DEVELOPMENT OF THE VASCULAR SYSTEM. 



799 



Fig. 304. 



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 re- 

 ceives all the right intercostal veins, and becomes 

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

 small branch of communication from its fellow 

 of the left side (Fig. 303, 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 (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 innominata above, is known as the 

 superior intercostal vein. The left canal of 

 Cuvier has by this time entirely disappeared ; 

 so that all the venous blood now enters the 

 heart by the superior and the inferior vena cava. 

 But the original vertebral veins are still con- 

 tinuous 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 in- 

 tercostal vein inosculates below with the lesser 

 azygous, before it crosses to the right side. 



There are still two parts of the circulatory 

 apparatus, the development of which presents 



peculiarities sufficiently important to be described separately. These 

 are, first, the liver and the ductus venosus, and secondly, the heart and 

 ductus arteriosus. 



The Hepatic Circulation and Ductus Ve- 

 nosus. The liver appears at a very early 

 period, in the upper part of the abdomen, as 

 a mass of glandular and vascular tissue, de- 

 veloped around the upper portion of the 

 omphalo-mesenteric vein, just below its ter- 

 mination in the heart (Fig. 305). As soon 

 as the organ has attained a considerable 

 size, the omphalo-mesenteric vein ( i ) breaks 

 up in its interior into a capillary plexus, Earlv form of the HEPATIC 



. J 1 CIKCULATION.- 1. Omphalo- 



the vessels of Which again unite into a mesenteric vein. 2. Hepatic vein. 



VenOUS trunk, which Conveys the blood to 3 - Heart. The dotted line show* 



, , m , . . . the situation of the future um- 



the Heart. I he omphalo-mesenteric vein biiicai vein. 



Adult condition of the 

 VENOUS SYSTEM. 1. 

 Right auricle of the heart. 

 2. Vena cava superior. 3,3. 

 Jugular veins. 4, 4. Subcla- 

 vian veins. 5. Vena cava 

 inferior. 6, 6. Iliac veins. 

 7. Lumbar veins. 8. Vena 

 azygos major. 9. Vena 

 azygos minor. 10. Superior 

 intercostal vein. 



Fig. 305. 



