515 



DEVELOPMENT OF THE VASCULAR SYSTEM 



septal cusps of the valves are developed as downward prolongations of the septum 

 intermedium (Fig. 467). The aortic and pulmonary semilunar valves are formed 

 from four endocardial thickenings an anterior, a posterior, and two lateral 

 which appear at the proximal end of the truncus arteriosus. As the aortic septum 

 grows downward it divides each of the lateral thickenings into two, thus giving 

 rise to six thickenings the rudiments of the semilunar valves three at the aortic 

 and three at the pulmonary orifice (Fig. 471). 



Further Development of the Arteries. Recent observations show that practi- 

 cally none of the main vessels of the adult arise as such in the embryo. In the site 

 of each vessel a capillary network forms, and by the enlargement of definite paths 

 in this the larger arteries and veins are developed. The branches of the main 

 arteries are not always simple modifications of the vessels of the capillary network, 

 but may arise as new outgrowths from the enlarged stem. 



External carotid 



Ventral aorta 



Internal carotid 

 Common carotid 



Aortic arch 



Ductus arteriosus 

 Vertebral artery 



Subclavian artery 

 Left pulmonary artery 



Right subclavian 

 artery 



Bight pulmonary 

 artery 



FIG. 473. Scheme of the aortic arches and their destination. (Modified from Kollmann.) 



has been seen (page 506) that each primitive aorta consists of a ventral and 

 a dorsal part which are continuous through the first aortic arch. The dorsal aortae 

 at first run backward separately on either side of the notochord, but about the 

 third \veek they fuse from about the level of the fourth thoracic to that of the fourth 

 lumbar segment to form a single trunk, the descending aorta. The first aortic 

 arches run through the mandibular arches, and behind them five additional pairs 

 are developed within the visceral arches; so that, in all, six pairs of aortic arches 

 are formed (Figs. 472, 473). The first and second arches pass between the ventral 

 and dorsal aortee, while the others arise at first by a common trunk from the truncus 

 arteriosus, but end separately in the dorsal aortse. As the neck elongates, the 

 ventral aortse are drawn out, and the third and fourth arches arise directly from 

 these vessels. 



In fishes these arches persist and give off branches to the gills, in which the 

 blood is oxygenated. In mammals some of them remain as permanent structures, 

 while others disappear or become obliterated (Fig. 473) . 



