934 GENERATION. 



with the liver as the omphalo-mesenteric vein, and its blood passes through the liver 

 before it reaches the central organ of the circulation. As the omphalo-mesenteric vein 

 atrophies, the mesenteric vein, bringing the blood from the intestinal canal, is developed, 

 and this penetrates the liver, becoming, finally, 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 penetrates 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 that portion of the two inferior cardinal veins which 

 remains constitutes 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 canal of Cuvier, as the upper extremities are developed, enlarges and be- 

 comes the vena cava descendens, receiving, finally, all the blood from the head and the 

 superior extremities. The left canal of Cuvier undergoes atrophy and finally disappears. 

 The upper portion of the superior cardinal veins is developed into the jugulars and sub- 

 clavians 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 the Heart. The central enlargement of the vascular system in the 

 first circulation, which becomes the heart, is twisted upon itself by a single turn. The 

 portion connected with the cephalic extremity of the embryon gives origin to the 

 arterial system, and the portion connected with the caudal extremity receives the tilood 

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

 thickened, while the walls of the venous portion remain comparatively thin. There then 

 appears a constriction, which partly separates the auricular from the ventricular portion. 

 At a certain period of development, the heart presents a single auricle and a single ven- 

 tricle. 



The division of the heart into two ventricles appears before the two auricles are sepa- 

 rated. This is effected by a septum, which gradually extends from the apex of the heart 

 upward toward the auricular portion. At the seventh week, there is a large opening be- 

 tween the two ventricles. This gradually closes from below upward, the heart becomes 

 more pointed, and the separation of the two ventricles is complete at about the end of the 

 second month. 



At about the end of the second month, a septum begins to be formed between the 

 auricles. This extends from the base of the heart toward the ventricles, but it leaves an 

 opening between the two sides (the foramen ovale, or the foramen of Botal) which per- 

 sists during the whole of foetal life. At the anterior edge of the opening of the vena 

 cava ascendens into the right auricle, there is a membranous fold, which projects into 

 the auricle. This is the valve of Eustachius, and it divides the right auricle incompletely 

 into two portions. 



During the sixth week, the heart is vertical and situated in the median line, with the 

 aorta arising from the centre of its base. At the end of the second month, it is raised 

 up by the development of the liver, and its point presents forward. During the fourth 

 month, it is twisted slightly upon its axis, and the point presents to the left. At this 

 time, the auricular portion is larger than the ventricles ; but the auricles diminish in their 

 relative capacity during the latter half of intra-uterine life. The pericardium makes its 

 appearance during the ninth week. 



Early in intra-uterine life, the relative size of the heart is very great. At the second 

 month, its weight, in proportion to the weight of the body, is 1 to 50. This proportion, 

 however, gradually diminishes until, at birth, the ratio is 1 to 120. The proportionate 



