DEVELOPMENT OF THE PHARYNX AND NECK. 



37 



and entrance of the ductus arteriosus. The fifth arch on the left 

 side is represented in its entirety by the pulmonary artery and 

 ductus arteriosus. The fifth arch on the right side disappears, 

 in the greater part of its extent at least. Probably the 

 right pulmonary artery is formed from the inner part of this 

 arch. 



All the aortic arches are not present at the same time ; some 

 have only a brief period of existence. One of these transient 

 arches is said to appear between what are usually described as 

 the 4th and 5th arches. If this is always the case then the 

 pulmonary arteries should be described as derived not from the 

 5th but from the 6th aortic arches. 



Subclavian Arteries. — The visceral arches with their arteries 

 are well developed before the limb buds appear. When, at the 

 end of the third week, the buds grow out to form the upper 

 extremities, the artery which supplies each bud grows out from 



innominate 

 carotid 



vert.— 



subclau. 



constriction 

 right put. art 



5th right arch • — 

 right dorsal aorta 



vert, 

 vbclav. 

 constriction 

 arch, left side 



left pulmonary art. 

 'eft dorsal aorta 



Fig 29 —The condition of the Eight and Left Dorsal Aortae in a 6th week human 

 foetus (After His.) The right arch disappears beyond the origin of the right 

 subclavian ; a constriction may appear at the corresponding point on the left side. 



the dorsal aortae opposite the 4th arches (Fig. 29). This artery 

 forms the entire subclavian on the left side, but only its third 

 stage on the right. 



The Arch of the Aorta on the Right Side.— In birds it 

 is the 4th right arch which forms the aortic arch, and 

 this occasionally happens in man. In amphibians both the 

 right and left 4th arches persist as aortic arches. The two 



