The Fetal Circulation ' 329 



through the umbilicus, to pass forward to the posterior border 

 of the liver, where it unites with the hepatic portal vein, so that 

 the combined vessel now carries blood from the intestine, from 

 the vitelline veins of the earlier period and from the placenta. 



For a period of time, generally extending to the date of birth, 

 the blood may pass either through the capillary system of the 

 liver or, entering the ductus venosus, may pass directly to the 

 posterior vena cava without going through that gland. 



The blood which is carried to the heart by the posterior vena 

 cava is largely arterial in character, that is, it has given up its 

 carbon dioxide and other waste material in the placenta and has 

 derived, from the blood of the mother, oxygen and nutritive 

 materials, which these veins carry to the heart. Into this 

 posterior vena cava, there also enters the blood which has been 

 to some degree favorably modified by passing through the kidneys 

 and the liver, in which glands certain deleterious substances 

 have been removed. The blood from the posterior vena cava 

 consequently presents a marked contrast to that from the anterior 

 vessel, which returns only the blood sent through the arteries to 

 the head, neck and anterior limbs after it has performed its 

 nutritive office to those parts and, consequently, comes back 

 charged with waste materials, without having received in its 

 course any additional nutrient matter or having undergone any 

 purification in passing through the tissues or organs. The 

 blood brought by the anterior vena cava passes into the right 

 auricle and thence into the right ventricle, from which it is 

 driven along the pulmonary artery. Since the lungs are not yet 

 functioning, only a small portion of it enters these organs and 

 virtually all of it pas.ses through the ductus arteriosus to the 

 dorsal, or posterior, aorta. Connecting with the latter at an 

 acute angle, the blood is directed backward toward the posterior 

 part of the body, whence it largely passes to the placenta, through 

 the umbilical arteries, where it is relieved of its waste matters 

 and, in exchange, receives oxygen and nutrient material. On 

 the return of this arterial blood from the placental capillaries, 

 along the umbilic veins, with its small admixture of venous 

 blood along with that which has been somewhat modified by 

 passing through the kidneys and liver, it re-enters the right 

 auricle, to at once pass over into the left auricle and thence into 



