THE FUNCTIONS OF LIFE. 35 



the blood by the organs of Circulation, consisting of 

 the Heart, which impels it through a system of 

 pipes called Arteries, and receives it back again by 

 means of another set of tubes called Veins. In the 

 third place it is necessary that the circulating blood 

 should continually undergo purification by the che- 

 mical action of oxygen : a purpose which is an- 

 swered by the function of Respiration. The fourth 

 stage of nutrition relates to the more immediate 

 application of this purified material to the wants of 

 the system, to the extension of the organs, to the 

 reparation of their losses, and to the restoration of 

 their exhausted powers. 



Life, then, consists of a continued series of actions 

 and reactions, ever varying, yet constantly tending 

 to definite ends. Most of the parts of which the body 

 consists undergo continual and progressive changes 

 in their dimensions, figure, arrangement, and com- 

 position. The materials which have been united 

 together and fashioned into the several organs, 

 are themselves successively removed and replaced 

 by others, which again are, in their turn, discarded, 

 and new materials substituted, though v/ithout any 

 perceptible change of external form. Perpetual 

 mutation appears to constitute the fundamental law 

 of living nature ; and it has been further decreed 

 by the power which gave the first impulse of ani- 

 mation to this organized fabric, that its movements 

 and its powers shall be limited in their duration, 

 and that, even when they are not destroyed by 

 extraneous causes, after continuing for a certain 

 period, they shall come to a close. The law of 

 Mortality, to which all the beings that have received 

 the gift of life are subjected, is a necessary conse- 



