DIGESTIVE ORGANS OF MAMMALIA. 223 



dual transformation into a frog, this canal be- 

 comes much reduced in its length ; so that when 

 the animal has attained its perfect form, it 

 makes but a single convolution in the abdominal 

 cavity. 



A similar correspondence exists between the 

 length of the canal, and the nature of the food 

 in the class of Birds. At the termination of the 

 small intestine there are usually found two caeca, 

 which in the gallinaceous and the aquatic fowls, 

 are of great length : those of the ostrich contain 

 in their interior a spiral valve. Sir E. Home is 

 of opinion that in these animals the functions 

 of the pyloric portion of the stomach are per- 

 formed by the upper part of the intestine. 



In the intestines of the Mammalia contrivances 

 are employed with the apparent intention of 

 preventing their contents from passing along too 

 hastily : these contrivances are most effectual in 

 animals whose food is vegetable, and contains 

 little nourishment, so that the whole of what the 

 food is capable of yielding is extracted from 

 them. Sir E. Home observes that the colon, or 

 large intestine of animals which live upon the 

 same species of food, is of greater length in 

 proportion to the scantiness of the supply. Thus 

 the length of the colon of the Elephant, which 

 inhabits the fertile woods of Asia, is only 262 

 feet ; while in the Dromedary, which dwells in 

 the arid deserts of Arabia, it is 42. This con- 



