694 



REPRODUCTION. 



FlG. 241. 



jugular and both subclavian veins, 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, is subsequently obliterated, while that 

 on the right side becomes excessively enlarged, forming the lower 

 extremity of the vena cava superior. 



The superior and inferior venaB cavae, accordingly, do not correspond 

 with each other so far as regards their origin. The superior 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 from the inferior limbs. 



The remaining vertebral veins finally assume the condition shown 

 in Fig. 241, which is the adult form of the venous circulation. At 

 the lower part of the abdomen the vertebral veins send inward trans- 

 verse branches of communication to the vena 

 cava inferior, between the points at which they 

 receive the intercostal veins. These branches 

 of communication become the lumbar veins ( 7 ), 

 which in the adult communicate with each other 

 by arched branches, a short distance to the side 

 of the vena cava. Above the level of the lum- 

 bar arches, the vertebral veins retain their origi- 

 nal direction. That upon the right side still 

 receives all the right intercostal veins, and be- 

 comes the vena azygos major ( 8 ). It also receives 

 from its fellow of the left side a small branch of 

 communication (Fig. 240, c), which 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, becom- 

 ing, in this way, the vena azygos minor (Fig. 

 241, 9 ). The six or seven upper intercostal 

 veins on the left side still empty, as before, into 

 their own vertebral vein ( 10 ), which, joining the 

 left vena in nominata above, is known as the 

 auricle of the heart. 2. superior intercostal vein. The left canal of 

 J u "ir p r4su,a 3 : Ccr has by this time disappeared; so that 

 vian veins. 5. Vena cava all the venous blood now enters the heart by 



inferior. 6. 6. Iliac veins. , r . T , , . ,. -.-> , 



7. Lumbar veins, s. Vena the superior and the interior vena cava. But 

 azygos major. 9. Vena azy- the original vertebral veins are still continuous 



gos minor. 10. Superior in- ,, , ,, , , ,. . . , , . 



tercostai vein. throughout, though much diminished in size at 



certain points ; since both the greater and lesser 



azygous veins inosculate below with the superior lumbar veins, and 

 the superior intercostal vein inosculates below with the lesser azygous 

 vein, before it crosses to the right side. 



There are two parts of the circulatory apparatus, the development 

 of which is sufficiently important to be described separately. These 



