838 GENERATION. 



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, en- 

 larges and becomes the vena cava descendens, receiving finally all the blood 

 from the head and the superior extremities. The left canal of Cuvier under- 

 goes atrophy and disappears. The upper portion of the superior cardinal 

 veins is 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 the Heart. The central enlargement of the vascular sys- 

 tem 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 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 comparatively thin. There then appears a con- 

 striction, which partly separates the auricular from the ventricular portion. 

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

 a single ventricle. 



The division of the heart into two ventricles appears before the two auri- 

 cles are separated. 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 between 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 persists 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 is 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 



