64 CIRCULATION OF THE BLOOD 



distributing spirit and a higher form of vital activity, 

 while the veins distributed nourishment and the lowest 

 forms of vital activity. The arteries arose from the heart, 

 and the veins, it was thought, from the liver. The words 

 artery and vein meant something different of old to what 

 they mean to us. The two systems, arterial and venous, 

 were recognised to be intimately connected. Both were 

 linked with the heart, though the relation of the arteries 

 was the closer. One branch of the venous system, 

 however, opened into the right cavity of the heart. 

 The vessel, therefore, that arose from that cavity and 

 linked it with the lung was thus really a branch of the 

 venous system, and was therefore a " vein." It had thick 

 walls like an artery and was therefore called the arterial 

 vein. It is the vessel we now call the pulmonary artery. 



A companion to this vessel connected also the left 

 cavity of the heart with the lung. As a derivative of the 

 left side of the heart from which the aorta arose, it was 

 regarded as an " artery." Its walls, however, were thin 

 like a vein, and it was called, therefore, the venal artery. 

 It is the vessel we now call the pulmonary vein. 



A result of the discovery of Harvey was the intro- 

 duction of a new nomenclature. For us who look at the 

 circulation from his point of view, new names are needed. 

 The arterial vein has become the pulmonary artery, and 

 the venal artery has become the pulmonary vein. For 

 us an artery is a vessel which takes blood from the heart 

 and a vein is a vessel which takes blood to the heart. 

 For the men of Harvey's day arteries were distinguished 

 from veins by containing a different kind of blood. They 

 had no idea of the constant and massive change of arterial 

 into venous, and of venous into arterial blood. Our 

 conception of the difference between artery and vein 

 is controlled by our idea of the circulation introduced 

 by Harvey. A new nomenclature was therefore needed 



