i86 TEXT-BOOK OF PHYSIOLOGY 



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 penetrating 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, uniting with the splenic and gastric veins, forms the portal 

 vein. The nerve elements in the intestinal wall consist of two plexuses, one 

 (Auerbach's) lying between the muscle coats, the other (Meissner's) lying in the 

 submucous coat. To this nerve net, composed of nerve cells and nerve processes, 

 found in connection with the muscle coats of the stomach, of the small and of the 

 large intestine as well, the term myenteric plexus has been given. The lymphatics, 

 which originate in the mucous and muscle coats, are very abundant. They unite 

 to form those vessels seen in the mesentery and empty into the thoracic duct. 



Intestinal Glands. The gland apparatus of the intestine by which the 

 intestinal juice is secreted consists of the duodenal (Brunner's) and the intestinal 

 (Lieberkuhn's) glands. 



The duodenal glands are situated beneath the mucous membrane and open by a 

 short wide duct on its free surface. They are racemose glands lined by nucleated 

 epithelium. The secretion of these glands is clear, slightly viscid, and alkaline. 

 Its chemic composition and functions are unknown. 



The intestinal glands or follicles are distributed throughout the entire mucous 

 membrane in enormous numbers. They are formed mainly by an inversion of the 

 mucous membrane and hence open on its free surface. Each tubule consists of a 

 thin basement membrane lined by a layer of spheric epithelial cells, some of which 

 undergo distention by mucin and become converted into mucous or goblet cells. 

 The epithelial secreting cells consist of granular protoplasm containing a well- 

 defined nucleus. The intestinal follicles constitute the apparatus which secretes 

 the chief portion of the intestinal juice. 



The surface of the mucous membrane presents throughout its entire extent 

 fine filiform or conical processes termed villi. The structure and function of the 

 villi will be considered in connection with the absorption of food materials. 



The pancreas and liver are developed, during embryonic life, from the walls of 

 the intestine and are anatomically and physiologically associated with it. 



PANCREAS 



The Pancreas. -This gland is long, narrow and flattened and is situated 

 deep in the abdominal cavity, lying just behind the stomach. It measures 



