OBJECTS OF NUTRITION. 3 



identified with the living substance. Thus we 

 often find stnictures 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 mate- 

 rials is also wanted for the reparation of occa- 

 sional injuries, to which, in the course of its long 

 career, the body is unavoidably exposed. Like 

 a ship fitted out for a long voyage, and fortified 

 against the various dangers of tempests, of ice- 

 bergs, 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 exigencies of the occasion. 



Any one of the circumstances above enume- 

 rated would of itself be sufficient to establish the 

 necessity of supplies of nourishment for the 

 maintenance of life. But there are other consi- 

 derations, equally important in a physiological 

 point of view, and derived from the essential 

 nature of organization, which also produce a 

 continual demand for these supplies ; and these 

 I shall now endeavour briefly to explain. 



Constant and progressive change appears to 

 be one of the leading characteristics of life ; and 

 the materials which are to be endowed with vi- 

 tality must therefore be selected and arranged 

 with a view to their continual modification, cor- 



