,S76 



COMPARATIVE ANATOMY 



aortic arches have degenerated. Consequently, while all six aortic arches 

 arise, they are not present simultaneously in the human embryo. 



Only three of the six embryonic aortic arches are represented in the 

 adult, the third, fourth, and in part sixth. The right and left third aortic 

 arches become respectively the root of the right and left internal carotid 

 arteries, the external carotids coming from the ventral aorta. The left 

 fourth aortic arch becomes the arch of the aorta of the adult, while the 

 right forms the right subclavian artery. The dorsal aorta between the 

 third and fourth arches degenerates and disappears. Posterior branches 

 of the sixth pair of aortic arches connect with the lungs and form the 

 pulmonary arteries. Until birth, the remainder of the left sixth aortic 

 arch persists as the ductus arteriosus connecting the pulmonary artery 





X- RAMUS OORSALIS 



\\ ARTERY---^. 



-VX - SEGMENTAL SOMATIC / / 



INTERMEDIATE ART 



VISCERAL ARTERY"- 



" RAMUS VENTR AUS" 



A DIAGRAM OF ARTERIES IN HEAD REGION. B ARTERIES IN TRUNK REGION. 



Fig. 315. — Diagrams illustrating the relations of somatic and visceral arteries in A, 

 Head and B, Trunk. The coelom in the head region is embryonic and transient. 

 (Redrawn from Corning, after Young and Robinson in Cunningham's Anatomy.) 



with the dorsal aorta. The vessel which connects the right pulmonary 

 artery with the right dorsal aorta degenerates. The fifth pair of arches 

 degenerate soon after their appearance. 



Changes in the Embryonic Circulation. The vascular system of a 

 1.5 mm. to 2.0 mm. embryo includes paired dorsal aortae from which are 

 given off intersegmental vessels to the body-wall, and a series of vitelline 

 arteries to the yolk-sac. Blood is returned to the sinus venosus of the 

 heart through vitelline veins from the yolk-sac and paired umbilical veins 

 from the placenta. 



In embryos of 2.0 mm. to 2.5 mm., blood is returned from the head to 

 the heart by the paired precardinal veins. At this stage, six pairs of 

 vitelline arteries carry blood to the yolk-sac. In embryos of 2.6 mm., 

 postcardinal veins join the ptecardinal veins to form the common cardinal 

 veins, the ductus Cuvieri. These, together with the umbilical and vitel- 

 line veins, return blood to the sinus venosus. Also, at this stage, the 

 paired dorsal aortae begin to unite into a median unpaired trunk. In the 

 head region, however, this fusion does not occur, and the paired dorsal 

 aortae persist as the internal carotids of the adult body. The vitelline 



