INTRODUCTORY : THE ANIMAL ORGANISM 7 



considerably in composition from that of a living man, and 

 the difference is increased by its digestion. In the course 

 of incorporation the food has therefore to undergo 

 chemical changes by which it is converted into the 

 substances which compose the body, and these changes 

 it undergoes by the activity of the living matter itself. 

 That is to say, the living substance has the power of 

 making, out of unlike materials, additional matter of its 

 own composition. The process by which this is done is 

 known as assimilation. 



Both absorption and assimilation are processes in which 



work is done, and therefore involve the use of 



(c) Results of _ energy, but their net result is to add to the 



0) Repair. ' number of complex molecules, and therefore to 



the amount of energy, in the body. Part, at 



least, of the new complex material is used in the repair 



of the waste caused by disintegration. 



It will be seen that disintegration and its complementary 

 m t b ii assimilation constitute a series of chemical 



changes, continually taking place in the body, 

 whereby there is kept up a continual evolution of energy. 

 These changes, regarded as a whole, are known as 

 metabolism, the disintegrative changes being known as 

 katabolism and the assimilative as anabolism. 



The incorporation of new material has, however, a further 

 effect than the mere repair of waste. Throughout the body 

 of a young animal, and in such parts as the 

 w " roots of the hair and nails even in age, in- 

 corporation takes place in excess of waste, so that growth 

 occurs. Both in repair and in growth the new material is 

 not added in layers to a surface, like that which is taken up 

 by a crystal, but is placed between the existing particles, as 

 a substance is taken into solution. Growth, moreover, is 

 a very complex architectural process in which the intricate 

 structure of the body is built up out of many materials. 

 The meaning of the growth of the body will be better 

 understood when we have studied reproduction. 



The outward expression of the incorporation of food is 

 to be found, as we have seen, in growth. Now growth is 

 followed, sooner or later, by reproduction. That is to say, 

 a portion of the body breaks off to form a new individual 



