264- 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



proximal portion of the right sixth arch forms the stem of the right pulmonary 

 artery, but the proximal portion of the left arch is incorporated in the pulmonary- 

 trunk. 



The aortic arches of the' embryo are of especial importance comparatively. Five 

 arches are formed in connection with the gills of adult fishes. In adult tailed amphibia, 

 three or four arches, and in some reptiles two arches, are represented on either side. In 

 birds the right, in mammals the left fourth arch persists as the arch of the aorta. 



The different courses of the recurrent laryngeal nerves are easily explained. The vagus 

 early gives off paired branches which reach the larynx by passing caudal to the primitive 

 fourth aortic arches. When the latter, through growth changes, descend into the chest, 

 loops of both nerves are carried with them. Hence, after the transformation of the fourth 

 arches, the left recurrent nerve remains looped around the arch of the aorta, the right around 

 the right subclavian artery (cf. Fig. 273). 



^External carotid 



Innominate 



artery 



Right sub- 

 clavian artery 



Right pul- 

 monary artery 



Trunk of pul- 

 monary artery 



Internal carotid 

 Common carotid 



Aortic arch 



Ductus arteriosus 

 Vertebral artery 

 Subclavian artery 



Left pulmonary artery 

 Ventral aorta 



Fig. 273.— Diagram showing the aortic arches and their derivatives in human embryos. 



Branches of the Dorsal Aorta.— From the primitive aortae arise: (1) dorsal, 

 (2) lateral, and (3) ventral branches (Fig. 274). 



1. The dorsal branches are intersegmental and develop small dorsal and large 

 ventral rami. From the dorsal rami are given off neural branches which bifurcate 

 and form dorsal and ventral spinal arteries. 



Origin of the Vertebral Arteries and Basilar Artery. — As we have seen (Fig. 

 271), the internal carotids are recurved cranially in the S mm. embryo and anas- 

 tomose with the first two pairs of dorsal intersegmental arteries. By longitudinal 

 postcostal anastomoses (Fig. 274) of the dorsal rami of the first seven pairs of dor- 

 sal intersegmental arteries the vertebral arteries arise (Fig. 275). The original 

 trunks of the first six pairs are lost so that the vertebrals take their origin with the 



