33 Biological Chemistry. 



for animal heat and locomotion. Synthetic processes also 

 occur in the animal body during the formation of new 

 tissues in the adolescent individual and in the repair of 

 mature tissues in the adult, but such processes are quanti- 

 tatively of subordinate character when compared with the 

 amount of degradative action necessary for the maintenance 

 of bodily heat and of locomotion. 



If the products derivable from excreta and the dead 

 bodies of animals were returned to the atmosphere and the 

 soil they could be utilized for plant growth, and thus 

 supply fresh material for animal nutrition. The same 

 matter would thus pass alternately from animal to plant, 

 and it would at first sight appear that, as regards carbon 

 and nitrogen at any rate (the other materials such as 

 phosphorus and sulphur having been omitted from the dis- 

 cussion), there need never be a dearth of animal foodstuff 

 so long as the rays of the sun were available for the 

 supply of the necessary energy for synthesis. In actual 

 practice it is found, however, that the passage of matter 

 from plant to animal and back again is not accomplished 

 without considerable loss, especially as regards nitrogenous 

 material. This nitrogenous loss can be accounted for in 

 various ways. As already stated, ammonia is one of the 

 chief products of the bacterial action on organic nitrogenous 

 substances, and considerable quantities of this substance 

 escape into the air, and are thus lost to the soil. Further- 

 more, nitrates are readily washed out of the soil by the 

 rain and rendered unavailable as plant food. Small 

 amounts of nitrogen are also set free from complex nitro- 

 genous compounds by the action of bacteria. The general 

 cycle of nitrogenous changes has been represented by 

 Dr. E. J. Russell (" Fertility of the Soil," Cambridge, 1913, 

 p. 25) by the following diagram : 



