674 DEVELOPMENT OF THE CIRCULATORY APPARATUS. 



Fig. 261. 



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

 is below the subclavian, remains as a small branch of the vena in- 

 nominata, receiving the six or seven upper intercostal veins ; while 

 on the right side it becomes excessively enlarged, receiving the 

 blood of both jugulars and both subclavians, and is converted into 

 the vena cava superior. 



The left canal of Cuvier, by which the left vertebral vein at first 

 communicates with the heart, subsequently becomes atrophied and 

 disappears; while on the right side it becomes excessively enlarged, 

 and forms the lower extremity of the vena cava superior. 



The superior and inferior venae cavae, accordingly, do not cor- 

 respond with each other so far as regards their 

 mode of origin, and are not to be regarded as 

 analogous veins. For the superior vena cava 

 is one of the original vertebral veins; while 

 the inferior vena cava is a totally distinct vein, 

 of new formation, resulting from the union of 

 the two lateral trunks coming from the infe- 

 rior extremities. 



The remainder of the vertebral veins finally 

 assume the condition shown in Fig. 261, which 

 is the complete or adult form of the venous 

 circulation. At the lower part of the abdomen, 

 the vertebral veins send inward small trans- 

 verse branches, which communicate with the 

 vena cava inferior, between the points at which 

 they receive the intercostal veins. These 

 branches of communication, by increasing in 

 size, become the lumbar veins (7), which, in the 

 adult condition, communicate with each other 

 by arched branches, a short distance to the side 

 of the vena cava. Above the level of the 

 lumbar arches, the vertebral veins retain their 

 original direction. That upon the right side 

 still receives all the right intercostal veins, and 

 becomes the vena azygos major (s). It also 

 receives a small branch of communication from 

 its fellow of the left side (Fig. 260, c), and this branch soon enlarges 

 to such an extent as to bring over to the vena azygos major all the 

 blood of the five or six lower intercostal veins of the left side, 

 becoming, in this way, the vena azygos minor (a). The six or seven 



Adult condition of Vi> 

 NOUS SYSTEM. 1. Right 

 auricle of heart. 2. Vena 

 cava superior. 3, 3. Jugular 

 veins. 4,4. Subclavian veins 

 :>. Vena cava inferior. 6, 6 

 Iliac veins. 7. Lumbar veins 

 S. Vena azygos major. 9 

 Vena azygos minor. 10. Sa 

 perior intercostal vein. 



