THE DIGESTIVE SYSTEM 



313 



by the presence of numerous villi which cover its inner surface and give 

 it a velvety appearance. Somewhat arbitrarily thiee regions are dis- 

 tinguished, duodenun, jejunum, and ileum. The duodenum, the anterior 

 portion of the small intestine, averages about nine inches in length,, and 

 is characterized by the presence of tubulo-acinous glands located in the 

 submucosa and known as duodenal or Brunner's glands. The duodenal 

 glands secrete an alkaline mucus which neutralizes the acidity of the 

 food which enters the duodenum from the stomach. Zymogenic cells 

 are also found in the duodenal mucosa. 



PLEXUS OF AUERBACH^ 



PLEXUS OF MEISSNERIk 



PERITONEUM 



Fig. 262. — A stereogram of a portion of the small intestine, showing the arrangement 

 of sympathetic neurons in the plexuses of Meissner and Auerbach. Motor cells are 

 shown in black, sensory cells with white nuclei. (Redrawn after Kahn's "Das Leben 

 Des Menschen," W. Keller & Co.) 



The jejtmimi, which forms two-fifths of the remainder of the small 

 intestine, contains numerous transverse crescentic folds, the valvulae 

 conniventes, covered with large villi. These serve to retard the passage 

 of food and also to increase the absorptive surface. In the ileum, the 

 crescentic folds disappear, and villi become smaller and more scattered. 

 Lymph nodules are abundant in the tunica propria. 



The four layers of tissue characteristic of the alimentary canal are 

 present in the small intestine. The mucous epitheHum is of the simple 

 columnar sort, and each cell has a striated border on its inner free surface. 

 Throughout the entire length of the intestine are numerous tubular 

 mucus-secreting glands, perpendicular to the surface of the intestine, 

 the intestinal glands or crjrpts of Lieberkuehn. Goblet-shaped cells 

 distended with mucus are abundant in the walls of these glands. The 

 secretions of these glands are said to stimulate peristalsis of the intestine 

 as well as lubricate its surface. 



