264 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



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 primit ive 

 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). 



Branches of the Dorsal Aorta. From the primitive aortae arise: (i) 

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



i. 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. 



Postcostal anastomosis 



^ ^ _ . _ . ^ . ^Precostal anastomosis 



Dorsal ramus. 



Ventral \ 



-Lateral (visceral) artery 

 Ventral (splanchnic) artery 



Ventral anastomosis 



FIG. 274. Diagram of the trunk, in transverse section, showing the arrangement of the aortic 



branches. 



As we have seen (Fig. 271), the internal carotids are recurved cranially 

 in the 5 mm. embryo and anastomose 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 dorsal intersegmental 

 arteries, the vertebral arteries arise (Fig. 27.5). The original trunks of the 

 first six pairs. are lost, so that the vertebrals take their origin with the 

 subclavians from the seventh pair of intersegmental arteries (Fig. 276). 

 In embryos of 9 mm. the vertebrals in the region of the metencephalon 

 fuse to form a single, median ventral vessel, the basilar artery, which thus 

 is connected cranially (by way of the circulus arteriosus) with the internal 

 carotids, caudad with the vertebral arteries. 



The internal carotids (Fig. 271), after giving off the ophthalmic arteries, give rise 

 cranially to the anterior cerebral artery, from which arise later the middle cerebral artery 



