As evolution proceeded, the muscular connection between auricles and ventricles gradually diminished. In the piscine heart, junctional fibers are present and conduct equally well in either direction at the whole circumference of the auriculoventricular border. In the amphibian heart, some parts of the junctional tissue conduct better than others; in some sections, reversed, or ventriculoauricular, conduction is even more rapid than normal, or auriculoventricular, conduction. Differences in conductivity are even more pronounced in the reptilian heart, and a reduction in the amount of conduction tissue has occurred.1 Differentiation and specialization are still more evident in the avian heart, and in the mammalian heart the relatively small auriculoventricular conduction system furnishes the only morphologic connection between auricles and ventricles. Thus, functional differentiation may be assumed to have preceded anatomic alteration and reduction.
A second communication between auricles and ventricles in man has been suggested as an atavistic, inherited anomaly occurring
DAVID SCHERF. AN EXPERIMENTAL STUDY OF RECIPROCATING RHYTHM. Arch Intern Med (Chic). 1941;67(2):372–382. doi:10.1001/archinte.1941.00200020134008