PHYSIOLOGY. 125 



feet of evacuation to this head, perhaps not improperly. I have said 

 that the whole of animal fibres, those of the simple solid, and more 

 especially those of the moving fibre, are under a constant state of 

 tension, which gives irritation ; and this is in a sort of balance 

 over the whole system, so that if you diminish it in one part, 

 you do it over the whole ; so that by relaxing some particular 

 portions, we relax the whole ; and in this way we explain the 

 effects of pediluvium, which is a means of inducing sleep, and 

 our present mode of fomenting the lower extremities, which 

 proves an evident cause of sleep upon many occasions. Perhaps 

 this head is more general than may be imagined, and applies 

 wherever we take off any irritation. The evacuations operate 

 more upon the brain itself, but they also operate by the general 

 relaxation of the system ; and even the gratification of vehement 

 desires, formerly mentioned, and the giving a certain satiety or 

 complacency to the system, may be considered as relaxing or 

 taking off the irritation that was formerly present. 



" But I go to the last cause, and I say c Any violent, fre- 

 quent, or long continued exercise of the animal power? In 

 CXI 1 1. I have explained those circumstances of muscular 

 action or contraction, which induce an uneasiness and state of 

 debility in the muscles. I have only summed up those circum- 

 stances here more shortly, by saying any violent and unusual 

 exertion, frequently repeated ; or, independent of that, if con- 

 tinued beyond a certain time. The continued exercise of the ani- 

 mal power, with respect to particular muscles, induces uneasiness 

 and debility ; so, with respect to the brain, it induces sleep. Ac- 

 cording to Haller c naturaliter somnus sequitur vigilium et 

 laborem corporis humani.' It is this very circumstance, this 

 peculiar cause of sleep, which deserves to be considered as the 

 chief ; it is this, however, that has supported the hypothesis 

 which I have taken pains to refute, of a secretion, and of the 

 action of the body depending upon that." 



CXXIX. On the other hand, it appears, that a certain de- 

 gree of heat, all sensations of impression, impressions analo- 

 gous to those which produce sensation, all sensations which lead 

 to thought and action, and the increased impetus of the blood 

 in the vessels of the brain, are all of them causes favouring or 

 inducing a state of waking. " It is remarkable, that though 



