276 



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 vense cavae; right 

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

 aortae. 



Dorsal subdavian 



Vena ulnaris 

 prima 



V. linguo-facialis' 



V. linguo-facialis 

 Ant. cardinal vein 



Com. cardinal vein 



Dorsal subdavian 

 vein 



V . ulnaris prima 



V. thoraco-epigastrica Post, cardinal vein 



I 



V. thoraco- 

 epigastrica 



Ventral sub- 

 daman vein 



V. cephalica 



D 



V. linguo-facialis V. jugularis 

 externa 



V. cephalica 



V. card. ant. 



V. card. com. 

 V. card post. 



V. jugularis interna 



\ 



V. linguo-facialis 



V.jug. anterior 



V. anonyma 

 dextra 



V. anonyma 



sinistra 

 V. mammaria 



int - 



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 



