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HAND-BOOK OF PHYSIOLOGY. 



arteries supplying the gills, while in Amphibia three pairs persist through- 

 out life. 



In Reptiles, Birds, and Mammals, further transformations occur. 



In Reptiles the fourth pair remains throughout life as the permanent 

 right and left aorta; in Birds the right one remains as the permanent 

 aorta, curving over the right bronchus instead of the left as in Mammals. 



In Mammals the left fourth aortic arch develops into the perma- 

 nent aorta, the right one remaining as the subclavian artery of that side. 

 Thus the subclavian artery on the right side corresponds to the aortic arch 



Cl 



-pn 



FIG. 439. Diagram of the aortic arches in a mammal, showing transformations which give rise 

 to the permanent arterial vessels. A, primitive arterial stem or aortic bulb, now divided into A, the 

 ascending part of the aortic arch, and p, the pulmonary; a a', right and left aortic roots; A', de- 

 scending aorta; 1, 2, 3, 4, 5, the five primitive aortic or branchial arches; /, //, ///, IV, the four 

 branchial clefts which, for the sake of clearness, have been omitted on the right side. The perma- 

 nent systemic vessels are deeply, the pulmonary arteries, lightly shaded ; the parts of the primitive 

 arches which are transitory are simply outlined; c, placed between the permanent common carotid 

 arteries; c e, external carotid arteries; c i, internal carotid arteries; s, right subclavian, rising from 

 the right aortic root beyond the fifth arch; v, right vertebral from the same, opposite the fourth 

 arch; v' s', left vertebral and subclavian arteries rising together from the left, or permanent aortic 

 root, opposite the fourth arch; p, pulmonary arteries rising together from the left fifth arch; d, 

 outer or back part of left fifth arch, forming ductus arteriosus; p n, p n, right and left pneumogas- 

 tric nerves, descending in front of aortic arches, with their recurrent branches represented diagram- 

 matically as passing behind, to illustrate the relations of these nerves respectively to the right sub- 

 clavian artery (4), and the arch of the aorta and ductus arteriosus (d). (Allen Thomson, after 

 Rathke.) 



on the left, and this homology is further confirmed by the fact that the 

 recurrent laryngeal nerve hooks under the subclavian on the right side, 

 and the aortic arch on the left. 



The third aortic arch remains as the external carotid artery, while the 

 fifth disappears on the right side, but on the left forms the pulmonary ar- 

 tery. The distal end of this arch originally opens into the descending aorta, 

 and this communication (which is permanent throughout life in many rep- 

 tiles on both sides of the body) remains throughout foetal life under the 

 name of ductus arteriosus: the branches of the pulmonary artery to the 



