COMPARATIVE ANATOMY OF THE CARDIAC FORAMEN OVALE IN THE PINNIPEDIA

The structure of the cardiac foramen ovale from eight genera of pinnip eds was studied using the scanning electron microscope. Specimens were obtained from fetuses or neonates of the Californian sea lion (Zaloph us californianus), Antarctic fur seal (Arctocephalus gazella), walrus (Obenus rosmarus), grey seal (Halichoerus gryphus), ringed seal (Phoca hispida), bearded seal (Erignathus barbatus), Weddell seal (Leptonych otes weddelli), and crabeater seal (Lobodon carcinophagus). In each sp ecies, the structure that permits oxygenated blood from the placenta f lowing in the caudal vena cava to pass directly into the left side of the heart, the foramen ovale, when viewed from the terminal part of th e caudal vena cava had the appearance of the entrance to a short tunne l. A thin fold of tissue, the developed remains of the septum primum, projected from the caudal edge of the foramen ovale into the lumen of the left atrium. It constituted about 75% of the inner surface of the tunnel, and was generally unfenestrated. The wall of the interatrial s eptum contributed the ''floor.'' The distal end of the tunnel was stra ight-edged. In most cases the septum primum was long enough to cover t he foramen ovale. The siting of pulmonary veins in the roof of the lef t atrium appeared to be such that drainage from them after birth would press the septum primum over the foramen opening, thereby functionall y closing it. Collapses of the tunnel was seen in all the neonatal sea ls, and in the 1-month-old neonate the fold of tissue was anchored to the interatrial septum along the surface of the crista dividens, which lay in the left atrium. Cellular protrusions and thread formation may play a role in the closure of the foramen ovale.