IN THE ANIMAL ORGANISM. 227 



is confined within narrow limits. It is directly 

 proportional to the force required for these involun- 

 tary motions. 



Now, although we may suppose that the living 

 muscular tissue, with a sufficient supply of food, 

 never loses its capacity of growth ; that this form 

 of vital manifestation is continually effective ; this 

 cannot apply to those parts of the body whose avail- 

 able vital force has been expended in producing me- 

 chanical effects. For the waste of matter, in con- 

 sequence of motion and laborious exertion, is ex- 

 tremely various in different individuals. 



If we reflect, that the slightest motion of a finger 

 consumes force ; that in consequence of the force 

 expended, a corresponding portion of muscle dimi- 

 nishes in volume ; it is obvious, that an equilibrium 

 between supply and waste of matter (in living tissues) 

 can only occur when the portion separated or ex- 

 pelled in a lifeless form is, at the same instant in 

 which it loses its vital condition, restored in another 

 part. 



The capacity of growth or increase in mass de- 

 pends on the momentum of force belonging to each 

 part ; and must be capable of continued manifesta- 

 tion (if there be a sufficient supply of nourishment), 

 as long as it does not lose this momentum, by ex- 

 pending it, for example, in producing motion. 



In all circumstances, the growth itself is restricted 

 to the time ; that is to say, it cannot be unlimited 

 in a limited time. 



