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HANDBOOK OF PHY3IOLOGY. 



division of the heart; but this relation is gradually reversed as develop- 

 ment proceeds. Moreover, all through foetal life, the walls of the right 

 ventricle are of very much the same thickness as those of the left, which 

 may probably be explained by the fact that in the foetus the right ventri- 

 cle has to propel the blood from the pulmonary artery into the aorta, 

 and thence into the placenta, while in the adult it only drives the blood 

 through the lungs. 



Arteries. The primitive aorta arises from the bulbus arteriosus and 

 divides into two branches which arch backwards, one on each side of the 

 foregut and unite again behind it, and in front of the notochord into a 

 single vessel. 



This gives off the two omphalo-mesenteric arteries, which distribute 

 branches all over the yolk-sac; this area vasctilosa in the chick attaining 



pn 



FIG. 476. Diagram of the aortic arches in the 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; /, II, III, 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; c, placed between the permanent com- 

 mon carotid arteries; ce, external carotic arteries; ci, 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 per- 

 manent 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; pn,pn', 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.) 



a large development, and being limited all round by a vessel known as 

 the sinus terminalis. 



The blood is collected by the venous channels, and returned through 

 the omphalo-mesenteric veins to the heart. 



Behind this pair of primitive aortic arches, four more pairs make 



