CIRCULATION OP THE BLOOD 90 



artery and its continuation, named the ductus arteriosus, 

 into the aorta; the left, in like manner, charged by the con- 

 traction of its auricle, which has received its supply through 

 the foramen ovale from the vena cava, contracting, and 

 projecting the blood through the root of the aorta into the 

 trunk of that vessel. 



In embryos, consequently, whilst the lungs are yet in 

 a state of inaction, performing no function, subject to no 

 motion any more than if they had not been present, nature 

 uses the two ventricles of the heart as if they formed but 

 one, for the transmission of the blood. The condition of 

 the embryos of those animals which have lungs, whilst these 

 organs are yet in abeyance and not employed, is the same 

 as that of those animals which have no lungs. 



So it clearly appears in the case of the foetus that the 

 heart by its action transfers the blood from the vena cava 

 into the aorta, and that by a route as obvious and open, 

 as if in the adult the two ventricles were made to com- 

 municate by the removal of their septum. We therefore 

 find that in the greater number of animals in all, indeed, 

 at a certain period of their existence the channels for the 

 transmission of the blood through the heart are conspicuous. 

 But we have to inquire why in some creatures those, 

 namely, that have warm blood, and that have attained to the 

 adult age, man among the number we should not con- 

 clude that the same thing is accomplished through the sub- 

 stance of the lungs, which in the embryo, and at a time when 

 the function of these organs is in abeyance, nature effects 

 by the direct passages described, and which, indeed, she 

 seems compelled to adopt through want of a passage by the 

 lungs; or why it should be better (for nature always does 

 that which is best) that she should close up the various 

 open routes which she had formerly made use of in the 

 embryo and foetus, and still uses in all other animals. Not 

 only does she thereby open up no new apparent channels 

 for the passages of the blood, but she even shuts up those 

 which formerly existed. 



And now the discussion is brought to this point, that they 

 who inquire into the ways by which the blood reaches the 

 left ventricle of the heart and pulmonary veins from the 



