AN ANATOMICAL STUDY ON THE 



of animals with lungs, while the lungs are not used, 

 as in those animals themselves without lungs. 



So it seems obviously true in the fetus that the 

 heart by its beat transfers blood from the vena cava 

 to the aorta by as open a passage as if in the adult, 

 as I have said, the two ventricles were united by 

 removing the septum. Since these ways for the pas- 

 sage of blood are so conspicuous in the majority of 

 animals, — indeed in all at certain times, — we must 

 examine another matter. Why may we not conclude 

 that this passage is made through the substance of 

 the lungs in warm-blooded adult animals as man? 

 Nature made these ways in the embryo at a time when 

 the lungs were not used, apparently because of the 

 lack of a passage through them. Why is it better, for 

 Nature always does what is best, to close completely 

 to the passage of blood in adolescence those open 

 ways which are used in the embryos of so many 

 animals, without opening any others for this transfer 

 of blood? 



The situation is such that those who seek the ways 

 in man by which blood reaches the pulmonary vein 

 and left ventricle from the vena cava, will do best to 

 proceed by animal experimentation.^ Here the reason 



^ Most of Harvey's doctrine was developed from studies in compara- 

 tive anatomy and physiology. He was acutely aware of the value of 

 animal experimentation, which had already been specifically recom- 

 mended by Vesalius (1514-1564) and Realdus Columbus (1516-1559). 

 There is evidence that Harvey deplored the suffering involved in ani- 

 mal experimentation, and that he spoke feelingly on it. (S. Weir 

 Mitchell, Some Recently Discovered Letters of William Harvey, Phila., 



[58I 



