476 THE SENSORIAL FUNCTIONS. 



powers and the mechanism of the instruments em- 

 ployed,* I have already treated at sufficient 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 sleep, 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 exhaustion of the sensorium is 

 seldom so complete as to preclude 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, sug- 

 gested by internal irritations, and which the mind 

 is bereft of the power to control, in consequence of 

 the absence of all impressions from the external 



* 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 than twelve successive processes, as may be seen by the fol- 

 lowing 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, per- 

 ception, association, and volition. Then, again, there is another 

 physical change taking place in the sensorium, immediately conse- 

 quent on the mental act of volition : this is followed by the propa- 

 gation of the impression downwards along the motor nerve ; then 

 an impression is made on the muscle ; and lastly we obtain the con- 

 traction of the muscle, which is the object of the whole series of 

 operatiuns. 



