376 THE SENSORIAL FUNCTIONS. 



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 

 required 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 treated at suITicient length in the first part of 

 this work. 



Every excitement of the sensorial powers is, sooner or 

 later, followed by a proportional degree of exhaustion; and 

 when this has reached a certain point, a suspension of the 

 exercise of these faculties takes place, constituting the state 

 of sleej:), during which, by the continued renovating action 

 of the vital functions, these powers are recruited, and ren- 

 dered again adequate to the purposes for which they were 

 bestowed. In the ordinary state of sleep, however, the ex- 

 haustion of the sensorium is seldom so complete as to pre- 

 clude its being excited by internal causes of irritation, which 

 would be scarcely sensible during our waking hours: and 

 hence arise dreams, which are trains of ideas, suggested by 

 internal irritations, and which the mind is bereft of the 

 power to control, in consequence of the absence of all im- 



♦ A voluntary action, occurnng' as the immediate consequence of the ap- 

 plication of an external agent to an organ of the senses, thougli apparently 

 a simple phenomenon, implies the occurrence of no less than twelve succes- 

 sive processes, as may be seen by the following enumeration. First, there 

 is the modifying action of the organ of tlie 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, associa- 

 tion, and volition. Then, again, there is another physical change taking 

 place in the sensorium, immediately consequent on the mental act of voli- 

 tion: this is followed by the propagation of the impression downwards along 

 tlie motor nei've; then an impression is made on the muscle; and, lastly, we 

 obtain the contraction of the muscle, which is the object of the whole series 

 of operations. 



