Arterial Anomalies 13 
A. Absence or Imperfect Development of the Aortico-Pulmonary 
Septum 
Irregularities in the development of the proximal portion of 
the aorta and the pulmonary arteries can only be appreciated by 
comparison with the various embryonal stages. As briefly re- 
viewed on page 10 the process of development of the aortico-pul- 
monary septum is seen to be a complicated one and probably it is 
dependent on many factors. That some of these are coupled with 
the development of the heart there can be no doubt; this is sug- 
gested by the fact that in case of absence of the septum (per- 
sistent truncus communis) the ventricular septum is frequently 
rudimentary and is always defective at the base. The common 
trunk may open from the right side of the common ventricle as in 
an early stage of development or when the heart is well developed 
it may open from both ventricles over the defect in the interven- 
tricular septum. When the aortico-pulmonary septum is entirely 
absent the common trunk gives off the two pulmonary arteries 
separately from its lateral or dorsal wall and continues as the 
normal ascending aorta; see fig. 5. The common trunk is gen- 
erally considered as the homologue of the aorta and the case is 
referred to as lacking a pulmonary artery, but this is incorrect 
if we are dealing with a truncus arteriosus in which the septum 
has failed to develop. It is the structure from which both the 
aorta and pulmonary arteries are differentiated and consequently 
represents both in an imperfect state of development. If there 
is no other defect in the region but the absence of a septum the 
pulmonary arteries will be given off one on each side of the com- 
mon trunk. This condition is extremely rare, Keith (1909). 
The septum may begin to develop normally, then be arrested ; 
in such cases a truncus communis arises from the heart, then 
divides into an aorta and pulmonary trunk, Clarke (1885). The 
septum may be present but defective ; in Rokitansky’s case (1875) 
the external appearance was very much like that of the case just 
cited, but a remnant of septum was present in the common trunk. 
If the factors producing the anomaly have only a minor influence, 
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