CHAPTER XXIII. 



NUTRITION. 



1. NUTRITION may be considered as (the completion of 

 the functions of digestion.) It is that process by which the 

 waste of the organs is repaired, and by which their develop- 

 ment and growth are maintained. Respiration, digestion, 

 circulation, absorption, and secretion, are but separate links 

 in the chain of nutrition ; which would be instantly de- 

 stroyed by the absence of any one of them. 



2. In the construction of a machine, or an instrument, 

 designed to last for many years, Ihe mechanist seeks for^the 

 most durable materials.} In making a watch, for instance, 

 he forms the wheels of brass, the spring and barrel-chain of 

 steel, and for the pivot, which is subject to incessant friction, 

 he employs the hardest of all materials the diamond. The 

 necessity for this arises from the fact, tha^such instruments 

 do not contain, within themselves, the power of repairing 

 their own lossesj 



3. But far different is the case with the animal machine. 

 fin order to qualify it for exercising the functions of life, it 



must be so constructed as to render it capable of continual 

 alterations, displacements and adjustments ; and these sub- 

 ject to continual variation, according to the stage of growth, 

 and also to the different circumstances in which it may be 

 placed j Instead, therefore, of a few elementary bodies, or 

 their simpler combinations, nature has employed such com- 

 pounds as admit of greater change, and a more variable 

 proportion of ingredients, and greater diversity in the mode 

 of combination. It is nutrition that moulds these plastic 

 materials, and forms these ever-changing compounds ; and 

 so preserves the animal machine, amid all the varying 

 changes of condition to which it is subject. 



