SO MANUAL OF ELEMENTARY ZOOLOGY 



prey, which they are the better adapted to do by being 

 directed backwards. In the stomach the food 

 meets the gastric juice, secreted by the glands 

 of the mucous membrane of the stomach. This juice is 

 acid and contains an organic substance known as pepsin, 

 which turns proteins into more soluble materials. Here 

 the food is killed, disinfected of harmful bacteria by the 

 acid, and partly dissolved by the action of the pepsin. 

 Pepsin is an example of an exceedingly important class 

 of substances, found in living bodies, which resemble the 

 " catalytic agents " of the chemist in having the power of 

 bringing about changes in other substances without them- 

 selves undergoing change, and of doing this even though 

 they be present in very small quantities in a large mass of 

 the substance acted upon. These agents are called ferments 

 or enzymes. From time to time a ring of muscle known as 

 the pyloric sphincter, which guards the opening of the 

 duodenum, relaxes and lets through partly digested food 

 into the intestine, where it meets three alkaline juices, the 

 bile, the pancreatic juice, and a juice known as the succus 

 entericus, which is secreted by the intestinal wall. Of these 

 the pancreatic juice is the most powerful, dissolving all 

 three classes of organic food-stuffs (proteins, fats, and 

 carbohydrates, see p. 6), each by means of a distinct 

 enzyme. The action of the bile and succus entericus is 

 subsidiary to that of the pancreatic juice, and the bile is 

 also partly an excretion. The food thus rendered diffusible 

 is absorbed by the activity of the intestinal lining. The 

 movement of the food along the alimentary canal is brought 

 about by the contraction of a muscular layer in its wall 

 (see Fig. 45), waves of contraction passing down its length. 

 This process is called peristalsis. Finally the undigested 

 portion of the food passes through the rectum and cloaca 

 to the exterior as the faeces. 



The secretion of bile is not the only function of the liver. 

 That organ is the great chemical workshop and 

 the uver 8 ° f storehouse of the body. In it a part of the 

 excess of carbohydrate and fatty food taken 

 during the summer is stored for use during the winter sleep 

 and the breeding season. The fat is stored in droplets, the 

 carbohydrate in the form of glycogen or animal starch, which, 



