DEVELOPMENT OF THE CIRCULATORY SYSTEM 845 



the two ducts of Cuvier into the heart. As development advances, the 

 liver is formed in the course of this vessel a short distance below the 

 heart, and the vein ramifies in its substance; so that the blood of 

 the omphalo-mesenteric vein passes through the liver before it goes to 

 the heart. The omphalo-mesenteric vein is obliterated as the umbilical 

 vein makes its appearance. The blood from the umbilical vein is at 

 first emptied directly into the heart ; but this vessel soon establishes the 

 same relations *with the liver as the omphalo-mesenteric vein, and 

 its blood passes through the liver before it reaches the heart. As 

 the omphalo-mesenteric vein atrophies, the mesenteric vein, bringing 

 the blood from the intestinal canal, is developed, and this penetrates the 

 liver, finally becoming the portal vein. 



As the lower extremities are developed, the inferior vena cava makes 

 its appearance between the two inferior cardinal veins. This vessel 

 receives an anastomosing branch from the umbilical vein before it pene- 

 trates the liver, and this branch is the ductus venosus. As the inferior 

 vena cava increases in size, it communicates below with the two inferior 

 cardinal veins ; and the portions of the two inferior cardinal veins which 

 remain constitute the two iliac veins. The inferior cardinal veins, 

 between that portion which forms the iliac veins and the heart, finally 

 become the right and the left azygos veins. 



The right duct of Cuvier, as the upper extremities are developed, 

 enlarges and becomes the vena cava descendens, finally receiving the 

 blood from the head and the superior extremities. The left duct of 

 Cuvier diminishes in size and remains as the coronary sinus. The 

 upper portions of the superior cardinal veins are developed into the 

 jugulars and subclavians on the two sides. As the lower portion of 

 the left cardinal vein and the left canal of Cuvier atrophy, a venous 

 trunk appears, connecting the left subclavian with the right canal of 

 Cuvier. This increases in size and becomes the left vena innominata, 

 which connects the left subclavian and internal jugular with the vena 

 cava descendens. 



Development of tJie Heart. The central enlargement of the vascular 

 system in the first circulation, which becomes the heart, is twisted on 

 itself by a single turn. The portion connected with the cephalic ex- 

 tremity of the embryo gives origin to the arterial system, and the 

 portion connected with the caudal extremity receives the blood from 

 the venous system. The walls of the arterial portion of the heart soon 

 become thickened, while the walls of the venous portion remain com- 

 paratively thin. There then appears a constriction, which partly sepa- 

 rates the auricular from the ventricular portion. At a certain period of 

 development the heart presents a single auricle and a single ventricle. 



