CIRCULATION OF THE BLOOD. 133 



tricuspid valves, is forced into the pulmonary artery, which 

 stands open to receive it, and is immediately distended with it. 

 Once in the pulmonary artery, the blood cannot return, by 

 reason of the sigmoid valves ; and then the lungs, alternately 

 expanded and contracted during inspiration 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 ven- 

 tricle, and all regress being prevented by the mitral valves, the 

 blood is projected into the aorta, and consequently impelled into 

 all the arteries of the body. The arteries, filled by this sudden 

 push, as they cannot discharge themselves so speedily, are dis- 

 tended ; 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 

 necessary, 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 arterial 

 system, and then absorbed and imbibed from every part by the 

 veins, it returns through these in a continuous stream. That all 

 this is so, sense assures us ; and necessary inference from the 



