COMPARATIVE ANATOMY OF THE CARDIAC FORAMEN OVALE IN CATS (FELIDAE), DOGS (CANIDAE), BEARS (URSIDAE) AND HYAENAS (HYAENIDAE)

The structure of the foramen ovale from 16 species representing 4 carn ivore families, the Felidae, Canidae, Ursidae and Hyaenidae, was studi ed using the scanning electron microscope. The Felidae were represente d by 9 domestic cat fetuses (Felis catus), 2 snow leopard neonates (Un cia uncia), an ocelot neonate (Leopardus pardalis), 2 lion neonates (P anthera lee), a panther neonate (Panthera pardus) and 3 tigers (Neofel is tigris), comprising 2 fetuses and a neonate. The Canidae were repre sented by a golden jackal neonate (Canis aureus), a newborn wolf (Cani s lupus), 8 domestic dog fetuses (Canis familiaris), 3 red fox neonate s (Vulpes vulpes) and a dhole neonate (Cuon alpinus). The Ursidae were represented by a brown bear neonate (Ursus arctos), a day-old grizzly bear cub (Ursus arctos horribilis), a polar bear neonate (Ursus marit imus), and 2 additional bear fetuses (species unknown). The Hyaenidae were represented by a striped hyaena neonate (Hyaena hyaena). In each species, the foramen ovale, when viewed from the terminal part of the caudal vena cava, had the appearance of a short tunnel. A thin fold of tissue, the developed remains of the embryonic septum primum, extende d from the distal end of the caudal vena cava for a variable distance into the lumen of the left atrium and contributed towards the 'tunnel' appearance in all specimens. It constituted a large proportion of the tube, and its distal end was straight-edged. There was fibrous materi al underlying the endothelium of the flap, the apparent morphology of which suggested that it comprised cardiac muscle. In each species, the pulmonary veins drained from close beside the interatrial septum and this venous drainage seemed to be directed along the length of the fla p of tissue on its dorsal surface. Collapse of the 'tunnel' of the sep tum primum effected closure of the foramen ovale in most of the neonat es studied. When closed, the tissue flap appeared to be anchored to th e interatrial septum along the surface of the crista dividens which la y in the left atrium. The straight edge at the end of the 'tunnel' rem nant was not usually attached and, when gently lifted, formed a small blind-ended pouch ending at the attachment of the flap to the crista d ividens.