DEVELOPMENT OF THE HEAET. 435 



ceiving, 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 subclavians 

 on the two sides. As the lower portion of the left cardinal 

 vein and the left canal of Cuvier atrophy, a venous trunk ap- 

 pears, connecting the left subclavian with the right canal of 

 Cuvier. This increases in size and becomes the left vena in- 

 nominata, 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 blood from the venous sys- 

 tem. The walls of the arterial portion of the heart soon be- 

 come thickened, while the walls of the venous portion re- 

 main comparatively thin. There then appears a constriction, 

 which partly separates the auricular from the ventricular por- 

 tion. 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 be- 

 fore the two auricles 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. 1 



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 leaves an open- 

 ing between the two sides, the foramen ovale, or the foramen 



1 BURDACH, Traite dc physiologic, Paris, 1838, tome iii., p. 514. 



