Heart and Blood 51 



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 following lan- 

 guage : 1 " 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 undoubtedly very narrow passages. Now 

 if the mouth of the vena arteriosa, or pulmonary artery, 

 had stood in like manner 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 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 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 com- 

 pressed on every side, a certain portion of it distils into 

 the pulmonary veins by the minute orifices mentioned." 



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



