THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



223 



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

 I veins, including the subdavian veins, open at first into the posterior cardinals 

 (Fig. 193). 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. 195). The bilateral symmetry is 

 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. 194, B, and 195). The portion of the left 

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



Ant. cardinal 



Duct of Cuvier 

 Subclavian 



Inf. vena cava 



Post, cardinal 



Iliac 



.. Ant. cardinal 

 (int. jugular) 



Ext. jugular 



Subclavian 

 Duct of Cuvier 



Inf. vena cava 

 -Post, cardinal 



- Post, cardinal 

 Subcardinal 



Iliac 



A B 



FIG. 194. Diagrams of two stages in the development of the anterior and posterior cardinal veins, 

 the subcardinal veins (revehent veins of the primitive kidney), and the inferior vena cava. 

 The small branches of the cardinals and subcardinals ramify in the primitive kidneys 

 (mesonephroi). Slightly modified from Hochstetter. 



communciates with the intracranial sinuses. The anastomosis itself be- 

 comes the left innominate vein. The portion of the left cardinal between the 

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

 of the sinus venosus together form the coronary sinus (Fig. 196). On the 

 right side the more distal part of the cardinal becomes the internal jugular 

 vein; the portion between the subclavian and the anastomosis (left innomi- 

 nate vein) becomes the right innominate vein ; and the common stem formed 

 by the latter and the left innominate constitutes the superior vena cava 

 which opens into the right atrium (see p. 205) . The external jugular vein 

 on each side appears later than the superior cardinal as an independent 



