CHAPTER V, 

 THE FROG {continued) : waste and repair of substance — 



THE DIGESTIVE ORGANS — NUTRITION. 



Waste and Eepair. — The effects of prolonged muscular 

 exertion are familiar to every one. Sooner or later sensa- 

 tions of fatigue, hunger, and thirst are produced, accom- 

 panied 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 j this is always moist, /'.f., 

 contains a good deal of water, and is further distinguished 

 by containing a considerable volume of the gas carbon 

 dioxide, or carbonic acid (CO^). 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 (CONgHj) and uric acid (C^H^N^O^). 

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



