DEVELOPMENT OF THE VEINS 



273 



jugular veins of the adult, while the external jugulars are new veins 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 hes mesial to the cerebral nerves. Later jt,,is 

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

 (Figs. 283 and 284). In 11 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 cavernosus and receives the ophthalmic vein from 

 the eye and the atiterior 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 



Middle cerebral vein 



Posterior cerebral vein 



Sinus cavernosus 

 Anterior cerebral vein 



N. hypoglossus 

 V. capitis lateralis 



Fig. 283. — Veins of the head of a 9 mm. human embryo (after Mall). X 9. 



posterior cerebral vein from the myelencephalon emerges through the jugular foramen and is 

 drained with the others by the v. capitis laterahs into the internal jugular (Fig. 284 B). 

 Soon the cerebral veins reach the dorsal median line (Fig. 284 C), and longitudinal anas- 

 tomoses 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 transverse sinuses. 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 petrosal sinus, but the inferior petrosal sinus is 

 formed as a new channel median to the internal ear. The anterior cerebral vein becomes the 

 superficial middle cerebral of adult anatomy. A more detailed account of these changes may 

 be found in the original work of Mall (Amer. Jour. Anat., vol. 4, 1905). 



