232 



CHORDATE ANATOMY 



Small Intestine. The small intestine extends, gradually diminishing 

 in diameter, from the pylorus to the ileocolic valve of the colon. The 

 small intestine is distinguished not only by its smaller diameter but also 

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

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

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

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



"VILLI 



---PLICA CIRCULARIS 



S^- LYMPHATIC 



BLOOD VESSELS 



■VILLUS 



CRYPT 



MUSCULARIS MUCOSAE 



■SUBMUCOSA 



CIRC. MUSCLE 



"LONG. MUSCLE 

 -SEROSA 



Fig. 2i8. — A longitudinal section of the human jejunum, showing in cross section one 

 of the circular plicae (valvulae conniventes). XiS- 



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. 



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

 intestine, contains numerous transverse crescentic folds, the plicae or 

 valvulae conniventes, covered with large villi. See Fig. 218. These plicae 

 serve to retard the passage of food and also to increase the absorptive 



