28 THE FUNCTIONS OF ANIMALS. 



a cell consists of a relatively stable living framework, and of a change- 

 ful content enclosed by it. 



Now, many physiologists regard the framework as the genuine living 

 protoplasm, and the contents as the material upon which it acts. " The 

 framework is the acting part, which lives, and is stable ; the content is 

 the acted-on part, which has never lived, and is labile, that is, in a 

 state of metabolism or chemical transformation." This view naturally 

 leads those who adopt it to regard protoplasm as a sort of ferment acting 

 on less complex material which is brought to it, and which forms the 

 really changeful part of each cell. You will remember that the strange 

 characteristic of a ferment is that it can act on other substances without 

 being itself 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 these respects, 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 destructive 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 disruptive metabolism. 



Anabolism 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 two-fold 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 dis- 

 assimilation. 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 sensa- 

 tion of white, or of red, or of yellow, it is supposed that in one of the 

 three kinds of visual substance disassimilation is preponderant. When 

 we have a sensation 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 



