ANATOMY OF THE F(ETUS. 259 



oxygenized blood from the placenta is brought by the um- 

 bilical vein to the inferior vena cava, where it of course 

 mixes with the impure blood from the lower extremities ; 

 entering the right auricle, its current is directed by the 

 large Eustachian valve toward the foramen ovale, through 

 which it passes into the left auricle. The blood from the 

 head and upper extremities, which is returned by the supe- 

 rior vena cava as venous blood, enters the right auricle, and 

 passes by the auriculo-ventricular opening into the right 

 ventricle ; the crossing of these two currents in the right 

 auricle is permitted by the construction of the Eustachian 

 valve, though, to a certain extent, the two streams must 

 intermingle. 



We thus see that the blood brought to the right auricle 

 has two sources of exit, viz : the foramen ovale and the 

 auricnlo-ventricular orifice. By following the blood from 

 the cavities into which these orifices open, as if it had 

 started originally from these points, we shall at once com- 

 prehend the foetal circulation. Thus, that from the right 

 ventricle must enter the pulmonary artery which goes to 

 the lungs; but the lungs are solid and impervious, and 

 nature has therefore provided an exit for it by the ductus 

 arteriosus, through which it reaches the aorta at a point 

 bej^ond the origin of the brachio-cephalic branches, and 

 is carried to the body and lower extremities. Then, again, 

 the blood which is in the left auricle can go only to the left 

 ventricle, and from there it must go to the aorta. This 

 portion of the blood being that from the placenta, and 

 therefore the most richly oxygenized, is able to enter the 

 brachio-cephalic trunks, which the other current could not 

 do, and supplies the important parts to which they are 

 distributed. Beyond the origin of the brachio-cephalic 

 trunks that portion of the blood which has not entered 

 those vessels, joins with the current from the ductus arte- 

 riosus, helps to supply the body and lower extremities, and 

 is returned to the placenta by the hypogastric arteries. 



