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brain, in the spermatic or preparing arteries and veins, 

 and in the umbilical arteries and veins. I shall now, 

 therefore, for your sake, my learned friend, enter some- 

 what more at large into my reasons for rejecting the 

 vulgar notion of the anastomoses, and explain my own 

 conjectures concerning the mode of transition of the 

 blood from the minute arteries into the finest veins. 



All reasonable medical men, both of ancient and 

 modern times, have believed in a mutual transfusion, 

 or accession and recession of the blood between the 

 arteries and the veins ; and for the sake of permitting 

 this, they have imagined certain inconspicuous openings, 

 or obscure foramina, through which the blood flowed 

 hither and thither, moving out of one vessel and re- 

 turning to it again. Wherefore it is not wonderful that 

 Riolanus should in various places find that in the 

 ancients which is in harmony with the doctrine of a cir- 

 culation. For a circulation in such sort teaches nothing 

 more than that the blood flows incessantly from the veins 

 into the arteries, and from the arteries back again into 

 the veins. But as the ancients thought that this move- 

 ment took place indeterminately, by a kind of accident, 

 in one and the same place, and through the same 

 channels, I imagine that they therefore found them- 

 selves compelled to adopt a system of anastomoses, 

 or fine mouths mutually conjoined, and serving both 

 systems of vessels indifferently. But the circulation 

 which I discovered teaches clearly that there is a 

 necessary outward and backward flow of the blood, 

 and this at different times and places, and through 

 other and yet other channels and passages ; that this 

 flow is determinate also, and for the sake of a certain 

 end, and is accomplished in virtue of parts contrived 

 for the purpose with consummate forecast and most 

 admirable art. So that the doctrine of the motion of 

 the blood from the veins into the arteries, which 

 antiquity only understood in the way of conjecture, 

 and which it also spoke of in confused and indefinite 



