OF NUTRITION; OR, WHY WE GROW. 53 



effect of the yeast-plant is analogous to many pro- 

 cesses in the animal body. For example, there is 

 reason to believe that the limbs are powerfully 

 developed by exercise, and that muscles waste if riot 

 kept in use. But the action of a muscle depends 

 upon an energetic decomposition in it, and in this 

 more energetic decomposition of the active than of 

 the inactive muscle, we may easily recognize the 

 cause of its greater vital development. The stimuli 

 which call it into functional activity produce chemical 

 changes in it, as the yeast does in fermentable 

 liquids ; and the larger growth consequent thereon 

 is like the more abundant development of the yeast 

 cells in actively fermenting fluids. 



This effect may be illustrated mechanically. The 

 pendulum rises by the force of its fall, and will be 

 made to rise the higher by any impulse which makes 

 its fall more rapid. This aspect of the subject is 

 further illustrated in the Appendix. 



Recognizing this dependence of nutrition on decay, 

 we have in our hands a clue which will guide us 

 through the labyrinth of the vital phenomena. For 

 the most striking, and at the first view the most 



