272 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



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, as five arches 

 are formed in connection with the gills of adult fishes, three are represented on either side in 

 adult amphibia and reptiles, while in birds the right, in mammals the left, fourth arch persists 

 as the arch of the aorta. 



From the primitive aortae arise dorsal, lateral and ventral branches (Fig. 267). 

 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. Those of each side anastomose 

 longitudinally in the median line and give rise to the dorsal and ventral median 



Post, transverse 

 anastomosis 



Dorset/ ramu, 

 of dorsal 



at orrery 

 \Pre costa-l' 

 [afia.s'tomosis 



f entro- 



lateral 

 , visceral 

 I artery 



Lat ramus 

 of. Ventral 

 aiv -dorsa.1 

 mtersegm, 

 . artery 

 D. splanchnic 



H-eneural anastomosis 



Post- costal anastomosis 

 o f clorsa.1 ramus of dorsaJ. 

 't'hfef-sejfnertfai astery 



Dorsal intcrseyfnental 

 artery 



Dorsal aorta. 



ventral splanchnic 

 artery 



Ventral anastomosis of 



ventral div of dorsal intersegmental artery 



FIG. 267. A diagram showing the arteries of the trunk in transverse section. 



spinal arteries. The dorsal rami also form lateral anastomoses dorsal and ven- 

 tral to the transverse processes of the vertebrae. 



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

 264), the internal carotids are recurved cranially in the 5 mm. embryo and anas- 

 tomose with the first two pairs of dorsal intersegmental arteries. The ventral 

 longitudinal anastomosis of the dorsal rami of the first seven pairs of dorsal in- 

 tersegmental arteries gives rise to the vertebral arteries (Fig. 268, A). The trunks 

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

 clavians from the seventh pair of intersegmental arteries (Fig. 268, B). 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 with 

 the internal carotids, caudad with the vertebral arteries. 



