364 PRACTICE OF PHYSIC. 



and variously modify the action of the heart. I suppose, there- 

 fore, a force very constantly during life exerted in the brain, 

 with respect to the moving fibres of the heart, as well as of 

 every part of the body ; which force I shall call the Energy of 

 the Brain ; and which I suppose may be, on different occasions, 

 stronger or weaker with respect to the heart. " See Vol. I. p. 80." 



MCLXXVI. Admitting these propositions, it will be obvi- 

 ous, that, if I can explain in what manner the first set of re- 

 mote causes (MCLXXIV.) diminish the energy of the brain, 

 I shall at the same time explain in what manner these causes 

 occasion a syncope. 



MCLXXVII. To do this, I observe, that one of the most 

 evident of the remote causes of syncope is a hoemorrhagy, or an 

 evacuation of blood, whether spontaneous or artificial. And, 

 as it is very manifest, that the energy of the brain depends upon 

 a certain fulness and tension of its blood-vessels, for which na- 

 ture seems to have industriously provided by such a conforma- 

 tion of those blood-vessels as retards the motion of the blood 

 both in the arteries and veins of the brain ; so we can readily 

 perceive, that evacuations of blood, by taking off the fulness 

 and tension of the blood-vessels of the brain, and thereby dimi- 

 nishing its energy with respect to the heart, may occasion a 

 syncope. In many persons a small evacuation of blood will 

 have this effect ; and in such cases there is often a clear proof 

 of the manner in which the cause operates, from this circum- 

 stance, that the effect can be prevented by laying the body in a 

 horizontal posture ; which, by favouring the afflux of the blood 

 by the arteries, and retarding the return of it by the veins, pre- 

 serves the necessary fulness of the vessels of the brain. 



It is farther to be remarked here, that, not only an evacua- 

 tion of blood occasions syncope, but that even a change in the 

 distribution of the blood, whereby a larger portion of it flows 

 into one part of the system of blood-vessels, and consequently 

 less into others, may occasion a syncope. It is thus I explain 

 the syncope that readily occurs upon the evacuation of hy- 

 dropic waters, which had before filled the cavities of the abdo- 

 men or thorax. It is thus also I explain the syncope that some- 

 times happens on blood-letting, but which does not happen till 



