36 C. W. M. Poynter 
Several hypotheses have been offered in explanation of the ven- 
tral position of the proximal portion of the subclavian but none 
of them, it seems to me, are entirely without objection. The first 
assumes that the dorsal aortae unite ventral to the cesophagus. 
Such a process is difficult to understand in view of the way in 
which the common aorta normally develops; then too if a process 
so fundamental, apparently, as aortic development may present 
such an extreme irregularity we have no evidences of its having 
been found except in this one anomaly and always at this point. 
The second hypothesis, that of Rau (1890), offered as an ex- 
planation of the general class, conceives that the anomalous sub- 
clavian has arisen by early anastomoses with the right bronchial 
artery. This would account for the ventral position of the ves- 
sel, but we would have to assume a very high origin for the bron- 
chial and also that it developed much earlier than it seems to do 
normally. The right bronchial artery would then be a branch of 
the anomalous subclavian and no such condition has been re- 
ported (Holzapfel). 
The third hypothesis, which is favored by Holzapfel (1899), 
is that an anomalous anastomosis develops ventrally between the 
paired descending aortae. This anastomosis occurs early, then 
when the lungs are laid down their relation to this aberrant ves- 
sel will determine which type, 7. e., b or c, will develop. The ex- 
planation cannot be disproved but may be objected to on the 
ground that it denies an orderly development of the earliest vas- 
cular anlage and reaffirms the theory of Baader for this particu- 
lar anomaly, in the light of all the more recent investigations 
which suggest that a more exact explanation may be possible. 
The fourth hypothesis was offered by Banchi (1907) who 
thinks these cases represent a persistence of both fifth (not pul- 
monary ) arches with atrophy of the dorsal root of the right fourth 
arch and the right descending aorta. We may offer as an objec- 
tion to this that there is possibly still some controversy on the 
“fifth” arch, see page 8, but more important, that such an ex- 
planation necessitates an anomalous union between these arches 
ventrally, which is as great a difficulty as the general question ; or 
if we accept his explanation of this point we still have to account 
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