VOLUNTARY MOTION. 535 



those which are subservient to sensation,* made 

 to communicate directly with the sensorium, and 

 are thereby placed under the direct control and 

 guidance of the will. The mental act of volition 

 is doubtless accompanied by some corresponding 

 physical change in that part of the sensorium, 

 whence the motor nerves, or those distributed to 

 the muscles of voluntary motion, arise. Here, 

 then, we pass from mental phenomena to such 

 as are purely physical ; and the impression, 

 whatever may be its nature, originating in the 

 sensorium, is propagated along the course of the 

 nerve to those muscles, whose contraction is re- 

 quired for the production of the intended action. 

 Of the function of voluntary motion, as far as 

 concerns the moving powers and the mechanism 

 of the instruments employed,! I have already 



* On this subject I must refer the reader to the researches of 

 Sir Charles Bell, and Magendie, who have completely established 

 the distinction between these two classes of nerves. 



t A voluntary action, occurring as the immediate consequence 

 of the application of an external agent to an organ of the senses, 

 though apparently a simple phenomenon, implies the occurrence 

 of no less that twelve successive processes, as may be seen 

 by the following enumeration. First, there is the modifying 

 action of the organ of the sense, the refractions of the rays, for 

 instance, in the case of the eye : secondly, the impression made 

 on the extremity of the nerve : thirdly, the propagation of this 

 impression along the nerve : fourthly, the impression or physical 

 change in the sensorium. Next follow four kinds of mental 

 processes, namely, sensation, perception, association, and volition. 

 Then, again, there is another physical change taking place in the 

 sensorium, immediately consequent on the mental act of volition : 

 this is followed by the propagation of the impression downwards 



