OF FUNCTION; OR, HOW WE ACT. 19 



the various forces can be discerned and demon- 

 strated only by the application of known laws of 

 force. 



Two sources of difficulty, arising from this com- 

 plexity of the organic processes, may be specially 

 noticed. On the one hand, there are certain changes 

 which involve decomposition, and yet are probably 

 not ""tfttended with any functional activity. The 

 portions of the body which have given out their 

 force in function, may pass into still lower forms 

 of composition previous to their excretion as worn- 

 out materials : a process of decay may go on in 

 them, which does not manifest itself in any external 

 force. And, besides this, the decomposition which 

 is to bring into their orderly activity the various 

 structures, must itself be of an ordered and definite 

 character. Unregulated, or in excess, it would pro- 

 duce not function but disease ; as, indeed, we see 

 in our own mechanical contrivances : not every 

 possible expansion of the steam, but only that which 

 takes place in definite direction and amount, can 

 raise the piston. 



But, on the other hand, a still greater difficulty 



