CONGENITAL DISEASE OF THE HEAET. 261 



connexion with the left ventricle, it is the right ventricle which 

 atrophies since it has no function to perform. As a general rule 

 in congenital disease of the heart, those parts which in conse- 

 quence of the defect are thrown out of the circulatory circuit will 

 atrophy, whilst those which have increased work thrown upon 

 them will become hypertrophied. (Fig. 30.) 



When the pulmonary channel is obliterated the blood usually 

 reaches the lungs through the ductus arteriosus, and this may 

 also carry part of the blood when the pulmonary artery is 

 stenosed. Sometimes, however, the pulmonary blood supply is 

 carried by enlarged bronchial arteries, which, as is well known, 

 show considerable variations in their points of origin. 



Defects in the cardiac septa are very common in congenital 

 heart disease, and are often, as already explained, associated 

 with obstruction to the exit of blood from the right ventricle. 

 They may, however, occur without such obstruction. A know- 

 ledge of the development of the cardiac septa throws light upon 

 the positions in which septal defects are likely to be found. 



Defects in the auricular septum are occasionally very large, 

 the septum being represented merely by a crescentic fold at the 

 upper and back part of the auricular cavity. The fold in such cases 

 occupies the position at which the septum primum or primitive 

 septum of the auricle is known to make its first appearance. If 

 the septum primum has developed to a greater degree, but still 

 failed to fuse with that part of the ventricular septum which lies 

 between the auriculo-ventricular orifices, a defect will exist just 

 above the top of the latter. (Fig. 32.) This opening, between 

 the base of the tricuspid cusp and the position of the foramen 

 ovale appears to correspond in position with the ostium primum 

 of embryologists, for this ostium is between the lower part of the 

 downgrowing septum of the auricles and the two endocardial 

 projections which divide the common auricular canal into the 

 right and left auriculo-ventricular orifices. Normally the ostium 

 is closed by fusion of these structures. The substance of the 

 septum primum becomes fenestrated and finally widely perforated 

 in its upper part, giving rise to the ostium secundum, a part of 



