CIRCULATION OF THE BLOOD 97 



Farther, in this foramen ovale, from that part which regards 

 the pulmonary vein, there is a thin tough membrane, larger 

 than the opening, extended like an operctilum or cover; this 

 membrane in the adult blocking up the foramen, and ad- 

 hering on all sides, finally closes it up, and almost obliterates 

 every trace of it. In the foetus, however, this membrane is 

 so contrived that falling loosely upon itself, it permits a 

 ready access to the lungs and heart, yielding a passage to 

 the blood which is streaming from the cava, and hindering 

 the tide at the same time from flowing back into that vein. 

 All things, in short, permit us to believe that in the em- 

 bryo the blood must constantly pass by this foramen from 

 the vena cava into the pulmonary vein, and from thence 

 into the left auricle of the heart; and having once entered 

 there, it can never regurgitate. 



Another union is that by the pulmonary artery, and is 

 effected when that vessel divides into two branches after 

 its escape from the right ventricle of the heart. It is as if 

 to the two trunks already mentioned a third were super- 

 added, a kind of arterial canal, carried obliquely from the 

 pulmonary artery, to perforate and terminate in the great 

 artery or aorta. So that in the dissection of the embryo, 

 as it were, two aortas, or two roots of the great artery, 

 appear springing from the heart. This canal shrinks gradu- 

 ally after birth, and after a time becomes withered, and 

 finally almost removed, like the umbilical vessels. 



The arterial canal contains- no membrane or valve to direct 

 or impede the flow of blood in this or in that direction: for 

 at the root of the pulmonary artery, of which the arterial 

 canal is the continuation in the foetus, there are three semi- 

 lunar valves, which open from within outwards, and oppose 

 no obstacle to the blood flowing in this direction or from 

 the right ventricle into the pulmonary artery and aorta; 

 but they prevent all regurgitation from the aorta or pul- 

 monic vessels back upon the right ventricle; closing with 

 perfect accuracy, they oppose an effectual obstacle to every- 

 thing of the kind in the embryo. So that there is also rea- 

 son to believe that when the heart contracts, the blood is 

 regularly propelled by the canal or passage indicated from 

 the right ventricle into the aorta. 



(4) HC xxxvin 



