228 INTESTINAL DIGESTION. 



jejunum, and the third or csecal portion is distinctively 

 designated ileum. 



The Ileum is, on the whole, the narrowest portion of the 

 small intestine, but the thickest in its coats. 



Having now specially to describe the structure of the small 

 intestine, it may be taken as a whole, merely alluding to 

 local peculiarities. 



This portion of the alimentary canal has four coats, i.e., 

 peritoneal, muscular, cellular, and internal mucous. 



The first, or peritoneal, has nothing of peculiar, beyond its 

 enclosing a little triangular space all along the upper attached 

 border of the gut. The looseness of the peritoneal folds 

 attaching the small intestine is very marked: and Colin 

 notes, that the mesentery is proportionately larger in young 

 than in adult quadrupeds, so that the gradual shortening of 

 this explains the spontaneous reduction of exomphalus or 

 umbilical hernia in colts. 



The second, or muscular, coat is mostly developed at the 

 commencement of the duodenum and terminating portion of 

 the ileum. It consists of white involuntary fibres, arranged 

 so as to form an outer longitudinal layer and an inner cir- 

 cular one, both of which completely encircle the intestine. 



The third, or cellular, coat is similar to that of the stomach, 

 in being disposed in two layers, so as to connect the three 

 coats together. It is especially condensed on the inner sur- 

 face of the muscular coat, so as to take the appearance of a 

 fibrous tunic attached to the mucous lining by loose cellular 

 tissue. 



The fourth, or mucous, coat is thin, having a velvety 

 appearance, due to villi, peculiarly small in the intestines of 

 the horse, but remarkably developed in other animals, especi- 

 ally carnivora and fishes. The villi may be seen by a pocket 

 leu?, on a well- washed piece of intestinal mucous membrane. 



