CONGENITAL DISEASE OF THE HEAET. 263 



the completed ventricular septum may often be recognised. The 

 angle formed by the meeting of two of the cusps of the pulmo- 

 nary valve corresponds exactly in position with the angle between 

 two of the aortic cusps on the other side of the septum. The 

 cusps which are thus adjacent were continuous with each other 

 until the spiral septum divided the common passage above them 

 into the aorta and pulmonary artery. These cusps may be con- 

 veniently termed septal, and the anterior corresponding pair were 

 developed from the ventral endocardial cushion of the bulbus, 

 whilst the posterior corresponding pair arose from the dorsal 

 cushion. On the septal wall of the infundibulum a line or 

 furrow may sometimes be traced from the angle between the 

 septal cusps of the pulmonary valve downwards and backwards 

 to the undefended space. The portion of the septum in front of 

 this line appears as a thick muscular band passing downwards 

 and forwards ; this represents the false septum of the reptilian 

 heart. The portion behind the line passes downwards and 

 backwards, and becomes in part continuous with the supraven- 

 tricular crest already described. The line or furrow represents 

 in its upper part the fusion of the endocardial cushions which 

 divided the bulbus into the aortic and pulmonary orifices, and in 

 its lower part, which inclines towards the undefended space, 

 represents the line of fusion of the aortic septum with the 

 septum inferius. (Fig. 34.) 



The commonest site for a perforation of the interventricular 

 septum to occupy is immediately in front of the undefended 

 space. This corresponds to the lower part of the furrow of the 

 aortic septum. A perforation posterior to the undefended spot 

 is less common, and is usually associated with a defect in the 

 adjacent part of the auricular septum, since it lies at the point 

 of fusion of the septum inferius with the posterior endocardial 

 cushion of the auricular canal. The undefended space itself 

 may be defective in conjunction with septal defects in front of it 

 or behind it. (Fig. 33.) The rarest defects of all are in the 

 extreme fore part of the anterior region of the septum. 



Apertures in the region of the undefended space and 



