THE AKTEEIES. 417 



The aortic arches of the fourth pair, in the second branchial 

 arches, are the systemic arches, which by their union form 

 the dorsal aorta. At first the vessels of the two sides, right 

 and left, are of equal size, but from a very early period the left 

 one becomes the larger, and ultimately forms the arch of the 

 aorta in the adult. The right systemic arch persists for some 

 time, but ultimately disappears, with the exception of its 

 proximal part, which is said to give origin to the right sub- 

 clavian artery. 



The aortic arches of the fifth pair, in the third branchial 

 arches, are the pulmonary arches : from them the pulmonary 

 arteries arise as posteriorly directed branches. The pulmonary 

 arches retain their connections with the dorsal aortse throughout 

 the whole period of intra-uterine life, up to the time of birth ; these 

 connections having, as in the chick embryo, a most important 

 influence on the course of the circulation. At the time of birth, 

 the part of each pulmonary arch between the origin of the 

 pulmonary artery and the aorta (cf. Fig. 128), a part known as' 

 the ductus arteriosus or ductus Botalli, becomes obliterated ; 

 and from this time the blood driven into the pulmonary 

 arches by the right ventricle can no longer pass directly to 

 the aorta, but is all sent through the pulmonary arteries to the 

 lungs. 



Zimmermann has found traces, in rabbit embryos of the 

 eleventh day, of a pair of aortic arches between the systemic and 

 pulmonary arches. This observation, if confirmed by fiiture 

 investigation, will be of considerable interest, as showing that 

 the pulmonary arches of the rabbit are the sixth and not the 

 fifth pair, and that the pulmonary arteries therefore arise in the 

 rabbit fi'om the same pair of arches as in the frog ; in other 

 words, that the pulmonary arteries are strictly corresponding 

 structures in these two types. 



As regards the arteries of the trunk, the two dorsal aort© 

 are at first distinct along their whole length, and the allantoic 

 arteries appear as though they were direct posterior continuations 

 of the aortfe. Later on, the two aortse unite to form the definite 

 dorsal aorta, which is continued as a narrow median caudal 

 artery to the hinder end of the embryo ; the allantoic arteries 

 from this time appearing as branches of the aorta. 



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