Arterial Anomalies 9 
Reagan (1912) studied the pig and found numerous anasto- 
moses in the region between the fourth and pulmonic arches. 
He concluded that “A fifth vessel, very closely approximating a 
theoretically perfect aortic arch, can be demonstrated for the pig.” 
Coulter (1909) worked on the cat and found rudimentary ves- 
sels below the fourth arch and concluded that a complete fifth 
arch develops in the cat. 
Lewis (1903) was the first to question the fifth arch. He 
called attention to the irregular condition of the vessels between 
the fourth and fifth arches and concluded: “ The irregular small 
arteries around the fourth entodermal pouch do not, as Zimmer- 
man believed, form a distinct aortic arch.” Later (1905), he 
made reconstructions of the arches, together with the pouches in 
the rabbit, and showed not only a great irregularity of the vas- 
cular elements but that there was not sufficient evidence of a 
fifth entodermal pouch to warrant the claims of six arches, and 
that the question of the homology of the arch from which the 
pulmonary artery sprung was, in mammals, still an unsettled one. 
In the light of recent work on the early vessels of the embryo 
it is not sufficient simply to find a vessel to prove the existence of 
a fifth arch. From my study of rabbits and the negative evi- 
dence which the collected variations furnish, I am inclined to 
accept Dr. Lewis’s conclusions. The problem has been particu- 
larly covered recently by Bremer (1912), who says: 
While not wishing to go too deeply into the controversy on the presence 
or absence of a sixth aortic arch, I may say that it seems to me that the 
solution should come from further study of the entodermal pouches, of 
the branches of the nerves and the cartilages of the region. ... As far 
as the early development of the vessels is concerned there is nothing cer- 
tainly to prove the presence of an interpolated arch. 
Quite recently new interest has been given the pulmonary arch 
and arteries. If the pulmonic vessel is a true arch it has under- 
gone great modification and its interpretation is not as simple as 
had been supposed. Reagan (1912) says that both the pre-pul- 
monic caecum and the pulmonic vessel “seem to have been greatly 
modified, if they ever resembled closely the parts anterior which 
have generally been considered their homologues.” Of the pul- 
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