THE THORAX 



Caudahvards each primitive dorsal aorta furnishes the vitelline or omphalo- 

 mesenteric artery, which is its principal mode of termination. The tvvo 

 vitelline arteries pass to the yolk-sac. Another branch of each primitive 

 dorsal aorta, however, passes caudalwards, which, though small, is really 

 the continuation of the corresponding dorsal aorta. After retrogression of 

 the yolk-sac, the vitelline arteries are replaced by the allantoic arteries. 



At a subsequent period the two primitive dorsal aortae unite at about 

 the level of the future fourth thoracic vertebra, and so give rise to one vessel, 

 namely, the dorsal aorta. This vessel is continued onwards as the median 

 caudal aorta, which is ultimately known as the middle sacral artery. 



The arterial arches, as stated, are caUed first, second, third, fourth, fifth 



External Carotid 



Ventral Aorta 



Internal Carotid 

 Common Carotid 



Right Subclavian 

 Artery 



Right Pulmonary. 

 Artery 



Trunk of Pulmonary Artery 



Aortic Arch 



Ductus Arteriosus 

 \'ertebral Artery 



Left Pulmonary 

 Artery 



Fig. 414. — Scheme of the Aortic Arches and their Destination 



(MODIFIED FROM KOLLMANN) (FROM GrAY'S 'AnaTOMY'). 



(rudimentary and temporary), and sixth. As the development of the prin- 

 cipal arteries proceeds, important changes take place in these arches. Keep- 

 ing in view the fact that, according to some authorities, the arterial arches 

 are five in number, and not six, the following description will be equally 

 applicable to each view, provided the sixth arch of the view adopted is re- 

 garded as being equivalent to the fifth arch of the other view. 



The first, second, and fifth arches disappear, on either side. 



The sixth right arch disappears. 



The part of the left dorsal aorta between the dorsal roots of the third and 

 fourth left arches disappears. 



The part of the right dorsal aorta, which extends from the dorsal root of 

 the third right arch to the place of junction of the two dorsal aortas, disap- 



