ANABOLISM AND KATABOLISM. 23 



affected by the changes which it produces, and that it can 

 go on doing so continuously, with a power which has no 

 direct relation to its amount. In many ways, therefore, 

 living matter resembles a ferment. 



Somewhat different, however, is another idea, that the 

 protoplasm is itself the seat of constant change, that it is 

 constantly being unmade and remade. On the one hand, 

 more or less crude food passes into life by an ascending 

 series of assimilative or constructive chemical changes with 

 each of which the material becomes molecularly more 

 complex and more unstable. On the other hand, the 

 protoplasm, as it becomes active or a source of energy, 

 breaks down in a descending series of disruptive or de- 

 structive chemical changes ending in waste products. 



The former view, which considers protoplasm as a sort of 

 ferment, restricts the metabolism to the material on which 

 the protoplasm acts. The second view regards protoplasm 

 as the climax or central term of the constructive and dis- 

 ruptive metabolism. 



Anabolistti and Katabolism. — All physiologists are agreed 

 that in life there is a twofold process of waste and repair, of 

 discharge and restitution, of activity and recuperative rest. 

 But there is no certainty as to the precise nature of this 

 twofold process. 



In your future physiological studies, you will have to consider the 

 power that our eyes have of appreciating those different kinds of light 

 which give rise to sensations of colour. It was in studying these that 

 Professor Hering was led to an interesting theory of living matter. He 

 supposes that there exist in or about the retina three different " visual 

 substances," which we may call A, B, C. He supposes that each of these 

 is continually undergoing one of two kinds of metabolism. It is either 

 being built up by assimilation, or it is being broken down in disassimila- 

 tion. He supposes that each of these substances is affected by two kinds 

 of light, and that these two kinds of light have opposite influences on 

 the metabolism of the substance. When we have a sensation of white, 

 or of red, or of yellow, it is supposed that in one of the three kinds of 

 visual sulDstance disassimilation is preponderant. When we have a sen- 

 sation of black, or of green, or of blue, it is supposed that in one of the 

 three kinds of visual substances assimilation is preponderant. Excess 

 of disassimilation in A, gives us the sensation of white ; excess of assimil- 

 ation of A, gives us the sensation of black ; and similarly with red and 

 green for B, with yellow and blue for C. 



But generalising from his studies on colour sensation, Hering was led 

 to regard all life as an alternation of two kinds of activity, both induced 



