798 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



Fig. 302. 



VENOUS SYSTEM farther 

 advanced, showing the for- 

 mation of the iliac and sub- 

 clavian veins a. Vein of 

 new formation, which he- 

 comes the inferior vena cava. 

 b. Transverse hr-inoh of new 

 formation, which afterward 

 becomes the left vena inno- 

 minata. 



Fig. 303. 



Further development of 

 the VENOUS SYSTEM. 

 The vertebral veins are 

 much diminished in size, 

 and the canal of Cuvier, on 

 the left side, is gradually 

 disappearing, c. Transverse 

 branch of new formation, 

 which is to become the vena 

 azygos minor. 



left to right, and emptying into the right verte- 

 brtil 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 extre- 

 mity, still passes down the left vertebral vein 

 to the heart upon its own side, while a part 

 crosses over by the communicating branch (6), 

 and is finally conveyed to the heart b} 7 the 

 right descending vertebral. Soon afterward, 

 this branch of communication enlarges so 

 rapidly that it preponderates over the left 

 superior vertebral vein, from which it origi- 

 nated (Fig. 303), and, serving then to convey 

 all the blood from the left side of the head and 

 left upper extremity to the right side above the 

 heart, it becomes the left vena innominata. 



On the left side, that portion of the superior 

 vertebral vein, which is below the subclavian, 

 remains as a small branch of the vena innomi- 

 nata, receiving the six or seven upper inter- 

 costal veins ; while on the right side it becomes 

 excessively enlarged, receiving the blood of 

 both jugulars and both subclavians, and is con- 

 verted into the vena cava superior. 



The left canal of Cuvier, by which the left 

 vertebral vein at first communicates with the 

 heart, is subsequently atrophied and obliterated, 

 while on the right side it becomes excessively 

 enlarged, and forms the lower extremity of the 

 vena cava superior. 



The superior and inferior venae cavse, accord- 

 ingly, do not correspond with each other so far 

 as regards their mode of origin, and are not 

 to be regarded as analogous veins. The supe- 

 rior vena cava is one of the original vertebral 

 veins ; while the inferior vena cava is a vessel of 

 new formation, resulting from the union of two 

 lateral trunks coming from the inferior extre- 

 mities. 



The remainder of the vertebral veins finally 

 assume the condition shown in Fig. 304, which 

 is the complete or adult form of the venous 

 circulation. At the lower part of the abdo- 

 men, the vertebral veins send inward small 

 transverse branches of communication to the 

 vena cava inferior, between the points at which 

 they receive the intercostal veins. These 



