AORTIC ARCHES. 



701 



Though all these arches do not exist at the same time, still, in 

 describing the vessels which arise from them, they may be con- 

 veniently considered together. 



On the right side the fifth arch disappears completely. On the 

 left side the anterior root and neighboring part of the fifth 

 arch are transformed into the pulmonary artery ; the remaining 



FIG. 289. 



Diagram of the aortic arches ; the permanent vessels arising from them are shaded 

 darkly. (Allen Thomson, after Rathke.) 1, 2, 3, 4, 5. Primitive aortic arches of right 

 side. I, n, in, iv. Pharyngeal clefts of the left side, showing the relationship of the 

 clefts to the aortic arches. A. Aorta. P. Pulmonary artery, d. Ductus arteriosus. 

 a'. Left aortic root. a. Right aortic root. A'. Descending aorta, pn. pn'. Right and 

 left vagi. *.*'. Right and left subclavian arteries, v.v.' Right and left vertebral arteries. 

 c. Common carotid arteries, ce. External carotid, ci. d'. Right and left internal 

 carotid. 



part of this arch continues as the ductus arteriosus, which con- 

 nects the pulmonary artery with the permanent aorta. 



The fourth left arch, in mammalia, becomes the permanent 

 aorta. At the junction of the fourth and fifth left posterior 

 roots the left subclaviau artery is given off. In birds the right 

 fourth arch is transformed into the permanent aorta ; and in 

 examining the development of the aortic arch of the chick, it 



