OF FUNCTION; OK, HOW WE ACT. 21 



dently we do not take into account the phenomena 

 of thought, feeling, or will. These form another 

 subject. But, confining our attention to those 

 operations of the nervous system which are strictly 

 physical in their character, it may be observed, that 

 all the stimuli which excite them are adapted to 

 bringinto activity the repressed chemical affinities 

 of the elements. Thus the nervous force is called 

 into action by mechanical irritation, or motion, in 

 whatever form applied, by changes of temperature, 

 by chemical irritants, by electricity, light or sound, 

 and by the taste or smell of bodies. It is hardly 

 possible to perceive in these various agents any pro- 

 perty in common to which their influence on the 

 nervous system can with reason he referred, except 

 the power they all, so far as they are known to us, 

 possess of disturbing an unstable chemical equili- 

 brium. Acting upon a tissue in which the affinities 

 of the component elements are so delicately balanced, 

 and the inherent tendency to change so strong, as 

 in the nervous substance, it can hardly be otherwise 

 than that they should overthrow that balance, and 

 bring about a change of composition. "In com- 



