DIGESTION AND RESPIRATION 



523 



numerous minute, finger-shaped villi containing blood capillaries and 

 small lymphatic vessels. The villi protrude into the lumen and increase 

 the intestinal surface manyfold. Intestinal glands lie at their base. 

 Many mucus-producing goblet cells are present in the lining epithelium 

 and their secretion helps to lubricate the food and to protect the lining 

 of the intestine. 



When food enters the duodenum the liver and pancreas pour their 

 secretions into the gut and a series of reactions begins. Bile salts emul- 

 sify the fats, and lipase, produced by the pancreas and intestinal glands, 

 hydrolyzes them into fatty acids and glycerol. The pancreas also se- 

 cretes trypsinogen \vhich, in the presence of enterokinase secreted by 

 the intestine, is converted into trypsin. Chymotrypsinogen secreted 

 by the pancreas is changed into chymotrypsin in the presence of trypsin. 

 Trypsin and chymotrypsin split proteins and large polypeptides (pro- 

 teoses and peptones) into smaller groups of amino acids known as 

 peptides. Peptides are further split to individual amino acids by vari- 

 ous peptidases secreted by the pancreas and intestinal glands. Amylase 

 secreted by the pancreas, and to a lesser extent by intestinal glands, 



Table 3. DIGESTIVE ENZYMES 



The less important enzymes of a given region have been put in parentheses. 



