CONGENITAL DISEASE OF THE HEAET. 259 



a narrow strait of communication between the pulmonary trunk 

 and the sinus or body of the ventricle. 



The pulmonary cusps are developed at the distal end of the 

 bulbus arteriosus from endocardial projections known as cushions, 

 which give rise to the segments of both the aortic and the 

 pulmonary valves. The pulmonary orifice may become narrowed 

 or even completely occluded by developmental defects in this 

 position. 



The common arterial trunk beyond the valves is the conus 

 arteriosus of embryologists. This becomes subdivided into the 

 pulmonary artery and the aorta by the growth and fusion of 

 dorsal and ventral ridges which, commencing above at the origin 

 of the fifth branchial arch, grow downwards with a spiral twist 

 to unite with the endocardial cushions from which the semilunar 

 valves are derived. Narrowing or obliteration of the pulmonary 

 trunk may result from a faulty position of the spiral septum. 

 Defect in coalescence of the ridges which form the septum may 

 be the cause of certain rare communications between the aorta 

 and pulmonary artery, whilst complete failure of the septum will 

 result in the presence of a common trunk which conveys blood 

 to the lungs and the systemic vessels as well. 



Thus it comes about that congenital defects may occur on the 

 right side at the junction of the infundibulum with the sinus of 

 the right ventricle, a position normally indicated by the supra- 

 ventricular crest and lying just above the undefended space; or 

 in the infundibulum itself, or at the site of the pulmonary valve, 

 or in the pulmonary trunk between the valve and the ductus 

 arteriosus. Of these defects the commonest is one at the position 

 of the pulmonary cusps. (Fig. 28.) 



Congenital defects of the infundibulum or of the pulmonary 

 artery are often associated with defects in the ventricular or 

 auricular septa. Malpositions of the aorta and pulmonary artery 

 often coexist as well. The spiral septum of the conus arfceriosus 

 not only separates the pulmonary artery from the aorta, but 

 also, after fusion with the endocardial cushions of the bulbus 

 from which the semilunar valves are derived, projects downwards 



172 



