CIRCULATION OF THE BLOOD 103 



like manner continually open, and nature had found no con- 

 trivance for closing it when requisite, and opening it again, 

 it would have been impossible that the blood could ever 

 have passed by the invisible and delicate mouths, during 

 the contractions of the thorax, into the arteries; for all 

 things are not alike readily attracted or repelled; but that 

 which is light is more readily drawn in, the instrument being 

 dilated, and forced out again when it is contracted, than 

 that which is heavy; and in like manner is anything drawn 

 more rapidly along an ample conduit, and again driven 

 forth, than it is through a narrow tube. But when the 

 thorax is contracted the pulmonary veins, which are in the 

 lungs, being driven inwardly, and powerfully compressed 

 on every side, immediately force out some of the spirit they 

 contain, and at the same time assume a certain portion of 

 blood by those subtle mouths, a thing that could never come 

 to pass were the blood at liberty to flow back into the heart 

 through the great orifice of the pulmonary artery. But its 

 return through this great opening being prevented, when 

 it is compressed on every side, a certain portion of it distils 

 into the pulmonary veins by the minute orifices mentioned." 

 And shortly afterwards, in the next chapter, he says : " The 

 more the thorax contracts, the more it strives to force out 

 the blood, the more exactly do these membranes (viz., the 

 semilunar valves) close up the mouth of the vessel, and 

 suffer nothing to regurgitate." The same fact he has also 

 alluded to in a preceding part of the tenth chapter: "Were 

 there no valves, a three-fold inconvenience would result, so 

 that the blood would then perform this lengthened course 

 in vain; it would flow inwards during the disastoles of the 

 lungs and fill all their arteries; but in the systoles, in the 

 manner of the tide, it would ever and anon, like the 

 Euripus, flow backwards and forwards by the same way, 

 with a reciprocating motion, which would nowise suit the 

 blood. This, however, may seem a matter of little moment ; 

 but if it meantime appear that the function of respiration 

 suffer, then I think it would be looked upon as no trifle, etc." 

 Shortly afterwards he says: "And then a third inconveni- 

 ence, by no means to be thought lightly of, would follow, 

 were the blood moved backwards during the expirations, 



