CHAPTER SIXTH. 



NUTRITION. 



308. THE second class of functions are those which relate 

 to nutrition and the perpetuation of the species ; the functions 

 of vegetative or organic life. 



309. The increase of the Tolume of the body requires 

 additional materials. There is also an incessant waste of par- 

 ticles, which, haying become unfit for further use, require to be 

 carried out of the system. Every contraction of a muscle ex- 

 pends the energy of some particles, whose place must be sup- 

 plied by others. These supplies are derived from every natural 

 source, the animal, vegetable, and 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, its distribution to all parts, 

 and the assimilation and appropriation of it to the growth and 

 sustenance of the body, is called NTJTBITIOJS, in the widest 



V ' 



sense of the term. 



310. 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 equilibrium be- 

 tween 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 begins, and at last 

 the organism becomes exhausted, the functions cease, and 

 death ensues. 



311. The solids and fluids taken into the body as food are 

 subjected to a process called Digestion, by which the solid 

 portions are reduced to a fluid state, the nutritive particles 

 separated from the excrementitious, and the whole prepared to 

 become blood, bone, muscle, &c. The residue is afterwards 

 expelled, together with those particles of the body which re- 

 quire to be renewed, and those which have been derived from 

 the blood by several processes, termed Secretions. Matters in 

 a gaseous form are also received and expelled with the air we 



