24 C. W. M. Poynter 
In the latter case the development is not so regular; Breschet’s 
case is illustrated in fig. 30. 
Persistence of the right arch with atrophy of the left (ductus 
arteriosus) is not so unusual as persistence of both arches. 
When the development of the vascular arches is otherwise nor- 
mal a right pulmonic arch (ductus arteriosus) may persist; it 
would seem that if the normal developmental relations are main- 
tained it should join the proximal portion of the subclavian and 
represent in its distal part a segment of the right dorsal aorta. 
This would correspond to the position on the left or the position 
it occupies in cases of right arch persisting, Abernethy (1793) ; 
however in the cases of Gruber (1846), Jackson (1875) and Hil- 
drith (1880) the vessel joins the innominate just before its divi- 
sion. The explanation of this condition presents some difficul- 
ties especially in the light of other anomalies of the arch. 
A right arch persists occasionally in cases of right aortic arch, 
although this is not the rule: Abernethy (1793), Breschet (1826), 
Otto (1824), Ollivier (1861), Quain 1844) and Lockwood 
(1884). In these cases it passes from the right pulmonary to 
the aorta at a point below the origin of the right subclavian. In 
cases of situs viscerum transversus the left arch usually persists 
at least till birth, but in the cases of Arnold (1868) and Griffith 
(1891) the development of the right arch obtained as in the cases 
of right aortic arch above. 
Persistence of the left arch to the time of birth and its later 
obliteration was known to Galen and its place in the foetal cir- 
culation was described by Harvey. The persistence of the arch, 
ductus arteriosus Botalli, is not a rare anomaly and it is fre- 
quently combined with other developmental defects, having to do 
particularly with the circulation of the lungs. 
The cause of this anomaly has been sought indirectly in en- 
deavoring to discover the process which produces normal occlu- 
sion. Two main factors have been suggested as responsible for 
the obliteration of the ductus arteriosus: (a) lowering of the 
blood pressure in the ductus after birth and (0) the difference in 
the histological structure in the ductus and the connecting blood- 
vessels. Recently Faber (1912) concluded that the obliteration 
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