THE HEART. 573 



formed from the septum inferius (Fig. 244, SD), but it is com- 

 pleted above, partly by the lower edge of the interauricular sep- 

 tum, and partly by a prolongation of the aortic septum, which 

 divides the truncus arteriosus into systemic and pulmonary 

 trunks. 



The aortic septum grows back beyond the truncus arteriosus, 

 so as to project a certain distance into the ventricular cavity ; 

 it then fuses with the free lower edge of the interauricular sep- 

 tum, in such a way as to cut off the systemic trunk from the 

 right ventricle, and to place it in communication with the left 

 ventricle ; while finally the septum inferius extends so as to meet 

 and fuse with the interauricular septum, and so completes the 

 separation of the ventricles from each other. 



The valves of the heart. The outer flaps of the auriculo- 

 ventricular valves, both mitral and tricuspid, are formed from 

 the lower lips of the canalis auricularis, which hang down into 

 the ventricular cavity (Fig. 244) ; the inner flaps of the valves are 

 derived from the lower edge of the interauricular septum. The 

 valves are at first very thick and soft, and only later become 

 thin and membranous. 



The semilunar valves are formed, about the end of the fifth 

 week, as cushion-like thickenings of the endothelium, which soon 

 become hollowed out into pockets. 



2. The Arteries. 



The general plan of arrangement of the arteries in the 

 human embryo is the same as in other Vertebrates ; and has 

 already been described, in previous chapters, in the case of the 

 rabbit, the chick, and the frog. 



From the anterior end of the truncus arteriosus a series of 

 pairs of aortic arches arise, which run round the sides of the 

 pharynx, lying in the visceral arches (Fig. 243). On reaching 

 the dorsal surface of the pharynx, the aortic arches of each side 

 open into a longitudinal vessel, the aorta. The two aortae run 

 backwards along the body, ventral to the notochord ; they are 

 at first separate along their whole length, but early fuse together 

 in the hinder part of their course to form the definite dorsal aorta. 

 From the dorsal aorta, vitelline arteries are given off to the yolk- 

 sac ; and at the posterior end of the embryo the aorta divides 



