12 THE ANIMAL ORGANISM 



has been depleted and replaced by carbon dioxide, just as it 

 would be if a candle had been burnt in it. This loss of carbon 

 dioxide and the intake of oxygen which usually accompanies it 

 are characteristic of living animals. This gaseous exchange is 

 sometimes, especially in medical works, called respiration, 

 though this word is also used for the whole series of processes, 

 including the chemical reactions, by which energy is obtained. 

 In man and animals like him, gas exchange takes place through 

 the lungs, in breathing. If the breath be tested, it will be found 

 to have undergone the same changes as the air in a vessel in 

 which an animal has been stifled. Fishes and other aquatic 

 animals use the oxygen which is held in solution in the water 

 in which they Hve. They usually respire by means of structures 

 known as gills, which offer to the water a large surface upon 

 which gases can be exchanged ; of these we shall consider examples 

 when we study the crayfish and dogfish. The necessity for 

 renewing by aeration the dissolved oxygen in the water of an 

 aquarium is due to the respiration of the inhabitants. The nitro- 

 geneous w^aste matters may be identified by chemical analysis 

 in excreta such as the urine of man. The formation of water is 

 less easily demonstrated, because the bulk of the water lost to 

 the body has been taken in as such through the mouth to perform 

 certain indispensable functions, one of which is the washing out 

 of nitrogenous waste substances, which are harmful, but a careful 

 comparison of the quantities of water which enter and leave the 

 body shows that more goes out than has entered. 



COMPLICATIONS I V^EAR AND TEAR 



While the chemical processes by which energy is liberated in the 

 body are all of the general character which we have just outlined, 

 they are nevertheless varied in detail and extremely complicated. 

 Oxidation takes place, not by single reactions between oxygen and 

 the substances which are ultimately oxidised, but by chains of 

 reactions. These cannot here be described more fully. It must, 

 however, be mentioned that, besides those processes in which by 

 the disintegration of certain substances energy is liberated, there 

 is involved a considerable loss of material by wear and tear of 

 the more permanent part of the living matter in which the oxidised 

 substances are contained. 



