10 THE VITAL FUNCTIONS. 



parts of the frame, and evaporation is constantly tending to 

 exhaust the fluids. The repeated actions of the muscles in- 

 duce certain changes in these organs, both in their mecha- 

 nical properties and chemical composition, which impair 

 their powers of contraction, and which, if suffered to con- 

 tinue, would, in no long time, render them incapable of ex- 

 ercising their proper functions; and the same observation 

 applies also to the nerves, and to all the other systems of 

 organs. Provision must accordingly be made for remedy- 

 ing these constant causes of decay by the supply of those 

 peculiar materials, which the organs require for recruiting 

 their declining energies. 



It is obvious that the development of the organs, and ge- 

 neral growth of the body, must imply the continual addition 

 of new particles from foreign sources. Organic increase 

 consists not in the mere expansion of a texture previously 

 condensed, and the filling up of its interstices by inorganic 

 matter; but the new materials that are added must, for this 

 purpose, be incorporated with those which previously exist- 

 ed, and become identified with the living substance. Thus, 

 we often find structures forming in the bodies of animals of 

 a nature totally different from that of the part from which 

 they arise. 



In addition to these demands, a store of materials is also 

 wanted for the reparation of occasional injuries, to which, 

 in the course of its long career, the body is unavoidably ex- 

 posed. Like a ship fitted out for a long voyage, and forti- 

 fied against the various dangers of tempests, of icebergs, and 

 of shoals, the animal system, when launched into existence, 

 should be provided with a store of such materials as may be 

 wanted for the repair of accidental losses, and should also 

 contain within itself the latent source of those energies, 

 which may be called into action when demanded by the ex- 

 igencies of the occasion. 



Any one of the circumstances above enumerated would 

 of itself be sufficient to establish the necessity of supplies of 

 nourishment for the maintenance of life. But there are 



