2 7 6 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



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

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

 sistence of vessels usually obliterated, e. g., double superior or inferior venae cavas; right 

 aortic arch; permanent ductus arteriosus; (3) to incomplete development, e. g., double 

 aortae. 



Dorsal subdavian vei 



V. lingua-facial is 



V. linguo-facialis 

 Ant. cardinal win 



'om. cardinal win 



Dorsal subdavian 

 vein 



V . ulnaris prima 



V. thoraco-epigastrica Post, cardinal vein 



V, cephalica 



V. linguo-facialis V. jugularis 

 externa 



V. cephalica 



V. card. ant. 



V. card. com. 

 V. card post. 



V.brachialis' 



FIG. 285. Four reconstructions of the veins of the human right arm (after F. T. Lewis). 

 X about 15. A, 10 mm. embryo; B, 11.5 mm. embryo; C, 16 mm. embryo; D, 22.8 mm. 

 embryo. 



THE FETAL CIRCULATION AND CHANGES AT BIRTH 



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 small amount of venous blood brought in by the 

 portal vein. Thence 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 



