The<unk>  is the large quadrangular cavity located between the two venae cavae in the embryonic human heart. In the adult it is incorporated into the wall of the right atrium to form a smooth part called the sinus venarum, also know as the venarum sinus, which is separated from the rest of the atrium by a ridge of fibres called the crista terminalis.
In the embryo, the thin walls of the<unk>  are connected below with the right ventricle, and medially with the left atrium, but are free in the rest of their extent. It receives blood from the vitelline vein, umbilical vein and common cardinal vein.
It originally starts as a paired structure but shifts towards associating only with the right atrium as the embryonic heart develops. The left portion shrinks in size and eventually forms the coronary sinus and oblique vein of the left atrium, whereas the right part becomes incorporated into the right atrium to form the sinus venarum.
Sinus venosus
Ductus venosus
Brachiocephalic vein
Coronary sinus
Internal jugular vein
Cardiac veins
Descending aorta
Aortic sinus