NUTRITION. 



375 



the issue of the changes continually taking place among 

 its particles. 



The change of component particles, in which the nutri- 

 tion of organs consists, is most evidently shown when, in 

 growth, they maintain their form and other general charac- 

 ters, but increase in size. When, for example, a long 

 bone increases in circumference, and in the thickness of 

 its walls, while, at the same time, its medullary cavity 

 enlarges, it can only be by the addition of materials to its 

 exterior, and a coincident removal of them from the 

 interior of its wall ; and so it must be with the growth of 

 even the minutest portions of a tissue. And that a similar 

 change of particles takes place, even while parts retain a 

 perfect uniformity, may be proved, if it can be shown that 

 all the parts of the body are subject to waste and impair- 

 ment. 



In many parts, the removal of particles is evident. 

 Thus, as will be shown when speaking of secretion, the 

 elementary structures composing glands are the parts of 

 which the secretions are composed : each gland is con- 

 stantly casting off its cells, or their contents, in the 

 secretion which it forms : yet each gland maintains its 

 size and proper composition, because for every cell cast off 

 a new one is produced. So also the epidermis and all 

 such tissues are maintained. In the muscles, it seems 

 nearly certain, that each act of contraction is accompanied 

 with a change in the composition of the contracting tissue, 

 although the change from this cause is less rapid and 

 extensive than was once supposed. Thence, the develop- 

 ment of heat in acting muscles, and thence the discharge 

 of urea, carbonic acid, and water the ordinary products 

 of the decomposition of the animal tissues which fol- 

 lows all active muscular exercise. Indeed, the researches 

 of Helrnholtz almost demonstrate the chemical change 

 that muscles undergo after long-repeated contractions ; 

 yet the muscles retain their structure and composition? 



