DEVELOPMENT OF THE VASCULAR SYSTEM. 703 



right side, an oval depression, termed the fossa ovalis, indicating the 

 site of the original foramen ovale. The fossa ovalis is surrounded by a 

 slightly raised ring, the annulus ovalis, representing the edge of the 

 original inter-auricular septum. 



The foramen ovale is sometimes completely obliterated within a few 

 days after birth, but often continues partially pervious for several weeks 

 or months ; and it is not unfrequent to find a small aperture remaining 

 in adult life. In these instances, although the consolidation of the inter- 

 auricular septum is incomplete, yet no admixture of blood takes place 

 between the two sides of the heart. The oblique direction of the pas- 

 sage and its valvular arrangement prevent any regurgitation from left 

 to right ; and the complete filling of the left auricle with blood from 

 the lungs is a sufficient obstacle to the entrance of venous blood from 

 the right. 



