46 PRINCIPLES OF ANIMAL BIOLOGY 



changes occur regularly during cell division, the protoplasm being firmer 

 at the beginning of division, more liquid (less viscous) later on. 



Metabolism. — The protoplasm of a cell carries on all the general 

 processes of any living body. Within it occurs a multitude of complex 

 chemical reactions by which the protoplasm maintains and renews itself 

 and produces more protoplasm. Protoplasm digests food and for this 

 process secretes various chemical substances. When food is broken down 

 into simpler substances during digestion, it is absorbed and built up into 

 the living substances itself or perhaps is combined with oxygen for the pro- 

 duction of heat and motion. Protoplasm also respires, gets rid of waste 

 materials by the process of excretion, grows, is capable of movement, 

 and responds to changes in external conditions, or exhibits irritability. 

 The chemical processes involved in all these activities of protoplasm are 

 included under the term metabolism. 



Metabolism may be defined as the sum of all the chemical and physical 

 processes carried on within the protoplasm. It consists of two phases, 

 namely, the constructive phase or anahoUsm and the destructive phase 

 or catabolism. Anabolism includes all the processes concerned in the 

 growth and repair, or upbuilding, of protoplasm. It includes all processes 

 by which substances are transformed into reserves of food. Catabolism 

 includes all those processes opposed to anabolism. These are the proc- 

 esses by which protoplasm is broken down and the waste products 

 eliminated. Both anabolism and catabolism are continous processes. 

 As long as anabolic processes are in excess of catabolic processes, growth 

 occurs; but when catabolic processes are in excess a diminution in size 

 takes place. 



So far as metabolism of animals relates to food, it pursues the following 

 cycle in the economy of living things collectively. Organic food is first 

 made out of inorganic matter through the process of photosynthesis in 

 plants. These organic substances become the food of animals which 

 arc unable to subsist on inorganic food. The animals digest these foods, 

 and from the simpler digestive products build up their protoplasm 

 through the process of assimilation. To supply the energy required for 

 all this work the animal must secure oxygen by respiration. Waste 

 materials produced along the way are eliminated by excretion, and useful 

 products accessory to the general processes are elaborated by secretion. 

 One of the products of the food cycle is commonly growth. All these 

 processes are part of metabolism; they are described in the next seven 

 so(;tions. 



Photosynthesis. — Tlu; things which i)hints may take in are water 

 and salts from tiic soil, and oxygen and carbon dioxide (COo) from the 

 air (or water, in the case of aquatic plants). The fii-st three of these 

 are utilized in about the same way in plants as in animals. The carbon 



