THE CIRCULATION. 199 



the capillary walls and is taken up by the foetal red cor- 

 puscles. There is, of course, no mixing whatever of the 

 maternal and foetal bloods. The two remain perfectly dis- 

 tinct, and the transfer of nourishment and oxygen is made 

 through the delicate capillary walls. This placenta is, 

 therefore, the foetal lung. Running to this placenta through 

 the umbilical cord or naval stalk from the foetus is a large 

 artery arising from the abdominal aorta. This carries the 

 blood from the foetus to the placenta. Here it passes 

 through the placental capillaries and is gathered up in the 

 veins and brought back to the foetus through the umbilical 

 cord by the umbilical vein. This, although called a vein, 

 contains arterial blood, having just received in the placenta 

 both its new supply of nourishment and oxygen. Upon 

 reaching the foetus this umbilical vein flows through the 

 liver, and from the liver the blood reaches the ascending 

 vena cava, by means of which it is carried to the right 

 auricle. This pure blood, going through the vena cava 

 in the way just described, does not drop from the right 

 auricle into the right ventricle, but goes from the right 

 auricle at once into the left auricle through an opening in 

 the auricular septum called the foramen ovale. The course 

 of blood in this direction is facilitated by a peculiar flap or 

 valve on the right auricle so placed that it guides the cur- 

 rent of blood across into the left auricle. This little flap is 

 called the Eustachian valve. This transfer is materially 

 aided by the fact that the ascending vena cava flows around 

 to the back of the right auricle before emptying into it, and 

 so has its opening close to the auricular septum. The pure 

 blood so transferred to the left auricle now drops into the 

 left ventricle, and by its systoles is forced out through the 

 aorta. For reasons to be pointed out in a moment, most of 

 this pure blood goes to the head and neck through the 

 innominate and carotid arteries. Thus, these rather more 

 important portions of the body are supplied with the best 

 available blood. 



