood-vessels and nerves. In some cases (e. g., the lateral and cor- 

 onary ligaments of the liver) they are strengthened by fibrous tissue; in other cases 

 (e. g., the broad ligaments of the uterus) they contain also unstriped muscular tissue. 



THE PELVIC CAVITY 



The pelvis is the posterior part of the trunk. It incloses the pelvic cavity 

 (Cavum pelvis), which communicates in front with the abdominal cavity, the line 

 of demarcation being the pelvic brim or terminal line. 



The dorsal wall or roof is formed by the sacrum and first three coccygeal 

 vertebrae. The lateral walls are formed l)y the parts of the ilia behind the ilio- 

 pectineal lines and the sacro-sciatic ligaments. The ventral wall or floor is formed 

 by the pubic and ischial bones. The boundary of the outlet is formed by the third 

 coccygeal vertebra dorsally, the ischial arch ventrally, and the posterior edges of 

 the sacro-sciatic ligaments and the semimembranosus muscles laterally. The out- 

 let is closed by the perineal fascia; this consists of superficial and deep layers, 

 which are attached around the margin of the outlet and centrally to the organs at 

 the outlet — the anus and its muscles, the vulva (in the female), and the root of the 

 penis (in the male). 



The cavity contains the rectum, parts of the internal generative and urinary 



