76 THE VERTEBRATE ANIMAL: METABOLISM 



production of heat, and certain waste products. Growth occurs 

 in living organisms when assimilation goes on at a more rapid 

 rate than dissimilation. Organisms grow by the method of intus- 

 susception, or growth from within; they differ in this respect from 

 inorganic crystals, which grow by the process of accretion, or the 

 deposition of additional material on the surface of that already 

 present. Metabolic reactions are catalyzed by the enzymes typical 

 of protoplasm, and are made possible by the fundamental physico- 

 chemical organization of that system. The results of the reac- 

 tions of the cells of the body and their interactions are apparent 

 as the normal functioning of the animal, and all bodily func- 

 tions may be understood, in the final analysis, by reference to 

 metabolism. 



Irritability is the term applied to the capacity which protoplasm 

 possesses of responding by internal reaction to a stimulus, or 

 change in its environment. It is well known that non-living 

 things react in certain definite ways to certain changes in their 

 surroundings, as when a metal undergoes a particular amount of 

 contraction for a particular decrease in temperature. The reac- 

 tions of living organisms, however, involve both chemical and 

 physical factors, and, as we shall see later, are so much more com- 

 plicated as to be distinctive. 



Rs'produciion is that capacity upon which depends the continuity 

 of living organisms. Under varied conditions, certain parts of 

 organisms detach themselves, and, either alone or after union 

 with protoplasm of another organism of the same kind, give rise to 

 a new individual in all essential respects like the parent or parents. 

 Nothing comparable with reproduction is known to occur among 

 inanimate objects. 



Assimilation. — Nutrition. — A discussion of nutrition involves 

 consideration of the needs that food must supply and of the nature 

 of food. Food is necessary in order to enable the protoplasmic 

 system to replace those constituents that are constantly being 

 removed in dissimilative reactions, some of which are for the 

 transformation of energy and production of heat; that is, food 

 is necessary for maintenance or repair. In young organisms, 

 food also supplies the materials for growth. Food, therefore, 

 is the source of the raw materials necessary for the normal func- 

 tioning of the protoplasmic system. 



Consequently, the nature of food becomes clear. It must 



