THE LIVING SUBSTANCE 19 



We have seen that life amongst animals manifests itself in 

 various ways, viz. by movement and by different kinds of 

 secretion. Now each of these different ways implies that 

 a different kind of chemical decomposition is going on in the 

 cell, and the membranes separating the cells from one another 

 derive their importance from the fact that they regulate the 

 passage of materials in and out of the cells. Each cell is in 

 fact a chemical factory, and the possession of cellular structure 

 by an animal is really a sign of the number and variety of 

 chemical processes going on in it. We have so far alluded 

 chiefly to the decomposition or breaking down of complex 

 chemical substance, which occurs in the cell whether the 

 result of this be movement or secretion ; but, as we have 

 already hinted, a concurrent process of building up new living 

 substance out of the food is also going on, else life would soon 

 come to an end. This building up, which as we have seen 

 is effected by the aid of the chromatin, is termed anabolism, 

 whilst the decomposition is termed catabolism, and metabolism 

 is used to denote the sum total of all the chemical processes 

 occurring in the cell. 



Neither anabolism nor catabolism can take place in the 

 absence of oxygen, which must be constantly supplied to 

 the living substance, as it is constantly being used up in the 

 process of oxidation, which constitutes the principal item in the 

 process of decomposition and is also an element in the processes 

 of anabolism. The absorption of this necessary oxygen by the 

 living substance is termed respiration. To sum up, we shall 

 form the justest conception of the activities of living substance, 

 if we regard it as in a continual state of burning. Life is 

 a fire, as Buddha said long ago, and the scriptural story of the 

 bush which burned and was not consumed exactly expresses 

 the modern conception of the nature of life. 



If we have once grasped this conception, certain conclusions 

 will at once suggest themselves. The ordinary coal-fire 

 burning in our grates would soon be extinguished if the ashes 

 which result from the oxidation of the coal were not con- 

 stantly removed. So it is with life : the products of decom- 

 position, when they are so far broken down that they cannot 

 be used again to build up new protoplasm by the addition 

 of materials froin the, food, must be removed, or else they 



