1002 A MANUAL OF ANATOMY 



pears, except the small portion which contributes to the formation of the 

 right subclavian artery. 



Permanent Vessels. — The development of these vessels is here described, 

 in a caudo-cephalic order, or from behind forwards. 



The ventral portion of the sixth left arch gives rise to the right and left 

 pulmonary arteries respectively, which pass backwards to the lungs. 



The dorsal part of the sixth left arch — ^that is to say, the part beyond the 

 origin of the right and left pulmonary arteries — persists up to birth, and 

 forms the important vessel called the ductus arteriosus (duct of Botalli), 

 which joins the left dorsal aorta. After birth the ductus arteriosus undergoes 

 obliteration, and is converted into a fibrous cord, known as the ligamentum 

 ductus arteriosi. 



Fig. 415. 



-Development of Cervical, Segmental, and Vertebral 

 Arteries, 



The root, or proximal part, of the left ventral aorta — that is to say, the 

 part between the aortic bulb and the ventral root of the fourth left arch — forms 

 the part of the arch of the aorta between the innominate and left common 

 carotid arteries. 



The fourth left arch forms the part of the arch of the aorta between the 

 left common carotid artery and the point of entrance of the ductus arteriosus. 

 As development proceeds, the fourth left arch gradually descends, along Avith 

 the heart, into the cavity of the future thorax, and so comes to occupy a 

 lower plane than the fourth right arch. 



The root, or proximal part, of the right ventral aorta — that is to say, the 

 part between the aortic bulb and the ventral root of the fourth right arch — ■ 

 gives rise to the innominate artery. 



The fourth right arch contributes to the formation of the rie;ht subclavian 



