102 THE BIOLOGY OF AN ANIMAL. 



cell takes from the common carrier, the blood, only such ma- 

 terials as it needs, leading a somewhat independent life as to its 

 own nutrition. It co-operates with other cells* under the direc- 

 tion of the nervous system (co-ordinating mechanism), but to a 

 great degree is independent in its choice of food- -just as a sol- 

 dier in a well-fed army obeys orders for the common good, but 

 yet takes only what lie chooses from the daily ration supplied to 

 all. 



What takes place within the cell upon the entrance of the 

 food is almost wholly unknown, but somehow the food-matters, 



/ 



rich in potential energy, are built up into the living substance 

 probably by a series of constructive processes culminating in pro- 

 toplasm. Alongside these constructive processes (anabolism) a 

 continual destructive action goes on (katabolism) ; for living mat- 

 ter is decomposed and energy set free in every vital action, and 

 vitality or life is a continuous process. It must not be supposed, 

 however, that either the synthetic or the destructive process is a 

 single act. Both probably involve long and complicated chemi- 

 cal transformations but the precise nature of these changes is at 

 present almost wholly unknown. It is certain that the destruc- 

 tive action is in a general way a process of oxidation effected by 

 aid of the free oxygen taken in in respiration. We may be 

 sure, however, that it is not a case of simple combustion (i.e., the 

 protoplasm is not tc burnt"). It is more probably analogous to 

 an explosive action, the oxygen, first entering into a loose asso- 



i fj o o 



ciation with complex organic substances in the protoplasm, and 

 then suddenly combining with them under the appropriate stim- 

 ulus to form simpler and more highly -oxidized products. Of 

 the precise nature of the process we are quite ignorant. 



Outgo. Just as the income of the animal represents only the 

 first term in a series of constructive processes, so the outgo is 

 the last term of a series of destructive actions of which we really 

 know very little save through their results. The outgo is shown 



/ o o 



in the accompanying table. 



Both energy and matter leave the cells, and finally leave the 

 body the former as heat, work done, or energy still potential 

 (in urea and other organic matters) ; the latter as excretions, 

 which diffuse freely outwards through the skin and nephridial 

 surfaces. 



