1 62 Circulation of the Blood 



lungs, alternately expanded and contracted during in- 

 spiration and expiration, afford it passage by the proper 

 vessels into the pulmonary veins ; from the pulmonary 

 veins, the left auricle, acting equally and synchronously 

 with the right auricle, delivers the blood into the left 

 ventricle ; which acting harmoniously with the right 

 ventricle, and all regress being prevented by the mitral 

 valves, the blood is projected into the aorta, and con- 

 sequently impelled into all the arteries of the body. 

 The arteries, filled by this sudden push, as they cannot 

 discharge themselves so speedily, are distended ; they 

 receive a shock, or undergo their diastole. But as 

 this process goes on incessantly, I infer that the arteries 

 both of the lungs and of the body at large, under the 

 influence of such a multitude of strokes of the heart 

 and injections of blood, would finally become so over- 

 gorged and distended, that either any further injection 

 must cease, or the vessels would burst, or the whole 

 blood in the body would accumulate within them, were 

 there not an exit provided for it. 



The same reasoning is applicable to the ventricles of 

 the heart : distended by the ceaseless action of the 

 auricles, did they not disburthen themselves by the 

 channels of the arteries, they would by and by become 

 over-gorged, and be fixed and made incapable of all 

 motion. Now this, my conclusion, is true and neces- 

 sary, if my premises be true ; but that these are either 

 true or false, our senses must inform us, not our reason 

 ocular inspection, not any process of the mind. 



I maintain further, that the blood in the veins always 

 and everywhere flows from less to greater branches, and 

 from every part towards the heart; whence I gather 

 that the whole charge which the arteries receive, and 

 which is incessantly thrown into them, is delivered to 

 the veins, and flows back by them to the source whence 

 it came. In this way, indeed, is the circulation of the 

 blood established : by an efflux and reflux from and to 

 the heart ; the fluid being forcibly projected into the 



