OBJECTS OF NUTRITION. 11 



other considerations, equally important in a physiological 

 point of view, and derived from the essential nature of or- 

 ganization, 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 vitality must therefore be selected and 

 arranged with a view to their continual modification, cor- 

 responding to these ever varying changes of condition. The 

 artificer, whose aim is to construct a machine for perma- 

 nent use, and to secure it as much as possible from the de- 

 terioration arising from friction or 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 inces- 

 sant, he would employ the hardest of all materials, — the 

 diamond. Such a machine, once finished, being exempt 

 from almost every natural cause of decay, might remain for 

 an indefinite period in the same state. Far different are the 

 objects which must be had in view in the formation of or- 

 ganized structures. In order that these may be qualified 

 for exercising the functions of life, they must be capable of 

 continual alterations, displacements, and adjustments, vary- 

 ing perpetuall}'^, both in kind and in degree, according to 

 the progressive stages of their internal development, and to 

 the different circumstances which may arise in their exter- 

 nal condition. The materials which nature has employed 

 in their construction, are, therefore, neither the elementary 

 bodies, nor even their simpler and more permanent combi- 

 nations; but such of their compounds as are of a more plastic 

 nature, and which allow of a variable proportion of ingre- 

 dients, and of great diversity in the modes of their combi- 

 nation. So great is the complexity of these arrangements, 

 that although chemistry is fully competent to the analysis 

 of organized substances into their ultimate elements, no hu- 



