262 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



fifth pair in the series. From each dorsal, or descending aorta there 

 develop cranially the internal carotid arteries. These extend toward 

 the optic stalks where they bend dorsad and caudad, connecting 

 finally with the first intersegmental arteries of each side (Fig. 271). The 

 descending aortas are now fused to their extreme caudal ends and the 

 umbilical arteries take their origin ventrally. Twenty-seven pairs of 

 dorsal intersegmental arteries are present. From the seventh cervical pair 

 of these the subdavian arteries of the upper limbs arise. Of the ventral 

 vitelline vessels three are now prominent : the cceliac artery in the stomach- 

 pancreas region, the vitelline, or superior mesenteric, in the small intestine 

 region, and the inferior mesenteric of the large intestine region. 



A 



Aortic arch 

 Aortic arch 2 



Aortic arch i 



Dorsal aorta 



Aortic arch 4 

 Aortic arch 6 



Esophagus 

 Trachea 



Pulmonary artery 

 Ventral aorta Bulbus cordis 



B 



Aortic arch 3 



Int. carotid artery 

 Aortic arch 2 



External carotid 

 Aortic arch 6 

 Pulmonary artery 



Bulbus cordis 



1 ortic arch 4 



Aortic arch 5 



Dorsal aorta 



FIG. 272. Aortic arches of human embryos: A, of 5 mm.; B, of 7 mm. (after Tandler). I-IV 



pharyngeal pouches. 



DEVELOPMENT OF THE ARTERIES 



Transformation of the Aortic Arches. The ancestral aortic arches 

 are early transformed into more appropriate vessels. The third pair is 

 largest at 5 mm. (Fig. 272, A}; at 7 mm. the first and second aortic 

 arches are obliterated (Figs. 272 B and 273), but the descending and 



