362 



DIGESTIVE SYSTEM OF THE HORSE 



csscum at the lesser curvature of its base. The average diameter of the jejuno- 

 ileum is about two antl a half to three inches (ca. 6 to 7 cm.). In the cadaver one 

 finds most of the tube presenting irregular constricted and dilated parts. The 

 last three or four feet (ca. 1 meter) are usually tightly contracted, resembling some- 

 what the terminal part of the oesophagus. This part may be termed the ileum. 



The mesenteric part is connected with the dorsal abdominal wall by the great 

 mesentery. This is a wdde fan-shaped fold, consisting of tw^o layers of peritoneum, 

 between which the vessels and nerves reach the Ijowel; it also contains the mesen- 

 teric lymph glands and some fat. The visceral border of the mesentery contains 

 the intestine, while the parietal border or root (Radix mesenterii) is attached to a 

 small area around the great mesenteric trunk under the first and second lumbar 

 vertebrae. The root is thick, as it contains a large number of vessels and nerves 

 placed close together. The mesentery is short at first, but soon reaches a length 

 of one and a half to two feet (ca. 50 cm.) — sufficient to allow coils of the intestine 

 to reach the abdominal floor, the pelvic cavity, or even to descend into the scrotum 

 through the inguinal canal. Near its termination the intestine (ileum) leaves 



the border of the mesentery, so that 



Hepatic, 

 duct 



Pancreatic 

 duct 



Diverticulum 

 duodeni 



Fig. 264. — Diagram of Section of Diverticulum Duo- 

 deni OF Horse. 

 Solid line indicates mucous membrane. 



the latter has a free edge which passes 

 to the caecum. 



Structure. — The wall consists of 

 four coats — serous, muscular, sub- 

 mucous, and mucous, enumerated 

 from without inward. 



The serous coat is complete ex- 

 cept at the mesenteric edge, where 

 the vessels and nerves reach the 

 bowel. 



The muscular coat consists of 

 an outer longitudinal and an inner 

 circular layer, the latter being the 

 thicker. In the last few feet of the 

 intestine the muscular coat is very 

 thick, and being usually firmly con- 

 tracted in the dead subject, gives the impression that this part of the bowel is of 

 smaller caliber; such, however, is not the case during life. 



The submucous coat is a layer of areolar tissue in which the vessels and nerves 

 ramify. It contains also the duodenal glands and the bases of the solitary glands 

 and Peyer's patches. 



The mucous membrane is soft and velvety. It has a grayish or yellowish-red 

 color and is very vascular. About five or six inches from the pylorus it forms a 

 pouch, the diverticulum duodeni, in which the pancreatic and hepatic ducts open. 

 On a small papilla opposite this is the termination of the accessory pancreatic duct. 

 At the ileo-ca)cal opening the nuicous mem]:)rane projects slightly into the cavity 

 of the ciecum, forming the ileo-caecal valve. The free surface is thickly beset with 

 villi, smull i)rojections of the mucous membrane which can be seen w^ell by placing 

 a piece of the membrane in water. They are relatively short and thick and have a 

 distinct neck in the horse. Each contains a central lymph-vessel (lacteal), and 

 around this a plexus of capillaries, lymphoid tissue, and unstriped muscle-fibers. 

 They are important agents in absorption from the contents of the intestine. The 

 epithelium is columnar, with many goblet cells. Underneath the basement mem- 

 brane is a layer of unstriped muscle-fibers, the muscularis mucosae. 

 The glands of the small intestine are of three kinds: 



1. The intestinal glands (Cdanduhr intestinales Lieberkuehni) are found 

 throughout. They are simple tubular glands which open between the villi. 



