530 THE THORAX OF THE FCETUS. 



In the septum between the two auricles, is a foramen of suf- 

 ficient size to permit the passage of a large quill, which inclines 

 to the oval form, with its longest diameter vertical when the 

 body is erect. On the left side of the septum, a valve, formed 

 by the lining membranes, is connected to this foramen ; and 

 allows a free passage to a fluid moving from the right auricle 

 to the left ; but prevents the passage of a fluid from the left to 

 the right. This structure is evidently calculated to allow some 

 of the blood which flows into the right auricle from the two 

 vense cavae to pass into the left auricle of the heart, instead of 

 going into the right ventricle. As the contents of the left 

 auricle pass into the left ventricle, and from thence into the 

 aorta, it is obvious that the blood which passes from the right 

 auricle into the left through this foramen, must be transmitted 

 from the system of the vena cava to the system of the aorta, 

 without going through the lungs, as it must necessarily do in 

 subjects who do not enjoy the foetal structure. 



— The valve, with which in the foetus the foramen ovale 

 is provided on the side of the left auricle, is of a semilunar 

 shape and called the valve of Botal ; it has a convex border, 

 adherent, and turned downwards ; and a concave border, free, 

 and turned upwards. The angles resulting from the union 

 of these borders are at birth attached to each side of the fora- 

 men about a quarter of an inch distant from each other. The 

 valve makes its appearance in the foetus at the third month of 

 intra-uterine existence, and gradually increases in size, so as 

 to more than cover the foramen at the period of birth. When 

 the child breaths and the lungs become filled with blood, the 

 fluid, entering the left auricle by the pulmonary veins, throws 

 down the valve against the septum auriculorum, to which its 

 free border usually becomes firmly united. 



— Occasionally, however, the union of the parts is found so 

 incomplete, even in old persons, as to allow a probe or even 

 the handle of a scalpel to be passed obliquely through the 

 opening : the obliquity of the orifice being such, as usually to 

 enable it to act as a perfect valve. A communication of this 

 sort, of greater or less magnitude between the auricles, exists 



