DEVELOPMENT OF THE HEPATIC CIKCULATION. 675 



upper intercostal veins on fhe left side still empty, as before, into 

 their own vertebral vein (10), which, joining the left vena innomi- 

 nata 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 or the inferior 

 vena cava. But the original vertebral veins are still continuous 

 throughout, though very 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 

 also inosculates below with the lesser azygous, just before it passes 

 over to the right side. 



There are still two parts of the circulatory apparatus, the deve- 

 lopment of which presents peculiarities sufficiently important to 

 be described separately. These are, first, the liver and the ductus 

 venosus, and secondly, the heart, with the ductus arteriosus. 



Development of the Hepatic Circulation and the Ductus Venosus. 

 The liver appears at a very early period in the upper part of the 

 abdomen, as a mass of glandular and vascular tissue, which is deve- 

 loped around the upper portion of the 

 omphalo-mesenteric vein, just below its Fi S- 



termination in the heart. (Fig. 262.) As 

 soon as the organ has attained a con- 

 siderable size, the omphalo-mesenteric 

 vein (i) breaks up in its interior into a 

 capillary plexus, the vessels of which 

 unite again into venous trunks, and so 

 convey the blood finally to the heart. 

 The omphalo-mesenteric vein below the Early form of HEPATIC 

 liver then becomes the portal vein ; while 



above the liver, and between that organ Heart. The dotted line shows the 



ill ' i n situation of the future umbilical 



and the heart, it receives the name of vein . 



the hepatic vein (2). The liver, accord- 



ingly, is at this time supplied with blood entirely by the portal vein, 



coming from the umbilical vesicle and the intestine ; and all the 



blood derived from this source must pass through the hepatic cir- 



culation befo're reaching the venous extremity of the heart. 



But soon afterward the allantois makes its appearance, and be- 

 comes rapidly developed into the placenta ; and the umbilical vein 

 coming from it joins the omphalo-mesenteric vein in the substance 

 of the liver, and takes part in the formation of the hepatic capillary 

 plexus. As the umbilical vesicle, however, becomes atrophied, and 



