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 
pee ge es A, BA ede 
* Cuvier’s Comp. Anat. sect. ix. 
