THE DEVELOPMENT OF THE VASCULAR SYSTEM 



251 



each side of the aorta. In the cervical region they lie dorsal to the branchial 

 arches and are called the anterior cardinal veins (Figs. 200 and 231). The 

 more caudal parts of the vessels are situated in the region of the developing 

 mesonephros and are called the posterior cardinal veins (Figs. 200 and 231). 

 At a point about opposite the heart the anterior and posterior cardinals on 

 each side unite to form a single vessel, the duct of Cuvier, which turns medially 

 through the septum transversum and opens into the sinus venosus (Figs. 

 200 and 216). Thus three primary sets of veins are formed at a very early 

 stage of development: (i) The omphalomesenteric veins; (2) the umbilical 

 veins; (3) the cardinal veins. 



The veins of the head and neck regions are derivatives of the anterior 

 cardinals. The proximal Darts of these vessels are present in embryos of 

 3.2 mm.; later they extend cranially along the ventro-lateral surface of the 



N.V N.VII N.IX 



Mid. cerebral vein 



Sup. cerebral vein 



Inf. cerebral vein 



Lat. vein of head 



FIG. 229. Veins of the head of a 9 mm. human embryo. Mall. 



brain on the medial side of the roots of the cranial nerves. The position 

 relative to the nerves is only temporary, however, for collaterals arising from 

 the veins pass to the lateral side of the nerves and enlarge to form the main 

 channels. The original channels atrophy except in the region of the trigemi- 

 nal nerves where they still remain on the medial side of the nerves as the 

 forerunners of the cavernous sinuses. The vessel thus formed laterally to the 

 cranial nerves (except the trigeminal) on each side of the brain is known as 

 the lateral vein of the head (vena later alis capitis) (Fig. 229.) The blood is 

 collected from the brain region by small vessels which unite to form three 

 main stems; one of these, the superior cerebral vein, opens into the cranial end 

 of the cavernous sinus; another, the middle cerebral vein, opens into the op- 

 posite end of the cavernous sinus; and the third, the inferior cerebral vein, 

 opens into the lateral vein of the head behind the ear vesicle (Figs. 229 and 



