56 DESCARTES. 



flows from the veins, and that from the arteries, 

 can only arise from this, that being rarefied, and, as 

 it were, distilled by passing through the heart, it is 

 thinner, and more vivid, and warmer immediately 

 after leaving the heart, in other words, when in the 

 arteries, than it was a short time before passing into 

 either, in other words, when it was in the veins; 

 and if attention be given, it will be found that this 

 difference is very marked only in the neighbourhood 

 of the heart ; and is not so evident in parts more 

 remote from it. In the next place, the consistency 

 of the coats of which the arterial vein and the great 

 artery are composed, sufficiently shows that the 

 blood is impelled against them with more force than 

 against the veins. And why should the left cavity 

 of the heart and the great artery be wider and 

 larger than the right cavity and the arterial vein, 

 were it not that the blood of the venous artery, hav 

 ing only been in the lungs after it has passed 

 through the heart, is thinner, and rarefies more 

 readily, and in a higher degree, than the blood 

 which proceeds immediately from the hollow vein? 

 And what can physicians conjecture from feeling 

 the pulse unless they know that according as the 

 blood changes its nature it can be rarefied by the 

 warmth of the heart, in a higher or lower degree, 

 and more or less quickly than before? And if it be 

 inquired how this heat is communicated to the other 

 members, must it not be admitted that this is 

 effected by means of the blood, which, passing 

 through the heart, is there heated anew, and thence 

 diffused over all the body? Whence it happens, 



