384 TEXTBOOK OF ZOOLOGY 



Classes of Digestive Enzymes 



1. Diastases or diastatic enzymes — split carbohydrates 



(a) Ptyalin in saliva 



(b) Amylase in pancreatic juice 



(c) Glycogenases — liver and muscles 

 Converts glycogen to glucose 



2. Lipase or lipolytic enzyme — splits fats 

 (a) Steapsin in pancreatic juice 



3. Inverting enzymes — convert disaccliarids to the less complex monosaccharids 

 (simple sugars) — intestinal juice 



(a) Maltase 



(b) Lactase 



(c) Sucrase (invertase) 



4. Proteases or proteolytic enzymes — split complex proteins 



(a) Pepsin in gastric juice 



(b) Trypsin in pancreatic juice — functions in small intestine 



(c) Erepsin in intestinal juice 



5. Clotting or coagulating enzyme 

 (a) Eennin in gastric juice 



In higher Metazoa digestion is accomplished principally extra- 

 cellularly through secretion of enzymes by certain groups of cells. 

 Such systems consist of: (1) an alimentary canal proper; and (2) 

 associated glands which discharge digestive juices into it. The 

 relative length of this canal varies considerably depending on the 

 habitual diet of the organism. In carnivores (flesh-eaters), such as 

 cats and dogs, it is from three to five times as long as the body; 

 while in herbivorous forms (plant-eaters), such as horses and cows, 

 it is over twenty times as long as the body. The length of the 

 human digestive tract is approximately ten times the length of the 

 body. The relative proportion of the internal absorptive surface of 

 the alimentary canal to the external surface of the body is signifi- 

 cant. In carnivorous animals it is about one-half the area of the 

 skin while in herbivorous animals it is about twice the area of the 

 skin. 



The process of digestion in man is quite well understood, and 

 it is fairly typical and general because of the omnivorous food 

 habits. The action of the several enzymes produced by different 

 glands is a very essential part of the process. The digestion of all 

 organic food materials is brought about by hydrolysis in the same 

 kind of chemical change. In hydrolysis the large molecules of pro- 

 tein, carbohydrate, or fat first combine with water and then split into 

 simpler products. Some foods may require more than one such 



