The
sinus venosus is a large quadrangular cavity which precedes the
atrium on the venous side of the
chordate heart. In mammals, it exists distinctly only in the
embryonic heart (where it is found between the two
venae cavae); however, the sinus venosus persists in the adult. In the adult, it is incorporated into the wall of the
right atrium to form a smooth part called the
sinus venarum, also known as the
venarum sinus, which is separated from the rest of the atrium by a ridge of fibres called the
crista terminalis. The sinus venosus also forms the
SA node and the
coronary sinus.