CHAPTER V 

 THE FROG (continued] : WASTE AND REPAIR OF SUBSTANCE 



THE DIGESTIVE ORGANS NUTRITION 



Waste and Repair. The effects of prolonged mus- 

 cular exertion are familiar to everyone. Sooner or later 

 sensations of fatigue, hunger, and thirst are produced, 

 accompanied by a loss of weight. Indeed, however 

 little exertion we make and however often we feed, our 

 weight always goes down between meals and rises again 

 when we take food. The loss of substance, of which 

 the diminution in weight is the index, takes place largely 

 in the form of perspiration, a fluid consisting of water 

 with certain organic and inorganic matters in solution. 

 A further loss is due to the air breathed out from the 

 lungs ; this is always moist, i.e., contains a good deal of 

 water, and is further distinguished by containing a con- 

 siderable volume of carbonic acid gas or carbon dioxide 

 (CO 2 ). Besides these two constant sources of loss, 

 there is an intermittent loss in the urine, which consists 

 of water containing certain matters in solution, the 

 most characteristic of which are two complex substances 

 called urea (CON 2 H 4 ) and uric acid (C 5 H 4 N 4 3 ). Both 

 of these, as well as carbon dioxide, act as poisons if 

 allowed to remain in the system. Lastly there is an 

 intermittent source of loss in the waste matters or 

 faeces which are passed out from the intestine. 



66 



