182 TEXT-BOOK OF PHYSIOLOGY. 



absorbable and assimilable compounds which collectively constitute intes- 

 tinal digestion. 



THE SMALL INTESTINE. 



The small intestine, in which this stage of digestion takes place, is a 

 convoluted tube, measuring about seven meters in length and 3.5 cm. in 

 diameter, and extending from the pyloric orifice of the stomach to the begin- 

 ning of the large intestine. 



The intestine consists of four coats: viz., serous, muscle, submucous, 

 and mucous. 



The serous coat is the most external and is formed by a reflection of 

 the general peritoneal membrane. It is, however, wanting in the duodenal 

 portion. 



The muscle coat, situated just beneath the former, surrounds the entire 

 intestine. It is composed of non-striated fibers, which are more abundant 

 and better developed in the upper than in the lower portions of the intestine. 

 The muscle coat consists of two layers of fibers: (i) an external or longitudinal, 

 and (2) an internal or circular layer. The longitudinal fibers are most 

 marked at that border of the intestine free from peritoneal attachment, 

 though they form a thin layer all over the intestine. The circular fibers are 

 much more numerous, and completely encircle the intestine throughout its 

 entire extent. It has been demonstrated that at the junction of the ileum 

 and colon, and surrounding the orifice, the ileo-colic, common to both, the 

 muscle-fibers are arranged in the form of, and play the part of, a sphincter 

 muscle, which has been termed the ileo-colic sphincter. It is usually 

 in a state of tonic contraction and regulates the passage of materials from 

 the small into the large intestine, and possibly also in the reverse direction 

 under special circumstances. 



The submucous coat consists of areolar tissue and serves to unite the 

 muscle with the mucous coat. A thin layer of muscle-fibers, the muscularis 

 mucosa, is placed on its inner surface. 



The mucous coat is soft and velvety in appearance and covered by a 

 single layer of columnar epithelium. Its entire surface is covered with 

 small conical projections termed villi. Throughout its entire extent, 

 with the exception of the lower portion of the ileum and the duodenum, 

 the mucous membrane presents a series of transverse folds the valvulae 

 conniventes, or valves of Kirkring. These folds vary from one-fourth to 

 half an inch in width and extend one-half to two-thirds of the distance around 

 the interior of the bowel. Each valve consists of two layers of the mucous 

 membrane permanently united by fibrous tissue. It is believed that the 

 valves retard to some extent the passage of the food through the intestine 

 and present a greater surface for absorption. 



Blood-vessels, Nerves, and Lymphatics. The blood-vessels of 

 the small intestine, which are very numerous, are derived mainly from 

 the superior mesenteric artery. After penetating the intestinal walls the 

 smaller vessels ramify in the submucous coat and send branches to the 

 muscle and mucous coats, supplying all their structures with blood. After 

 circulating through the capillary vessels the blood is returned by small 

 veins which subsequently unite to form the superior mesenteric vein, which, 



