28 C. W. M. Poynter 



corporated in the ventricular wall. This adjustment brings the 

 truncus arteriosus against the atrial wall ventrally and its open- 

 ing over the two ventricles. Later the canal of the bulbus cordis 

 and truncus arteriosus is divided by a septum, the aortic septum, 

 which in the truncus arteriosus produces the aorta and pulmonary 

 artery and in the bulbus grows down to meet the ventricular 

 septum and helps to complete the separation of the ventricles. 



The close association of the development of these structures 

 with the general development of the heart is indicated by the 

 frequency with which anomalies of their development are asso- 

 ciated with other cardiac irregularities, particularly defective 

 interventricular septum. In this group of cases 20 per cent, of 

 the hearts which showed complete lack of ventricular septum 

 also lacked an aortic septum. Approximately 2^ per cent, of all 

 defects of the ventricular septum were associated with irregu- 

 larities of the aortic septum. 



A. Persistent Truncus C omfnunis 



As seen above, the aorta and pulmonary artery are developed 

 by the growth of the aortic septum which begins as two distal 

 ridge-like thickenings in the lumen of the truncus communis ; 

 these grow out, fuse and so form a septum which normally takes 

 a spiral course. When the septum fails to develop the embryonal 

 truncus communis persists ; this common trunk is generally con- 

 sidered as the homologue of the aorta and the case is spoken of 

 as lacking the pulmonary artery, but this interpretation is incor- 

 rect. There is a condition of arrest of development of the pul- 

 monary artery to be spoken of later but it may be distinguished 

 from persistent truncus by the fact that the two pulmonary 

 arteries do not spring from the back of the common artery. The 

 persistent truncus may open from the right ventricle as in the 

 early embryo or from both ventricles over the defect in the inter- 

 ventricular septum. 



Various degrees of arrest of development less than complete 

 absence of the aortic septum have been reported, the septum may 

 begin its normal development then be arrested ; in such cases a 

 truncus communis arises from the heart and in a short distance 



