CHAPTER SIXTH. 



NUTRITION. 



200. THE second class of the functions of animal life 

 are those which relate to the maintenance of life and the 

 perpetuation of the species ; the functions of vegetative 

 life (59). 



201. The increase of the volume of the body must re- 

 quire additional materials. There is also an incessant waste 

 of particles which, having become unfit for further use, 

 are therefore carried out of the system. Every contraction 

 of a muscle expends the energy of some particles, whose 

 place must be supplied. These supplies are derived from 

 every natural source, the animal, vegetable, and even the 

 mineral kingdoms ; and are received under every variety of 

 solid, liquid, and gaseous form. Thus, there is a perpetual 

 interchange of substance between the animal body and the 

 world around. The conversion of these supplies into a 

 suitable material, and the appropriation of it to the growth 

 and sustenance of the body, is called NUTRITION. 



202. In early life, during the period of growth, the 

 amount of substances received is greater than that which is 

 lost. At a later period, when growth is completed, an equi- 

 librium between the matters received and those rejected, is 

 established. At a still later period, the equilibrium is again 

 disturbed, more is rejected than is retained, decrepitude be- 

 gins, and at last the organism becomes exhausted, the func- 

 tions cease, and death ensues. 



