54 CIRCULATION OF THE BLOOD 



by these very valves. From the right ventricle there is, 

 however, one exit, the pulmonary artery, or, as Harvey 

 calls it, the arterial vein. If it enters that vessel it cannot 

 regurgitate again, for ' ' there are three sigmoid or semi- 

 lunar valves at the orifice of the arterial vein [pulmonary 

 artery], which completely prevent blood sent into the 

 vessel from returning to the cavity of the heart." 



The whole passage which follows is saturated with Galen, 

 who had gained an idea of the action of these valves with- 

 out being able to draw the conclusions that Harvey 

 reached. ' From Galen," says Harvey, " that divine 

 man, that Father of Physicians, it clearly appears that 

 blood passes through the lungs from the arterial vein 

 [pulmonary artery] to the small branches of the venal 

 artery [pulmonary vein]." 



Now it may reasonably be doubted here whether 

 Harvey has rightly interpreted Galen. In saying that 

 in the works of Galen the matter clearly appears, he is 

 certainly giving Galen something much more than his 

 due. The passages in the works of Galen that refer to 

 this matter are neither clear nor are they even consistent 

 with each other. There is, however, one short para- 

 graph in Galen's very voluminous works which may bear 

 the interpretation that Harvey put on it. Whether 

 Galen meant it as Harvey interprets it we need not 

 discuss. The important thing, however, is that Harvey, 

 with all his over-respect for Galen, has the instincts of a 

 great investigator and interpreter, and can see the real 

 truth through Galen's verbiage. It is one of the many 

 cases in the history of science in which a truth half seen 

 or but dimly seen, and therefore imperfectly and obscurely 

 expressed by an earlier writer, awaited a later investigator 

 to grasp its nature and importance and to express it ade- 

 quately. To whom shall the honour be given ? It is a 

 question that can never be answered fully, because science 



