I94 TEXT-BOOK OF PHYSIOLOGY. 



and discharge of the intestinal fluids: e. g., pancreatic juice, bile, 

 and intestinal juice. Inasmuch as these fluids are alkaline in re- 

 action they exert a neutralizing and precipitating influence on vari- 

 ous constituents of the chyme. As soon as this has taken place, 

 gastric digestion ceases and those chemic changes are inaugurated 

 which eventuate in the transformation of all the remaining undigested 

 nutritive materials into absorbable and assimilable compounds which 

 collectively constitute intestinal 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 beginning of the large intestine. 



The intestine consists of four coats: viz., serous, muscular, sub- 

 mucous, and mucous. 



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

 of the general peritoneal membrane. 



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~~upf)er 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. 



The submucous coat consists of areolar tissue and serves to unite 

 the muscular 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 YJJJj^ Throughout its entire 

 extent, with the exception of the lowerportion of the ileum, 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-jf It is believed that the valves retard to some extent "the pas- 

 sage of the food through the intestine and present a greater surface 

 for absorption.^ 



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



