CHAP. V WASTE AND REPAIR 67 



allowed to remain in the system. Lastly there is an inter- 

 mittent source of loss in the waste matters or faeces which 

 are passed out from the intestine. 



These losses are made good in two ways. Firstly, by 

 breathing, in which process we constantly inhale pure air 

 and replace the poisonous carbonic acid by oxygen. Secondly 

 by eating and drinking, by which, at intervals, we make good 

 the loss of solids and liquids. Just as a clock is constantly 

 running down and has to be wound up in order to keep 

 it going, so our bodies run down by loss of substance 

 between meals, and require to be wound up by the repair of 

 substance which results from food and drink. 



The same thing is true of the frog. Every one of its 

 numerous and often vigorous movements is done at the 

 expense of a certain waste of substance. The various 

 tissues of the body are constantly undergoing a process of 

 wear and tear, expressed, not as in machines of human 

 construction, by a wearing away of surfaces and a loosening 

 of bolts and screws, but by a slow and almost imperceptible 

 dwindling, the lost material being carried off principally in 

 the form of water, carbon dioxide, and urea or some allied 

 compound containing nitrogen. 



Food of the Frog : general characteristics of the Diges- 

 tive Process. — As we have seen, the food of the frog 

 consists of worms, slugs, insects, and the like. These it 

 catches and swallows whole, the stomach often becoming 

 immensely distended with numbers of captured animals. 

 After remaining for some time in the stomach the carcases 

 are found to have undergone a marked change. Their soft 

 parts become softer and slimy and finally semi-fluid, and in 

 this way the food undergoes gradual disintegration. The 

 quantity of food in the stomach decreases, some of it is ' 

 passed into the intestine, which it traverses from duodenum 



F 2 



