THE CIRCULATORY SYSTEM 181 



secondary perforation of the septum atriorum and the formation 

 of the annulus ovalis or isthmus of Vieussens. 



THE ARTERIAL SYSTEM 



The arterial system of Man bears traces of primitive con- 

 ditions. It is indeed an astonishing fact, for example, that the 

 aortic arch system of the embryos of the higher Vertebrata, up 

 to Man himself, appears in the same manner as in the Anamnia. 

 Six pairs of aortic arches in all are formed in the young 

 Mammalian embryo, but the representatives of the first and 

 second of these and the vestige of the fifth degenerate early, 1 and 

 consequently only three pairs remain to undergo final transfor- 

 mation. 



[Conspicuous among the variations occurring in Man is the 

 occasional presence in the adult of paired aortic arches, the arch 

 of the right side, which usually disappears during development, 

 being retained. Twelve cases of double aortic arch have been 

 recorded in Man, 2 and this variation may be accompanied by the 

 obliteration and reduction to a fibrous band of the ordinarily 

 functional (left) arch, 3 the resulting condition of the parts being 

 essentially that characteristic of Birds.] In a similar manner, 

 many of the variations to which the vessels derivative of the 

 primitive arterial system of the human embyro are liable, can 

 only be explained by the fact that embryonic trunks, which 

 under normal conditions become occluded and vestigial, may 

 remain functional throughout life. In this respect the Anthro- 

 poids altogether agree with Man. 



On the inner surface of the abdominal wall in Man three 

 cord-like structures pass from near the bladder to the navel. 

 These are known as the ligamentum vesicale medium and the 

 ligamenta vesicalia lateralia. The first urachus corresponds with 

 the stalk of the allantois of the embryo ; the latter, however, 

 are the last vestiges of the umbilical or hypogastric arteries, 

 which during intra-uterine life, i.e. from about the time when the 

 posterior limbs are just beginning to appear as buds, convey the 



1 [The recent researches of Boas and others have proved that in all classes of 

 terrestrial Vertebrates the pulmonary artery is a derivative of the sixth aortic arch 

 (the fourth branchial), and that the arch in front of it is suppressed ; and Zim- 

 raermann has shown that Man himself is no exception to this rule (Vcrhandlg. 

 Internal. Medic. Congresses X., Berlin, 1891, Bd. ii., Abth. i. p. 145).] 



2 [Cf. Leboucq, Ann. Sci. Med. Gand., ]894, p. 7.] 



3 [Of. Morrison Watson, Jour. Anat. and Phys., vol. xi. p. 229.] 



