Oval fossa

The fossa ovalis (Oval fossa) is a shallow, oval-shaped depression on the septal wall of the right atrium (right surface of the interatrial septum). It represents the postnatal remnant of the foramen ovale, a fetal interatrial communication that allows blood to bypass the non-functional fetal lungs. In the adult dog, this opening is normally closed, leaving the fossa ovalis as a visible landmark.

Borders and related structures:

• The fossa ovalis is centrally located on the interatrial septum.

• It is surrounded by a raised muscular rim called the limbus (or annulus) of the fossa ovalis, which is most prominent dorsocranially.

• The thin floor of the fossa ovalis corresponds to the septum primum (fetal component), while the limbus represents the septum secundum.

• It lies ventral to the opening of the cranial vena cava and dorsomedial to the tricuspid valve.