256 



TEXT-BOOK OF EMBRYOLOGY. 



heart is situated in the cervical region, are comparatively short and receive 

 blood from the cervical region through segmental branches which belong only 

 to the most cranial of the cervical segments. The other segmental cervical 

 veins, including the subclavian veins, open at first into the posterior cardinals 

 (Fig. 231). Later, however, as the heart recedes into the thorax the anterior 

 cardinal veins are elongated and the segmental cervical veins, including the 

 subclavians, come to open into them <Fig. 233). The bilateral symmetry is 



Ext. jugular. 



Umbilical 



Omphalomesenteric 

 (vitelline) 



Mesonephros 



Subcardinal 



Duct of Cuvier 



Post, cardinal 



Fig. 231. — Diagram of the venous system of a human embryo of 2.6 mm. 

 Slightly modified from Kollmann's Atlas. 



then broken by an anastomosing vessel which extends obliquely across from a 

 point on the left cardinal about opposite the subclavian to a point nearer the 

 heart on the right subclavian (Figs. 232,8, and 233). The portion of the left 

 cardinal cranial to. the subclavian becomes the left internal jugular vein which 

 communicates with the intracranial sinuses. The anastomosis itself becomes 

 the left innominate vein. The portion of the left cardinal between the sub- 

 clavian and the duct of Cuvier, the duct of Cuvier itself, and the left horn of the 

 snius venosus together form the coronary sinus (Fig. 234). On the right side 



