DEVELOPMENT. 801 



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

 mammals. 



In mammals the left fourth aortic arch develops into the permanent 

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



ci 



fM 



Fig. 495. Diagram of the aortic arches in a mammal, showing transformations which give rise 

 to the permanent arterial vessels. A, primitive arterial stein 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', descending aorta; 1, 2, 3, 4, 5, the five primitive aortic or branchial arches; /, IT, III, IV, 

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

 The permanent 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 per- 

 manent 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 the left fifth arch, forming ductus 

 arteriosis ; p n, p n', right and left pneumogastric nerves descending in front of aortic arch, 

 with their recurrent branches represented diagrammatically as passing behind, to illustrate the 

 relations of these nerves respectively to the right subclavian artery (4) and the arch of the aorta 

 and ductus arteriosus (d). (Allen Thomson, after Rathke.) 



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 internal carotid artery, while 

 the fifth disappears on the right side, but on the left forms the pulmo- 

 nary artery. The distal end of this arch originally opens into the descend- 

 ing aorta, and this communication (which is permanent throughout 

 life in many reptiles on both sides of the body) remains through- 

 out fcetal life under the name of ductus arteriosus: the branches of the 

 pulmonary artery, to the right and left lung, are very small, and most 

 of the blood which is forced into the pulmonary artery passes through 

 the wide ductus arteriosus into the descending aorta. All these points 

 will become clear on reference to the accompanying diagram (fig. 495). 



