THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 583 



vena cava; it furnishes that portion of the latter which is 

 situated between the renal veins and the division into the two venae 

 iliacae communis (fig. 322 B and G ci' 2 ). 



While the abdominal part of the left cardinal vein (fig. 322 ca 3 ) 

 succumbs and the corresponding region of the right cardinal vein 

 produces the lower part of the inferior vena cava (c^ 2 ), their 

 thoracic portions persist in a reduced form, since they receive the 

 blood from the intercostal spaces (fig. 322 B ca). In this region 

 occurs still another and last metamorphosis, by which likewise an 

 asymmetry is brought about between the halves of the body. In the 

 thoracic part of the body the original conditions of the circulation are 

 altered by the degeneration of the left cava superior (fig. 322 C ess). 

 The direct flow of the left cardinal vein to the atrium is thereby 

 rendered more difficult, and finally ceases altogether, the tract desig- 

 nated by ca 2 undergoing complete degeneration. Meanwhile a trans- 

 verse anastomosis (hz l ), which has been formed in front of the 

 vertebral column and behind the aorta between the corresponding 

 vessels of both sides, receives the blood of the left side of the body 

 and transports it to the right side. In this manner the thoracic 

 part of the left cardinal vein and its anastomosis become the left 

 hemiazygos (hz and hz l ) ; the right and larger cardinal vein becomes 

 the azygos (az). 



Thus by many indirect ways is attained the permanent condition 

 of the venous system of the trunk, with its asymmetry and its 

 preponderance of the venous trunks in the right half of the body. 



A third series of metamorphoses, which we shall now take into 

 consideration, concerns the development of a liver circulation. 



The liver receives its blood in different stages of the embryonic 

 development from various sources : for a time from the vitelline 

 veins; during a second period from the umbilical veins; after 

 birth, finally, from the veins of the intestines the portal vein. 

 This threefold alteration finds its explanation in the conditions of 

 growth of the liver, the yolk-sac, and the placenta. As long as the 

 liver is small, the blood coming from the yolk-sac suffices for its 

 nourishment. But when it increases greatly in size the yolk-sac, 

 on the contrary, growing smaller other blood-vessels at this time, 

 the umbilical veins, must supply the deficiency. When, finally, at 

 birth the placental circulation ceases, the venous trunks of the 

 intestinal canal, which meanwhile have become very large, can 

 supply the needs. 



These circumstances must be kept in mind, in order to comprehend 



