OF SLEEP. SECT. XVIII. i$. 



of fome voluntary efforts to facilitate refpiration, as 

 uvhen we are awake\ And is further treated of in 

 Vol. II. .Ciafs I. 2. i. 2. on the Difeafes of the Vo* 

 luntary Power. Art. Somnus interruptus. 



15. We come now to thofe motions which' de- 

 pend on irritation. The motions of the arterial and 

 glandular fyftems continue in our fleep, proceeding 

 flower indeed, but ftronger and more uniformly, 

 than in our waking hours, when they are incommo- 

 ded by external ftimuli, or by the movements of vo- 

 lition ; the motions of the mufcles fubfervient to 

 refpiration continue to be flimulated into action, and 

 the other, internal fenfes of hunger, third, and luft, 

 are not only occafionally excited in our Hfleep, but 

 their irritative motions are fucceeded by their ufual 

 fenfations, and make a part of the farrago of our 

 dreams. Thefe fenfations of the want of air, of 

 hunger, third, and luft, in our dreams, contribute 

 to prove, that thefe nerves of the external fenfes arc 

 alfo alive and excitable in our fleep ; but as the fti- 

 inuli of external objects are either excluded from 

 them by the darknefs and filence of the night, or 

 their accefs to them is prevented by the fufpeniion of 

 volition, thefe nerves of fenfe fa.ll more readily 

 into their connexions with fenfation and with afib- 

 ciation ; becaufe much fenforial power, which dur- 

 ing the day was expended in moving the external 

 organs of fenfe in conlequence of irritation from 

 external ftimuli, or in confequence of volition, be- 

 comes now in fo'me degree accumulated, and renders 

 the internal or immediate organs of fenfe more ea- 

 jfily excitable by the other fenforial powers. Thus 

 in reipect to the eye, the irritation from external 

 ftimuli, and the power of volition during our wak- 

 ing hours, elevate the eye-lids, adapt the aperture 

 of the ins to the quantity of light, the focus of the 

 cry ftal line humour, and the angle of the optic axifes 

 to the diitaace of the object, ail which perpetual ac- 

 tivity 



