VIII.] THE EUSTACHIAN VALVE. 263 



which is now joined just 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 however the two can now be seen to be sepa- 

 rated 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 vense 

 cavse has apparently become merged. There is also on 

 the left side of the opening of the inferior cava a mem- 

 brane 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 cavse 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-circula- 

 tion 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 through the 

 third and fourth arches ; from these it is brought back 

 through the right auricle to the right ventricle, from 



