246 THE PHENOMENA OF MOTION 



supply of matter is, in the different periods of life, 

 unequal ; but, in the state of health, the available 

 vital force must always be considered as a constant 

 quantity, corresponding to the sum of living par- 

 ticles. 



Growth, or the increase of mass, stands, at every 

 age, in a fixed relation to the amount of vital force 

 consumed as moving power. 



The vital force, which is expended for mechanical 

 purposes, is subtracted from the sum of the force 

 available for the purpose of increase of mass. 



The active force, which is consumed in the body 

 in overcoming resistance (in causing increase of 

 mass), cannot, at the same time, be employed to 

 produce mechanical effects. 



Hence it follows necessarily, that w-hen, as in 

 childhood, the supply exceeds the waste of matter, 

 the mechanical effects produced must be less in the 

 same proportion. 



With the increase of mechanical efiects produced, 

 the capacity of increase of mass or of the supply of 

 waste in living tissues must diminish in the same 

 proportion. 



A perfect balance between the consumption of 

 vital force for supply of matter and that for me- 

 chanical effects occurs, therefore, only in the adult 

 state. It is at once recognized in the complete 

 supply of the matter consumed. In old age more 

 is wasted ; in childhood more is supplied than 

 wasted. 



