HEART AND BLOOD. 3/ 



membrane, however, is so contrived in the fetus, 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 embryo the blood must constantly pass by this 

 foramen from the vena cava into the arteria venosa, or pulmo- 

 nary 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 vena arteriosa, or 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 superadded, a kind 

 of arterial canal, carried obliquely from the vena arteriosa, or 

 pulmonary artery, to perforate and terminate in the arteria 

 magna or aorta. In the embryo, consequently, there are, as it 

 were, two aortas, or two roots of the arteria magna, springing 

 from the heart. This canalis arteriosus shrinks gradually after 

 birth, and is at length and finally almost entirely withered, 

 and removed, like the umbilical vessels. 



The canalis arteriosus contains no membrane or valve to di- 

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

 for at the root of the vena arteriosa, or pulmonary artery, of 

 which the canalis arteriosus is the continuation in the foetus, 

 there are three sigmoid or semilunar 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 ar- 

 tery and aorta ; but they prevent all regurgitation from the aorta 

 or pulmonic vessels back upon the right ventricle ; closing with 

 perfect accuracy, they oppose an effectual obstacle to everything 

 of the kind in the embryo. So that there is also reason to 

 believe that when the heart contracts, the blood is regularly 

 propelled by the canal or passage indicated from the right ven- 

 tricle into the aorta. 



What is commonly said in regard to these two great com- 

 munications, to wit, that they exist for the nutrition of the lungs, 

 is both improbable and inconsistent ; seeing that in the adult 

 they are closed up, abolished, and consolidated, although the 

 lungs, by reason of their heat and motion, must then be pre- 

 sumed to require a larger supply of nourishment. The same may 



