22 THE FUNCTIONS OF ANIMALS. 



Apart from any theory, it is certain that waste-products 

 are formed when work is done, and that living animals have 

 a marvellous power of rapid repair, of ceaselessly changing, 

 and yet remaining more or less the same. Theory begins 

 when we attempt to make the general idea of waste and 

 repair more precise. In the study of " protoplasrn," both 

 morphologist and physiologist have reached their strict 

 limits. Further analysis becomes physical and chemical, 

 and ends in the confession that protoplasm is a marvellous 

 form of matter in motion or a subtle kind of motion of 

 which we can form only a very vague conception. 



What is known in regard to the structure of protoplasm 

 does not help the physiologist very much. As we shall 

 afterwards see, the microscopists discover an intricate net- 

 work which pervades each unit of living matter, but no 

 physiologist dreams of explaining the life of a cell in terms 

 of its microscopically visible structure. Yet, as Professor 

 Burdon Sanderson says, " we still hold to the fundamental 

 principle that living matter acts by virtue of its structure, 

 provided the term structure be used in a sense which carries 

 it beyond the limits of anatomical investigation, i.e., beyond 

 the knowledge which can be attained either by the scalpel 

 or the microscope." But in the end this means that living 

 matter acts in virtue of its peculiar qualities, its characteristic 

 motion, of which we can form only a hypothetical con- 

 ception, and can give no scientific explanation. 



One general idea, however, the study of structure has 

 suggested, which the conclusions of physiologists corroborate. 

 ■ This idea is, that a cell consists of a relatively stable living 

 framework and of a changeful 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 



