Circulation of the Blood 161 



conjectural. I would ask of those who profess a 

 knowledge of the causes of all things, why the two 

 eyes keep constantly moving together, up or down, 

 to this side or to that, and not independently, one 

 looking this way, another that ; why the two auricles 

 of the heart contract simultaneously, and the like ? 

 Are fevers, pestilence, and the wonderful properties of 

 various medicines to be denied because their causes 

 are unknown? Who can tell us why the foetus in 

 utero, breathing no air up to the tenth month of its 

 existence, is yet not suffocated? Born in the course 

 of the seventh or eighth month, and having once 

 breathed, it is nevertheless speedily suffocated if its 

 respiration be interrupted. Why can the foetus still 

 contained within the uterus, or enveloped in the 

 membranes, live without respiration ; whilst once 

 exposed to the air, unless it breathes it inevitably 

 dies? l 



Observing that many hesitate to acknowledge the 

 circulation, and others oppose it, because, as I con- 

 ceive, they have not rightly understood me, I shall 

 here recapitulate briefly what I have said in my work 

 on the Motion of the Heart and Blood. The blood 

 contained in the veins, in its magazine, and where it 

 is collected in largest quantity, viz. in the vena cava, 

 close to the base of the heart and right auricle, gradually 

 increasing in temperature by its internal heat, and be- 

 coming attenuated, swells and rises like bodies in a 

 state of fermentation, whereby the auricle being dilated, 

 and then contracting, in virtue of its pulsative power, 

 forthwith delivers its charge into the right ventricle ; 

 which being filled, and the systole ensuing, the charge, 

 hindered from returning into the auricle by the tricuspid 

 valves, is forced into the pulmonary artery, which stands 

 open to receive it, and is immediately distended with 

 it. Once in the pulmonary artery, the blood cannot 

 return, by reason of the sigmoid valves ; and then the 



Vide Chapter VI. of the Disq. on the Motion of the Heart and Blood. 



M 



