DEVELOPMENT OF THE VEINS 271 



Of the umbilical veins, the right disappears early; the left persists 

 during fetal life, shifts to the median plane, and courses in the free edge 

 of the falciform ligament. After birth its lumen is closed, and from the 

 umbilicus to the liver it forms the ligamentum tere's. In early stages, 

 veins from the body wall drain into the umbilical veins. 



The Anterior Cardinal Veins and the Origin of the Superior Vena 

 Cava. The anterior cardinal veins consist each of two parts (Fig. 271): 



(1) the true anterior cardinals, located laterad in the segmented portion 

 of the head and neck and draining into the common cardinal veins; 



(2) the vena capitis medialis, extending into the unsegmented head proper 

 and running ventro-lateral to the brain wall. In embryos of 20 mm. there 

 has formed by anastomosis a large connection between the right and left 

 anterior cardinals, which carries the blood from the left side of the head 

 into the right vein (Fig. 282 C). Soon, the left anterior cardinal loses 

 its connection with the common cardinal on the left side; the remaining 

 portions become the accessory hemiazygos and the inconstant oblique 

 vein of the left atrium (Fig. 282 D). The proximal portion of the left 

 common cardinal, with the tranverse portion of the sinus venosus, persists 

 as the coronary sinus. The right common cardinal and the right anterior 

 cardinal vein, as far as its anastomosis with the left anterior cardinal 

 become the superior vena cava. The anastomosis itself forms the left 

 vena anonyma, while that portion of the right anterior cardinal between 

 the anastomosis and the right subclavian vein is known as the right 

 vena anonyma. The distal portions of the anterior cardinals become the 

 internal jugular veins of the adult, while the external jugular and sub- 

 clavian veins are new vessels which develop somewhat later. 



The vena capitis medialis (Fig. 271) is the continuation of the anterior cardinal vein 

 into the head of the embryo, where at first it lies mesial to the cerebral nerves. Later, it .is 

 partly shifted by anastomoses lateral to the cerebral nerves and forms the vena capitis 

 lateralis (Figs. 283 and 284). In n mm. embryos this emerges with the n. facialis, and, 

 caudal to the n. hypoglossus, becomes the internal jugular. Cranially, in the region of the 

 fifth nerve, the median vein of the head persists as the sinus caver nosus and receives the 

 ophthalmic vein from the eye and the anterior cerebral vein from the fore- and mid-brain 

 regions (Fig. 284 C). Between the n. trigeminus and the facialis, the middle cerebral vein 

 from the metencephalon (cerebellum) joins the v. capitis lateralis before it leaves the 

 cranium. More caudally, the posterior cerebral vein from the myelencephalon emerges 

 through the jugular foramen and is drained with the others by the v. caprtis lateralis into 

 the internal jugular (Fig. 284 B). Soon, the cerebral veins reach the dorsal median line 

 (Fig. 284 C), and longitudinal anastomoses are formed: (i) between the anterior and mid- 

 dle cerebral veins, giving rise to the superior sagittal sinus; and (2) between the middle 

 and posterior cerebral veins forming the greater part of the transverse sinsuses. In embryos 

 of 33 mm. the v. capitis lateralis disappears and the blood from the brain passes through the 

 superior sagittal and lateral sinuses and is drained by way of the jugular foramen into the 

 internal jugular vein (Fig. 284 C, D). The middle cerebral vein becomes the superior 



