KNOWLEDGE IN ANCIENT TIMES 13 



sent blood charged with nourishment, absorbed from the 

 intestines, together with natural spirits absorbed from 

 the liver, to all parts of the body. In the veins this 

 nutritive blood was thought to ebb and flow, up and down. 

 During this ebb and flow, the veins absorbed into their 

 blood impurities from all parts of the body. No idea of 

 circulation was involved. A large branch of the venous 

 system entered the right ventricle. 



Two parts of this venous system are of special 

 interest to us for the understanding of Galen's ideas. 

 One was the vessel that he called the arterial vein 

 which goes to the lung. It is the structure we now call 

 the pulmonary artery. This vessel when it reached the 

 lung discharged, as he thought, all the impurities that 

 had been brought by the venous system from the various 

 parts of the body. The impurities passed into the air- 

 tubes of the lung, and were thence exhaled into the outer 

 air. Galen knew that animals could not live indefinitely 

 in a closed space, and died if so confined. This event 

 was thought by him to be due to the impure and poisonous 

 substances conveyed by the breath to the air. 



The second branch or rather series of branches of 

 Galen's venous system that are of special interest for our 

 purpose had no real existence. They were entirely a 

 product of Galen's imagination. Their existence, how- 

 ever, was firmly believed for many centuries, and this 

 belief was one of the main obstacles to the true explana- 

 tion of the action of the heart. Galen thought that some 

 of the blood which entered the right ventricle passed 

 through the muscular wall or septum that lies between the 

 two ventricles, and so reached the left side of the heart. 

 Now it happens that on the walls of the septum which 

 face the cavity of the two ventricles are a large number of 

 blind pits. It was believed that these pits represented a 

 series of minute passages from one ventricle of the heart 



