OF FUNCTION; OR, HOW WE ACT. 13 



body. The application of cold, or heat, or friction, 

 alike will excite respiration. Any mechanical or 

 chemical irritation determines muscular contraction, 

 or will occasion in the nerves of special sense their 

 own peculiar sensations. These various agencies 

 operate, not by their own peculiar qualities, but by 

 disturbing an equilibrium, so that the same effect is 

 brought about in many ways. A sudden change 

 is the essential requisite. As almost any force will 

 cause a delicately-balanced body to fall, so almost 

 any change in the conditions of a living body, if 

 it be not fatal to its life, will bring its functional 

 activity into play. Anything that increases the 

 power of the chemical tendencies, or diminishes the 

 resistance to them, may have the same effect. 



To recapitulate : Chemical affinity is opposed, and 

 delicately balanced, by other force in the organic 

 body (as we oppose forces in a machine ; the elas- 

 ticity of heated steam by the tenacity of iron, for 

 example) ; and this affinity coming into play spon- 

 taneously or through the effect of stimuli which dis- 

 turb the equilibrium is the secret of the animal 

 functions. The body is not in this respect peculiar, 



