DEVELOPMENT OF THE VEINS 279 



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

 fetal life, shifts to the median line 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 teres. In early stages veins from the body wall drain into 

 the umbilical veins. 



The Anterior Cardinal Veins and the Origin of the Vena Cava Superior. — 

 The anterior cardinal veins consist each of two parts: (i) The true anterior cardi- 

 nals 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 em- 

 bryos 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. 274, C). Soon the left anterior cardinal loses its 

 connection with the common cardinal on the left side (Fig. 274, D). The proxi- 

 mal portion of the left common cardinal with the transverse 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 be- 

 come the superior vena cava. The anastomosis itself forms the left vena anonyma, 

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

 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 jugulars are new veins which develop much later. 



The vena capitis medialis 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 

 anastomosis lateral to the cerebral nerves and forms the vena capitis lateralis (Figs. 275, 276). 

 In 1 1 mm. embryos this emerges with the n. facialis and caudal to the n. hypoglossus becomes 

 the internal jugular. Cranially the median vein of the head persists as the sinus cavemosus 

 and receives the ophthalmic vein from the eye, and the middle cerebral vein from the fore- and 

 mid-brain regions. 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 for- 

 amen and is drained with the others by the v. capitis lateralis into the internal jugular (Fig. 

 276, B). Soon the three cerebral veins reach the dorsal median line (Fig. 276,0), and longitudinal 

 anastomoses are formed: (1) between the anterior and middle cerebral veins, giving rise to the 

 superior sagittal sinus; and (2) between the middle and posterior cerebral veins forming the greater 

 part of the lateral sinuses. In embryos of 3$ 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. 276, C, D). The middle cerebral 

 vein becomes the superior petrosal sinus, but the inferior petrosal sinus is formed as a new channel 

 median to the internal ear. For a more detailed account of the origin of the cephalic veins 

 the student is referred to the original work of Mall (Amer. Jour. Anat., vol. 4, 1905). 



