The least degree of defect of this septum is the persistence of the foramen ovale. A certain degree of this is very frequently present in adult hearts, but the aperture is usually so small or so valved that there is virtually no communication between the contents of the two auricles. On the other hand, the lesion may be connected with stenosis of the pulmonary artery or aorta, and in that case there is an actual and persistent communication between the two cavities?

In more extreme cases the septum itself is defective. It may be entirely wanting, or defective at its lower part. This lesion frequently coincides with a defect of the posterior ventricular septum.