686 DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



rent to the edges of the septum. The auricular septum in the adult 

 heart is, therefore, thinner at this spot than elsewhere ; and presents, 

 on the side of the right auricle, an oval depression, termed the fossa 

 ovalis, which indicates the site of the original foramen ovale. The 

 fossa ovalis is surrounded by a slightly raised ring, the annulus 

 ovalis, representing the curvilinear edge of the original auricular 

 septum. 



The foramen ovale is sometimes completely obliterated within a 

 few days after birth. It often, however, remains partially pervious 

 for several weeks or months. We have a specimen, taken from a 

 child of one year and nine months, in which the opening is still 

 very distinct; and it is not unfrequent to find a small aperture 

 existing even in adult life. In these instances, however, although 

 the adhesion and solidification of the auricular septum may not be 

 complete, yet no disturbance of the circulation results, and no ad- 

 mixture of blood takes place between the right and left sides of the 

 heart ; since the passage through the auricular septum is always 

 very oblique in its direction, and its valvular arrangement prevents 

 any regurgitation from left to right, while the complete filling of 

 the left auricle with pulmonary blood, as above mentioned, equally 

 opposes any passage from right to left. 



