FETAL CIRCULATION 277 



develops secondarily in connection with the ulnar border vein;"later, in embryos of 23 mm., 

 it anastomoses with the external jugular and finally drains into the axillary vein as in the 

 adult. With the development of the digits, the vv. cephalica el basilica become distinct, as in 

 embryos of 35 mm., but later are again connected by a plexus on the dorsum mani, as in the 

 adult (Evans in Keibel and Mall, vol. 2). 



In the lower extremity the fibular portion of the primitive border vein persists. Later 

 the 11. saphena magna arises separately from the posterior cardinal, gives off the vv. femoralis 

 and tibialis posterior, and annexes the fibular border vein at the level of the knee. Distal to 

 this junction the border vein persists as the v. tibialis anterior and probably the v. saphena 

 parva; proximally it becomes greatly reduced, forming the v. glutea inferior. 



Anomalies. — Anomalous blood vessels are of common occurrence. They may be 

 due: (1) to the choice of unusual paths in the primitive vascular plexuses; (2) to the persist- 

 ence of vessels usually obliterated, e. g., double superior venae cavse; right aortic arch; per- 

 manent ductus arteriosus; (3) to incomplete development, e. g., double (unfused) heart or 

 double dorsal aortae. 



FETAL CIRCULATION 



During fetal life oxygenated placental blood enters the embryo by way of the 

 large umbilical vein and is conveyed to the liver (Fig. 286) . There it mingles with 

 the smaU amount of venous blood brought in by the portal vein. It is carried 

 to the inferior vena cava either directly, through the ductus venosus, or indirectly 

 through the liver sinusoids and hepatic vein. The impure blood of the inferior 

 vena cava and portal vein affects but slightly the greater volume of pure placental 

 blood. Entering the right atrium it mingles somewhat with the venous blood 

 returned through the superior vena cava. It is said that the blood from the 

 inferior vena cava is directed by the valve of this vein through the foramen ovale 

 into the left atrium (following the path of the sounds in Figs. 262 to 264) which, 

 before birth, receives Httle venous blood from the lungs. This purer blood of the 

 left atrium enters the left ventricle, and is driven out through the aorta to be dis- 

 tributed chiefly to the head and upper extremities. 



The venous blood of the superior vena cava, shghtly mixed, is supposed to 

 pass from the right atrium into the right ventricle, whence it passes out by the 

 pulmonary artery. A small amount of this blood is conveyed to the lungs by the 

 pulmonary arteries, but, as the fetal lungs do not function, most of it enters the 

 dorsal aorta by way of the ductus arteriosus. Since the ductus is caudal to the 

 origin of the subclavian and carotid arteries, its less pure blood is distributed to 

 the trunk, visqera, and lower extremities. The placental circuit is completed by 

 the hypogastric, or umbilical, arteries by way of the umbiKcal cord. 



Pohlman (Anat. Rec, vol. 2, 190&) interprets „.s experiments to indicate that, con- 

 trary to the generally accepted view, there is a mingUng of the blood which enters the right 

 atrium through the two caval veins. If this occurs there would be no difference in the 

 quality of blood distributed to the various parts of the body. 



