82 



LETTEKS ON TREES. 



involving also the agency of oxygen to effect the dis- 

 integration and removal of the old. To this process 

 physiologists give the name of interstitial or molecular 

 nutrition. 



7. If the special end which the structure has to 

 serve be merely mechanical^ as is the case, for example, 

 with bone and cartilage, the general observation now 

 made embraces all that need be said as to its vital 

 relations. But if its end be higher than this, as is the 

 case with the secreting cells and glands (both vegetable 

 and animal), with the muscles also, and with the brain 

 and nerves of animals, it will have, superadded, a suit- 

 able specific vitality ; and then, there is this further 

 law, that every exercise of its proper function is more 

 or less exhaustive, as well of the general as of the 

 specific vitality of the structure ; and that just in pro- 

 portion to the amount and frequency of the exercise. 

 Every muscular effort we make, every exercise of 

 thought and will, exhausts the vitality of so much 

 brain and nerve and muscle — involving thereby the 

 diminution or loss of the specific power — entailing also 

 the death and removal of so many of their moelcules 

 — necessitating likewise their replacement by new 

 ones ; and that in the exact measure and proportion 

 of the acts of thought and volition and of muscular 

 exertion. But observe, further, as the subsequent 

 result of that exercise, that, under favourable circum- 

 stances, the restorative process outruns, within certain 



