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,’ 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,” and this variation may be accompanied by the 
obliteration and reduction to a fibrous band of the ordinarily 
functional (left) arch,’ 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 vesicaha 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, 7.c. 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- 
mermann has shown that Man himself is no exception to this rule (Verhandig. 
Internat. Medic. Congresses X., Berlin, 1891, Bd. ii., Abth. i. p. 145).] 
2 [Cf. Leboucq, Ann. Sct. Med. Gand., 1894, p. 7.] 
3 [Cf. Morrison Watson, Jour. Anat. and Phys., vol. xi. p. 229.] 
