OF THE CELL THEORY 167 



katabolic process, as the water falls from the crest 

 back into the basin, or as the living brain, muscle, 

 etc., become broken down into the various excretory 

 products. The up-hill or anabolic processes are 

 synthetic and require or absorb energy. The 

 water of the fountain will not rise of itself, but 

 must be forced above the level of the water of the 

 basin ; and similarly the digestion and assimilation 

 of food, the building-up of protoplasm, demand 

 an expenditure of energy on the part of the 

 animal. 



On the other hand, the downhill or katabolic 

 processes are analytic and are sources of energy ; 

 and just as the falling water of the fountain may be 

 used to drive a wheel or other machine, so in the 

 living body the katabolic changes, like a series of 

 explosions, give out energy which as muscular 

 movement or mental activity or in other ways may 

 be employed to do the work of the body. And 

 just as in a fountain, the energy required to raise 

 the water a given height is equal to the energy 

 liberated by the water in falling from the same 

 height ; arid if work is to be done by the fountain, 

 the water must be allowed to fall to a lower level 

 than that from which it was raised ; so also in the 

 living protoplasm of an animal, if work, muscular, 

 mental, or of other kind is to be performed, the 

 katabolic fall must be greater than the synthetic 

 rise ; or in other words the excretory matters 

 must be of simpler constitution than the food 

 taken in. 



Considerations of this kind which form the basis 



