300 NUTRITION. 



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 

 impairment. 



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 secre- 

 tions are composed : each gland is constantly 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 mus- 

 cles, it seems nearly certain, that each act of contraction is 

 accompanied with a change in the composition of the con- 

 tracting tissue, although the change from this cause is less 

 rapid and extensive than was once supposed. Thence, the 

 development of heat in acting muscles, and then the discharge 

 of urea, carbonic acid, and water the ordinary products of 

 the decomposition of the animal tissues which follows all 

 active muscular exercise. Indeed, the researches of Helm- 

 holtz almost demonstrate the chemical change that muscles 

 undergo after long-repeated contractions; yet the muscles 

 retain their structure and composition, because the particles 

 thus changed are replaced by new ones resembling those which 

 preceded them. So again, the increase of alkaline phosphates 

 discharged with the urine after great mental exertion, seems 

 to prove that the various acts of the nervous system are at- 

 tended with change in the composition of the nervous tissue ; 

 yet the condition of that tissue is maintained. In short, for 

 every tissue there is sufficient evidence of impairment in the 

 discharge of its functions : without such change, the produc- 

 tion or resistance of physical force is hardly conceivable : and 

 the proof as well as the purpose of the nutritive process ap- 

 pears in the repair or replacement of the changed particles ; 

 so that, notwithstanding its losses, each tissue is maintained 

 unchanged. 



But besides the impairment and change of composition to 

 which all parts are subject in the discharge of their natural 

 functions, an amount of impairment which will be in direct 

 proportion to their activity, they are all liable to decay and 

 degeneration of their particles, even while their natural actions 

 are not called forth. It may be proved, as Dr. Carpenter first 

 clearly showed, that every particle of the body is formed for a 

 certain period of existence in the ordinary condition of active 

 life ; at the end of which period, if not previously destroyed by 

 outward force or exercise, it degenerates and is absorbed, or 

 dies and is cast out. 



