DEVELOPMENT OF THE VEXOUS SYSTEM. 



673 



constituting another vein of new formation 

 (Fig. 259, a), which runs upward a little to the 

 right of the median line, and empties by itself 

 into the lower extremity of the heart. The 

 two branches, by means of which the veins of 

 the lower extremities thus unite, become after- 

 ward, by enlargement, the common iliac veins ; 

 while the single trunk (a) resulting from their 

 union becomes the vena cava inferior. Subse- 

 quently, the vena cava inferior becomes very 

 much larger than the vertebral veins ; and its 

 two branches of bifurcation are afterward re- 

 presented by the two iliacs. 



Above the level of the heart, the vertebral 

 and intercostal veins retain their relative size 

 until the development of the superior extremi- 

 ties has commenced. Then two of the inter- 

 costal veins increase in diameter (Fig. 259), and 

 become converted into the right and left sub- 

 clavians; while those portions of the vertebral 

 veins situated above the subclavians become the 

 right and left jugulars. Just below the junction 

 of the jugulars with the subclavians, a small 

 branch of communication now appears between 

 the two vertebrals(Fig. 259, b), passing over from 

 left to right, and emptying into the right verte- 

 bral vein a little above the level of the heart ; so 

 that a part of the blood coming from the left side 

 of the head, and the left upper extremity, still 

 passes down the left vertebral vein to the heart 

 upon its own side, while a part crosses over by 

 the communicating branch (I), and is finally 

 conveyed to the heart by the right descending 

 vertebral. Soon afterward, this branch of com- 

 munication enlarges so rapidly that it prepon- 

 derates altogether over the left superior verte- 

 bral vein, from which it originated (Fig. 260), 

 and, serving then to convey all the blood coming 

 from the left side of the head and left upper 

 extremity over to the right side above the heart, 

 it becomes the left vena innominate. 

 43 



Fig. 259. 



VENOCS STSTEM far- 

 ther advanced, showing 

 f 'i-riKition of iliac and sub- 

 clavian veins. n. Vein of 

 new formation, which be- 

 comes the inferior vena 

 cava. b. Transverse branch 

 of new formation, which 

 afterward become* the lefc 

 vena innominata. 



Fig. 260. 



Furthe development of 

 the V E .\ o c s SYSTEM. 

 The vertebral veins are 

 much diminished in size, 

 and the canal of Cuvier, on 

 the left side, is gradual y 

 disappearing, c. Trans- 

 verse branch of new forma- 

 tion, which is to become 

 the vena azygos minor. 



