42 MOTION OF THE 



we also, from what precedes, from dissections, and other argu- 

 ments, conceive the thing to be clear. But as there are some 

 who admit nothing unless upon authority, let them learn that 

 the truth I am contending for can be confirmed from Galen's 

 own words, namely, that not only may the blood be transmitted 

 from the pulmonary artery into the pulmonary veins, then into 

 the left ventricle of the heart, and from thence into the arteries 

 of the body, but that this is effected by the ceaseless pulsation 

 of the heart and the motion of the lungs in breathing. 



There are, as every one knows, three sigmoid or semilunar 

 valves situated at the orifice of the pulmonary artery, which 

 effectually prevent the blood sent into the vessel from returning 

 into the cavity of the heart. Now Galen, explaining the uses 

 of these valves, and the necessity for them, employs the follow- 

 ing language : l " There is everywhere a mutual anastomosis 

 and inosculation of the arteries with the veins, and they severally 

 transmit both blood and spirit, by certain invisible and un- 

 doubtedly very narrow passages. Now if the mouth of the 

 vena arteriosa, or pulmonary artery, had stood in like man- 

 ner continually open, and nature had found no contrivance 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 con- 

 duit, 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 com- 

 pressed 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 subtile 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 



1 De Usu partium, lib. vi, cap. 10. 



