Heart and Blood 47 



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 clearly, therefore, does it appear in the case of 

 the foetus, viz. 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 communicate by the removal 

 of their septum. Since, then, we find that in the 

 greater number of animals, in all, indeed, at a certain 

 period of their existence, the channels for the trans- 

 mission of the blood through the heart are so con- 

 spicuous, we have still to inquire wherefore in some 

 creatures those, namely, that have warm blood, and 

 that have attained to the adult age, man among the 

 number we should not conclude that the same thins 



O 



is accomplished through the substance 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 wherefore it should be better (for 

 nature always does that which is best) that she should 

 close up the various open routes which she had for- 

 merly made use of in the embryo and foetus, and still 

 uses in all other animals ; not only opening up no new 

 apparent channels for the passage of the blood, there- 

 fore, but even entirely shutting 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 pul- 

 monary veins from the vena cava, will pursue the 

 wisest course if they seek by dissection to discover the 

 causes why in the larger and more perfect animals of 

 mature age, nature has rather chosen to make the 



