52 Motion of the 



And shortly afterwards, in the very 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 sigmoid 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 diastoles 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, &c." And, again, and shortly 

 afterwards: "And then a third inconvenience, by no 

 means to be thought lightly of, would follow, were the 

 blood moved backwards during the expirations, had 

 not our Maker instituted those supplementary mem- 

 branes [the sigmoid valves]." Whence, in the eleventh 

 chapter, he concludes : " That they have all a common 

 use, (to wit, the valves,) and that it is to prevent 

 regurgitation or backward motion ; each, however, 

 having a proper function, the one set drawing matters 

 from the heart, and preventing their return, the other 

 drawing matters into the heart, and preventing their 

 escape from it. For nature never intended to distress 

 the heart with needless labour, neither to bring aught 

 into the organ which it had been better to have kept 

 away, nor to take from it again aught which it was 

 requisite should be brought. Since, then, there are 

 four orifices in all, two in either ventricle, one of these 

 induces, the other educes." And again he says : 

 " Farther, since there is one vessel, consisting of a 

 simple tunic, implanted in the heart, and another, 



