CH. LVIII.] 



THE FffiTAL CIRCULATION. 



835 



body) remains throughout foetal life under the name of the 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. The first 

 and second arches soon dis- 

 appear, but the third arches 

 and portions of the aortic 

 roots remain as the carotid 

 arteries (see fig. 655). 



As the umbilical vesicle 

 dwindles in size, the portion 

 of the omphalo-mesenteric 

 arteries outside the body gra- 

 dually disappears, but the part 

 inside the body remains as the 

 mesenteric arteries. 



Meanwhile with the growth 



pn 



Fig. 654.- Embryonic heart and vessels. 

 O, vein* ; A u, auricle ; V, ventricle ; 

 B, bulb ; t, a, 3, 4, 5, primitive aortic 

 arches. 



Fig. 655. 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 />, the pulmonary ; 

 a a', right and left aortic roots ; A', de- 

 scending aorta ; i, 2, j, 4, 5, the five 

 primitive aortic or branchial arches ; 

 7, //. ///, IV, the four branchial clefts 

 which, for the sake of clearness, have 

 been omitted on the right side. The per- 

 manent systemic vessels are deeply, the 

 pulmonary arteries lightly, shaded ; the 

 parts of the primitive arches which are 

 transitory are simply outlined ; e, placed 

 between the permanent common carotid 

 arteries ; e e, external carotid arteries ; 

 e i, internal carotid arteries; t, right 

 subclavian, rising from the right aortic 

 root beyond the tit t h arch ; , right verte- 

 bral from the vame, opposite the fourth 

 arch ; v' ', left vertebral and subclavian 

 arteries rising together from the left, or 

 permanent aortic root, opposite the fourth 

 arch ; ;>, pulmonary arteries rising toge- 

 ther from the left fifth arch ; "'. outer or 

 back part of left fifth arch, forming 

 ductUH arteriosus ;/./> n', right and 

 left pneumogastric nerves descending in 

 front of aortic arch, with their recurrent 

 branches represented diagrainmatically 

 a* passing behind, to illustrate the rela- 

 tions of these nerves respectively to the 

 right subclavian artery (4) and the arch 

 of the aorta and ductus arteriosui (d). 

 (Allen Thomson, after Rathke.) 



of the allantois two new arteries (umbilical) appear, and rapidly 

 increase in size till they are the largest branches of the aorta ; 



3 H 2 



