no 



manner it is supposed that we are then awake when the 

 nerves are braced and filled with .animal spirits, and 

 that when they are unbraced and empty we sleep. But 

 who can give any satisfactory account of sleep ? Some 

 ascribe it to the stoppage of the nerves, some to the 

 quiescency, and others to a deficiency of the animal 

 spirifs. The truth is, we are ignorant of the whole 

 affair ; and no more understand sleep than we do 

 death. 



But this we know, that during sleep several functions 

 are suspended, the organs of sense are at rest, the 

 muscles are quiescent, so that hardly any spirits flow 

 through them. The fibres of the nerves are little 

 changed, and an equilibrium obtains throughout* 

 There is no difference of pressure on the vessels, nor 

 of velocity in the humours, which circulate equally 

 through all the canals. Mean time, all disturbing 

 causes being at rest, the wasted humours are restored, 

 and the particles supplied, which were worn .oil' the 

 solids. 



We may observe farther, that when the head is hot 

 and the feet cold, we cannot sleep; that perspiration* 

 is twice a& great while we. sleep as while we are awake; 

 that too much sleep makes the senses dull, the memory 

 weak, and the whole body listless; that sleep will for. 

 a considerable time supply the place of meat and drink ;. 

 that a foetus sleeps always, children much, youths 

 more than adults ? and. they than old men* 



To speak a little more particularly. While^we are 

 awake there is a continual motion of the voluntary, 

 muscles, of the parts subservient to sense, and to the 

 affections, all which stimulate the nerves, blood-vessels^ 

 and heart. Thus the finer parts of the blood are con., 

 tinually wasted, whence weariness ensues; and if the 

 vigilance be continued^ a feverish heat and sensible 

 loss of strength. 



As the night advances, a weight falls on the large, 

 muscles and their tendons, and the mind becomes 

 heavy. The powers th lit hold the body erect, begin 

 to shrink from their oice ; the eye-lids clcse> the 



