NERVOUS SYSTEM. 157 



diately after the same organ has been acted upon by one of 

 greater intensity. In this case, the feeble impression scarce- 

 ly excites a perceptible sensation. Thus, weak sounds are 

 not distinctly heard, when they follow those which are much 

 louder ; and feebly illuminated objects are scarcely visible, 

 when the eye, immediately before, has been directed to those 

 which are placed in a stronger light. If we look at a dark 

 object on a white wall, and then direct the eye to another 

 part of the wall, we shall still continue to observe the fi- 

 gure of the dark object, but now become apparently more 

 bright than the wall itself. That part of the eye on which 

 the image of the black spot fell at first, experienced a kind 

 of repose, while the other parts on which the image of the 

 white wall fell, were in action. The part, therefore, which 

 was inactive, is now able to receive a stronger impression 

 from the colour of the wall, than the other parts already 

 fatigued by its influence *. 



b. Volition. The action of the nervous system in the 

 functions of volition, bears a close resemblance to its opera- 

 tions as a sensitive faculty. In the case of sensation, how- 

 ever, the action excited in the nerve was communicated to 

 the common sensorium ; in this of volition, the action excit- 

 ed in the nerve, is communicated to the muscles. 



The origin of volition may be traced to the impressions 

 communicated to the brain by the sensitive faculty, or to 

 changes taking place in the brain itself, independent of the 

 action of external objects. In the former case, the volition 

 follows the sensation, in some instances, after an observable 

 interval, in others, it appears almost instantaneously. In 

 the one there is an interruption of the circuit, if I may so 

 speak, by the interference of the mind ; in the other, the 

 mind appears to exercise little or no controul. But whether 



* CUVIER'S Comp. Anat. sect. ix. 



