ATRESIA OF THE AORTIC ORIFICE. 5 



This disposition of the valve, it seems to me, can only be 

 explained by the method of development of the interauricular 

 septum, as observed by Born^ in rabbit embryos, — namely, the 

 appearance of a second septum (septum secundum) which grows 

 downward and forward on the right side of the first septum, and 

 partly closes the foramen ovale, giving rise ordinarily to the 

 limbus Vieussenii, while the free edge of the first septum ordin- 

 arily forms the valve of the foramen ovale. In this instance, 

 however, the septum secundum appears to have grown entirely 

 across the lumen of the foramen, and thus forms the valve 

 of the foramen ovale, while the first septum forms a limbus 

 which appears in the left auricle. 



If this is the true explanation, it would tend to prove that 

 the development of this region is the same in man, and probably 

 in other mammalia, as in the rabbit. 



The obliteration of the aortic orifice can be explained, it 

 appears to me, by the faulty development of the valve in the 

 interauricular orifice. The valve effectually prevents the flow 

 of blood from the right auricle to the left, but allows it to pass 

 easily from the left auricle to the right. 



Such being the case, the very small amount of blood that is 

 returned from the lungs in early foetal life would pass through 

 to the right auricle. Consequently, the left ventricle would 

 have practically no blood to act upon, which fact would lead to 

 a faulty development, and probably to an insufficiency of the 

 auriculo-ventricular valve, so that when the flow of blood did 

 increase from the pulmonary veins, the left heart simply 

 impelled it into the right auricle in the line of least resistance. 



The aortic valve was kept always closed by the intra-arterial 

 pressure exerted by the right ventricle through the ductus 

 arteriosus, and the segments of the aortic valve fused. 



The portion of the aorta between the valves and the first 

 branch, innominate artery, was only needed for the supply of the 

 coronary arteries, and consequently only developed sufficiently 

 to meet their requirements. Of course, the blood current in 

 the aorta, as far as the junction with the ductus arteriosus, was 

 in a direction opposite to the usual one. 



1 "Entwickelung des Siuigethier-herzens," Archiv. fr. Mikr. Anat., Bd. 33, 

 1889. 



