CHAP, v WASTE AND REPAIR 67 



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

 breathing, in which process we constantly inhale pure 

 air and replace the poisonous carbon dioxide 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 be- 

 coming 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 to rectum, and 

 certain portions of it are finally ejected from the vent 

 in the form of faeces. 



F 2 



