116 STUDIES IN PHYSIOLOGY 



te r ri-um. After being strained between the leaflike plates 

 of mucous membrane in this sac, it enters the fourth and 

 last chamber, called the ab-o-ma'sum. This is the real diges- 

 tive portion of the stomach. The other sacs should be 

 regarded, like the crops of birds, as enlargements of the 

 esophagus that serve as storage reservoirs. The rennet 

 used in making cheese is prepared from the lining of the 

 abomasum of a calf. The wall of the rumen and reticulum 

 of cud-chewing animals is eaten in the form of tripe. The 

 intestines of the ox have the astonishing length of one hun- 

 dred and fifty feet. Since the food of the animal is wholly 

 vegetable, a much longer time is required for its digestion, 

 and hence the great extent of the intestines. Carnivorous 

 animals, on the other hand, have a relatively short alimen- 

 tary canal. 



Comparison of the Digestive Organs Studied. The striking 

 characteristic of the digestive apparatus of the earthworm 

 is its simplicity. A straight tube extends from one end of 

 the body to the other, with several enlargements in which 

 certain processes are carried on. Digestive glands corre- 

 sponding to the liver and pancreas of man are altogether 

 wanting. In the other animals that we have considered an 

 increasing complexity of structure is seen until we come to 

 the highly specialized alimentary canal of the ruminants. 

 In every example studied the digestive organs have become 

 specially fitted to digest the kind of food the animal eats. 



