VOL. XXXVJII.] PHILOSOPHICAL TRANSACTIONS. 583 



All the remote causes of sleep, or sleepiness, may be fully comprehended in 

 the 4 following particulars, and considered in the following order. 1. Exercise. 

 2. A too plentiful meal. 3. Drunkenness, or a too great quantity of fermented 

 liquors, or of their distilled spirits. 4. The whole tribe of narcotics, or sopo- 

 rifics, of which opium, and its several preparations are the chief. 



1. Exercise appears to waste all the fluids, and particularly the animal spirits, 

 the active instruments of all motion ; so that the remains are not sufficient for 

 the exigencies of the natural and vital functions ; and also to supply the de- 

 mands of voluntary motion, and to assist in sensation, and the operations of 

 the mind. And here it is proper to show how this waste necessarily brings on 

 sleep in a healthy person ; and how the natural and vital motions and functions, 

 of digestion, respiration, and circulation, notwithstanding this waste, do neces- 

 sarily go on in sleep, leading the remains of the spirits to their assistance, and 

 making the deficiency fall to the share of the animal or voluntary motions and 

 organs of sensation. 



In order to show this, let us observe what is very obvious, that when any 

 muscle is brought into action against our will, by a superior force, as when a 

 stronger man bends or extends my arm contrary to my will or inclination, the 

 benders or extensors of my arm swell and contract in the same manner, and the 

 afflux of the blood and spirits to the contracting muscles, is the same, as when 

 I do it voluntarily : therefore by any external or adventitious force, the blood 

 and spirits will be derived on the part thus forced into action. But all the 

 natural and vital parts have such an external or adventitious force continually 

 acting upon them. In the primae viae the weight and other qualities of our 

 food and drink, mixed with air and bile, excite the peristaltic motion, as 

 necessarily as the weight of a clock, or spring of a watch wound up, keeps the 

 wheels and pendulum, &c. in motion. The chyle forced from thence, together 

 with the blood returning into the heart, as necessarily set its clastic springs at 

 work, and the same blood and chyle forced into the arteries by it, make their 

 diastole and followihg systole unavoidable. The air by its elasticity, and the 

 whole weight of the atmosphere, forces itself into the elastic pipes and vesicles 

 of the lungs, and dilates them; which by their elasticity and mechanism, assisted 

 by various muscles, and the ribs and cartilages of the thorax, as necessarily re- 

 pel it in expiration. 



It is therefore evident, that all these natural and vital parts are acted on, and 

 set at work by an external adventitious and irresistible force, continually exciting 

 them whether we will or not, whether awake or asleep ; therefore the blood 

 and remaining spirits after labour, will be mechanically and necessarily led to 

 all these parts that are thus forced into action at all times, but especially most 



