LETTER IX. 



83 



limits, the destructive ; and that the structure acquires 

 ultimately an increase of bulk and power. 



8. All vital action, then, is exceedingly transient in 

 any structure. The greater the rapidity of its forma- 

 tion and growth, the more frequent the exercise of its 

 own peculiar function (if it have any), and the greater 

 the energy of this exercise, the shorter time does the 

 structure retain its vitality ; while its continued main- 

 tenance as a living structure can be accomplished 

 only by an incessant change and renewal of its consti- 

 tuent molecules. So true is it that we begin to die as 

 soon as we begin to live {nascentes morimur) ; that our 

 very life is nothing else but a succession of dying ; 

 that every day and hour wears away part of it ; and 

 that, so far as it is already spent, so far we are already 

 dead and buried — a truth, however, which has only 

 been recognised in all its fulness by physiologists 

 within these few years, and which was perhaps first 

 brought thus prominently into view as a principle in 

 physiology by Dr Carpenter. 



9. If these views as to the general nature of vitaUty 

 be correct, it will be a fair inference that the absence 

 of any such process of interstitial or molecular nutri- 

 tion in a tissue or organ, after its full development, 

 is equivalent to that tissue or organ being destitute 

 of vitality — to its being no longer the seat of any 

 vital change or action. It will be a proof that, 

 however it may retain its characteristic appear- 



