HEART AND BLOOD. 39 



more than if they had not been present, nature uses the two 

 ventricles of the heart as if they formed but one, for the trans- 

 mission 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 ob- 

 vious 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 conspicuous, 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 

 thing 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 formerly 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, therefore, 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 pulmonary veins from the vena cava, 

 will pursue the wisest course if they seek by dissection to dis- 

 cover the causes why in the larger and more perfect animals 

 of mature age, nature has rather chosen to make the blood per- 

 colate the parenchyma of the lungs, than as in other instances 

 chosen a direct and obvious course for I assume that no other 

 path or mode of transit can be entertained. It must be either 

 because the larger and more perfect animals are warmer, and 

 when adult their heat greater ignited, as I might say, and 

 requiring to be damped or mitigated ; therefore it may be that 



