The horizontal interventricular septum. Three cases with different ventriculoarterial connections.

Three hearts sharing the anatomical peculiarity of a horizontal interventricular septum are described. In one, both great vessels originate from the superior right ventricle, in the other the great arteries are connected normally, the third one shows a single aortic trunc. It is concluded that the horizontal interventricular septum has to be considered as an independent cardiac deformity occurring with most types of ventriculoarterial connections. Assuming rotation of the bulboventricular loop around a base-apex-axis as the basic ontogenetic event one can trace this morphology back to its original layout. It is, therefore, possible to classify each case according to the usual nomenclature regarding the horizontal septum and the upstairs-downstairs arrangement of the ventricles as an addition. This embryological event of rotation is considered the product of a restraightening process of the previously looped cardiac tube.