536 THE SENSORIAL FUNCTIONS. 



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 de- 

 gree 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 rendered 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 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 con- 

 sequence of the absence of all impressions froni 

 the external senses.* In many animals, a much 

 more general suspension of the actions of life, 

 extending even to the vital functions of respi- 

 ration and circulation, takes place during the 

 winter months, constituting what is termed 

 Hyhemation. 



along the motor nerve ; 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. 



* The only indications of dreaming given by the lower animals 

 occur in those possessed of the greatest intellectual powers, such 

 as the Dog, among quadrupeds, and the Parrot, among birds. 



