170 ESSENTIALS OF ZOOLOGY 



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 significant. 

 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 

 splitting. The splitting of the disaccJiaride, maltose, will serve as 

 an example of this process : 



Ci2 H22 Oil + H2 O =: 2 Ce H12 Og 

 (Malt sugar) (Water) (Glucose) 



The two molecules of glucose formed are in a form for ready 

 absorption. 



Gastric Digestion. — The tubular gasttnc glands located in the 

 mucous layer of the stomach secrete the acid gastric juice which is a 

 solution of 0.2 to 0.5 per cent hydrochloric acid and two important 

 enzymes, pepsin and rennin. The pepsin when present in the acid 

 medium brings about the splitting of complex proteins into inter- 

 mediate proteoses and peptones. Rennin causes the casein in milk 

 to coagulate. This is the first step in its digestion. It is claimed by 

 some that emulsified fats, such as cream, are partially digested by a 

 gastric lipase. The digesting mass or chyme in the stomach is con- 

 tinually churned and mixed by muscular activity of the walls. When 

 it becomes saturated (0.4 per cent) with acid and has been reduced 

 to the consistency of soup, it is discharged through the pylorus. 



Intestinal Digestion. — When the chyme is ejected through the 

 pylorus into the duodenum, the hydrochloric acid stimulates cer- 



