OBJECTS OF NUTRITION. 3 



fitted out for a long voyage, and fortified 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 acci- 

 dental 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 enumerated 

 would of itself be sufficient to establish the neces- 

 sity of supplies of nourishment for the maintenance 

 of life. But there are other considerations, equally 

 important in a physiological point of view, and 

 derived from the essential nature of organization, 

 which likewise create 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 vitality 

 must therefore be selected and arranged with a 

 view to their continual modification, corresponding 

 to these ever varying changes of condition. The 

 artificer, whose aim is to construct a machine for 

 permanent use, and to secure it as much as possible 

 from the deterioration arising from friction and 

 other causes of injury, would, of course, make 

 choice for that purpose of the most hard and durable 

 materials, such as the metals, or the denser stones. 

 In constructing a watch, for instance, he would 

 form the wheels of brass, the spring and the barrel- 

 chain of steel ; and for the pivot, where the motion 

 is to be incessant, he would employ the hardest of 



