CIRCULATION OF THE FCETUS. 223 



mixed together; the two auricles contract simultaneously, and not 

 one after the other; it is not probable that the vivified blood fur- 

 nished by the umbilical vein passes wholly to the superior half of 

 the body, and that the venous blood alone is expended in the re- 

 maining moiety. But seeing that the vena cava inferior,.surmounted 

 by the Eustachian valve, seems to be continuous wUh the foramen 

 ovale, rather than to open merely into the right auricle, and that the 

 cava superior opens opposite to the orifice of the right ventricle, and 

 on a plane which is rather in front of the inferior cava, it appears to 

 me we may conceive that the blood of the two vessels may really 

 pass into the left auricle and left ventricle, without necessarily mixing. 

 The simultaneous contraction of the auricles does not seem to oppose 

 this transfer; the blood of the inferior cava does not pass through 

 the foramen ovale during the contraction of the organ, nor does that 

 of the superior cava get into the ventricle during that action; if they 

 are both full of fluid at the moment the systole commences, what is 

 to prevent their passing it, without mixing, into the corresponding 

 cardiac ventricle? 



I believe, therefore, that Sabattier's theory is the best founded, 

 and that only a very small quantity of the blood poured into the right 

 auricle by the two cava? respectively is mixed there. 



572. However, it must not be supposed that the head and limbs 

 receive none but the blood brought to the heart by the umbilical 

 vein and its branches, nor that the abdomen and inferior extremities 

 are nourished only by the blood of the superior cava; on the one 

 hand, it would be absurd to suppose that that which is driven by the 

 left ventricle into the arch of the aorta, passes on into the carotid 

 and subclavian arteries, without some of it descending along the 

 thoracic aorta; and on the other, even were that the case, this blood 

 is no longer as pure as it was on leaving the placenta, for it is neces- 

 sarily mixed with the venous blood of the lower extremities and 

 abdomen. Further, the blood that passes along the aorta descen- 

 dens is not merely the blood of the ductus arteriosus, but with it 

 also is mixed that of the inferior cava. 



573. In the placenta. Some persons have supposed that tlie blood 

 brought back by the umbilical arteries is taken up by the uterine 

 veins, and proceeds to be revivified in the lungs of the mother, before 

 returning to the ovum; others have thought that only a portion of it 

 is absorbed, while the rest passes immediately into the capillaries of 

 the vein; that there is in some sense two circulations, one great cir- 

 culation, completely under the influence of the heart and lungs of 

 the mother, and one lesser circulation, the only one really belonging 

 to the foetus. What has been said above will, I think, suffice to 



