376 XUTEITIOX. 



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 attended 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 pro- 

 duction or resistance of physical force is hardly conceivable : 

 and the proof as well as the purpose of the nutritive pro- 

 cess appears 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. 



The simplest examples that can be adduced of this are 

 in the hair and teeth ; and it may be observed, that, in 

 the process which will now be described, all the great 

 features of the process of nutrition seem to be re- 

 presented.* 



An eyelash which naturally falls, or which can be drawn 



* These and other instances are related more in detail in Mr. Paget's 

 Lectures on Surgical Pathology, from which this chapter was originally 

 written. 



