VII.] THE EUSTACHIAN VALVE. 195 



at its entrance into the heart by the right vena cava superior, 

 is directed into the left auricle. The left vena cava superior 

 enters the right auricle independently ; between it and the 

 inferior vena cava is a small valve which directs its blood 

 entirely into the right auricle. 



On the sixteenth day the right vena cava superior, when 

 viewed from the exterior, still appears to join the inferior 

 vena cava before entering the heart; from the interior how- 

 ever the two can now be seen to be separated by a valve. 

 This valve, called the 'Eustachian valve,' extends to the 

 opening of the left vena cava superior, and into it the valve 

 which in the earlier stage separated the left superior and 

 inferior venae cavae has apparently become merged. There is 

 also on the left side of the opening of the inferior cava a 

 membrane, stretching over the foramen ovale, and serving as 

 a valve for that orifice. The blood from the inferior cava 

 still passes chiefly into the left auricle through the foramen 

 ovale ; while the blood from the other two vense cavae now 

 falls into the right auricle, being prevented from entering 

 the left chamber by the Eustachian valve. 



Hence, since at this period also the blood from the left 

 ventricle passes to a great extent to the anterior portion 

 of the body, there is a species of double-circulation going 

 on. The greater part of the blood from the allantois entering 

 the left auricle from the inferior vena cava passes into the 

 left ventricle and is thence sent chiefly to the head and 

 anterior extremities ; from these it is brought back through 

 the right auricle to the right ventricle, from whence it is 

 returned along the aorta to the allantois. 



From the seventeenth to the nineteenth day, the right 

 auricle becomes larger than the left. The large Eustachian 

 valve still prevents the blood from the superior cavae from 

 entering the left auricle, while it conducts the blood from the 

 inferior vena cava into that chamber through the foramen 

 ovale. The entrance of the inferior vena cava is however 

 further removed than it was from the foramen ovale, and the 

 increased flow of blood from the lungs prevents all the blood 

 of the inferior cava from entering into the left auricle. At 

 the same time the valve of the foramen ovale prevents the 

 blood in the left auricle from entering the right auricle. 



During the period from the seventh day onwards, the 



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