PRIMITIVE BLOOD VASCULAR SYSTEM 



271 



arteries arise the subclavian arteries of the upper limbs. Of the ventral vitelline 

 vessels three are now prominent, the cosliac artery in the stomach-pancreas re- 

 gion, the vitelline or superior mesenteric in the small intestine region and the in- 

 ferior mesenteric of the large intestine region. 



/ Of the aortic arches the third pair is largest at 5 mm. From the sixth-pair 

 -^xe-given-ofiLjthe- smaHrfu-lmonary arteries to the lungs. ' At 7 mm. the first and 

 second aortic arches are obliterated (Figs. 265, B, and 266), but the dorsal and ven- 

 tral aortse cranial to the third arch persist as parts of the internal and external 

 arteries respectively .7^ The third arches form the stems of the internal carotids, 

 while the ventral aortse between the third and fourth arches become the common 



Externa/ carotid- 



Ventral aorta. 



Eight subclavian 

 artery — — 



Riant 



pulmonar 



artery 



Trunk of 

 pulmonary 

 ' artery 



Interned carotid 

 Common carotid 

 Aortic arch 



Ductus arteriosus 

 Vertebral artery 



Subclavian artery 

 Left pulmonary 

 artery 



Xlorsa-t aorta. 



Fig. 266. — Diagram showing the aortic arches and their derivatives in human embryos. 



carotids./ In embryos of 15 mm. the bulbus cordis has been divided into the aor- 

 tic and pulmonary trunks so that the aorta opens into the left ventricle and the 

 pulmonary trunk into the right ventricle. [The dorsal aortae between the third 

 and fourth arches disappear, but the fourth arch on the left side persists as the 

 aortic arch of the adult. On the right side, the fourth aortic arch persists with 

 the descending aorta as far as the seventh intersegmental artery and forms part of 

 the right subclavian artery, which is thus longer than the left. On the right side, 

 the sixth arch between the origin of the right pulmonary artery and descending 

 aorta is early lost; on the left side, it persists as the ductus arteriosus and its lumen 

 is only obliterated after birth. The proximal portion of the right sixth arch 



