i86 TEXT-BOOK OF PHYSIOLOGY 



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 de- 

 veloped 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 fol ds 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. 



