260 CLINICAL APPLIED ANATOMY. 



into the ventricle as the " aortic septum " to complete the septum 

 between the ventricles. Consequently it is quite possible that 

 the septal defects and malpositions of the great vessels may be 

 due to the same unknown influences which cause the pulmonary 

 obstruction. However this may be, the septal defects and 

 arterial malpositions are usually looked upon as secondary 

 results of the pulmonary lesion and explained as follows : 



When stenosis of the pulmonary artery occurs before the 

 eighth week of foetal life, the ventricular septum is as yet 

 incomplete, and the excess of pressure in the right ventricle is 

 relieved by the passage of some blood across the top of the 

 septum into the left ventricle. As a result the septum fails to 

 complete its union with the elements which enter into the 

 formation of its upper part and remains unclosed. The con- 

 gestion in the right auricle is similarly relieved by the passage 

 of blood across the incomplete auricular partition. The high 

 pressure in the right ventricle dislocates the growing ventricular 

 septum to the left, so that both the aorta and the pulmonary 

 artery retain their connection with the right ventricle, this being 

 the portion of the common ventricular cavity with which they 

 communicate at their first formation. When there is a con- 

 siderable aperture in the ventricular septum, this may by itself 

 suffice to relieve the overpressure in the right cavities, and the 

 auricular septum may become complete. Should the ventricular 

 septum be entire, having completed its growth before the pul- 

 monary obstruction occurs, the auricular septum, which is 

 completed later than the ventricular, must of necessity remain 

 pervious, otherwise there is no sufficient exit for the blood from 

 the right cavities of the heart. 



Complete obliteration of the infundibular channel or of the 

 pulmonary artery is usually spoken of as atresia. The secondary 

 cardiac changes which result are similar to those which occur 

 with stenosis. The right ventricle carries on the systemic 

 circulation, the left becoming atrophied, since it fails to establish 

 its proper connection with the aorta. If, however, the ventricular 

 septum is complete and the aorta has established its normal 



