DEVELOPMENT OF THE VASCULAR SYSTEM. 693 



larger than the vertebral veins, and returns to the heart nearly ail the 

 blood from the lower half of the body. 



Above the level of the heart the vertebral and intercostal veins retain 

 their relative size until the development of the upper limbs has com- 

 menced. Then two of the intercostal veins increase in diameter (Fig. 

 240), and become the right and left subclavians ; while the vertebral 



FIG. 238. 



FIG. 239. 



FIG. 240. 



Diagram of the VENOUS 

 SYSTEM in its early con- 

 dition ; showing the ver- 

 tebral veins emptying 

 into the heart by two 

 lateral trunks, the "ca- 

 nals of Cuvier." 



VENOUS SYSTEM farther ad- 

 vanced, showing the iliac 

 and subclavian veins. a. 

 Vein of new formation, 

 which becomes the inferior 

 vena cava. b. Transverse 

 branch of new formation, 

 which becomes the left vena 

 innoniinata. 



Further development of the 

 VENOUS SYSTEM. The ver- 

 tebral veins are reduced in 

 size, and the canal of Cuvier, 

 on the left side, is disappear- 

 ing, c. Transverse branch 

 of nevr formation, which 

 becomes the vena azygos 

 minor. 



veins situated above them become the right and left jugular veins. 

 Just below the junction of the jugulars with the subclavians, a small 

 branch of communication now appears between the two vertebrals 

 (Fig. 239, 6), passing from left to right, and emptying into the right 

 vertebral vein a little above the heart ; so that a part of the blood 

 coming from the left side of the head, and the left upper limb, still 

 passes down the left vertebral vein to the heart on its own side, while 

 a part crosses over by the communicating branch (6), and reaches the 

 heart through the right vertebral vein. Soon afterward, this branch 

 of communication enlarges so rapidly that it preponderates over the 

 vertebral vein from which it originated (Fig. 240), and becomes the 

 left vena innominata. 



On the left side, that portion of the superior vertebral vein, which 

 is below the subclavian, remains as a branch of the vena innominata, 

 receiving the six or seven upper intercostal veins ; while on the right 

 side it becomes excessively enlarged, receiving the blood of both 



