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 aortas 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/ ramus_ 

 of dorsal 

 intersegment- 

 al artery 

 \rre coita-V 

 [anastomosis 



Ventro- 

 lateral 

 Visceral 

 I artery 



ILat. ramus 

 of, ventral 

 at v - dorsal 

 intersegmental 

 artery 



D- splanchnic 

 anastomosis 



Trene-ural anastomosis 



tost- costal anastomosis 

 o f dorsal ramus o f dorsal 

 'intersegmental artery 



Dorsal intersegmental 

 artery 



orsal 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- 

 clavian 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. 



