n] Circulation of the Blood. 33 



"The pulse of the arteries presents another difficulty. 

 " Of the vessels ending in the heart, some send into it the 

 " material which they carry, for instance the vena cava into 

 " the right ventricle, and the vein-like artery into the left ; some 

 " on the other hand carry material away from the heart, as for 

 " instance the aorta artery in the left ventricle and the artery- 

 " like vein nourishing the lung in the right. To each orifice are 

 " attached little membranes whose function is to secure that the 

 " orifices letting in do not lead out and that those leading out do 

 " not let in. It follows that when the heart contracts the arteries 

 " are dilated, and when it is dilated they are constricted; the two 

 " are not, it appears, constricted and dilated together. For when 

 11 the heart is dilated, it wishes that the orifices of the vessels 

 " which lead out should be shut so that material should not flow 

 " from the heart into the arteries, but that it should flow in 

 " this way when the heart contracts, the membranes gaping (and 

 " affording a passage). If therefore the arteries were dilated and 

 " constricted at the same time as the heart, it would follow that 

 " they would be dilated at the time when the material filling 

 "them from the heart was denied them, and constricted at a 

 " time when material was flowing into them from it. But it 

 "is manifest that this is impossible. To say therefore that 

 " the heart and arteries beat at different times is to deny one's 

 " senses and to doubt reason." 



He thus appears to have grasped the important truth, 

 hidden, it would seem, from all before him, that the heart, at 

 its systole, discharges its contents into the aorta (and pul- 

 monary artery), and at its diastole receives blood from the 

 vena cava (and pulmonary vein). 



Again, in his Medical Questions, he seems to have grasped 

 the facts of the flow from the arteries to the veins, and of the 

 flow along the veins to the heart. He says, Lib. n. Quaest. 17, 



"But the following matter seems worthy of consideration, 

 " the reason, namely, why veins when ligatured swell on the far 

 " side and not on the near side of the ligature. This is a fact 

 "well known by experience to those who let blood; for they 

 " place the ligature on the near side of the place of incision, 

 "not on the far side, because the veins swell on the far side, 

 f. l. 3 



