The =coronary sinus= (Sinus coronarius) is a very short bulbous trunk
which receives most of the blood from the wall of the heart. It is
situated just below the termination of the posterior vena cava and is
covered in part by a thin layer of ventricular muscle-fibers. It opens
into the right atrium just below the posterior vena cava. It is formed
by the union of two tributaries. The =great cardiac= or =left coronary
vein= (V. cordis magna) begins at the left side of the apex of the
heart, ascends in the left longitudinal groove and turns backward in the
coronary groove, in which it winds around the posterior border of the
heart to the right side and joins the coronary sinus. The =middle
cardiac= or =right coronary vein= (V. cordis media) begins on the right
side of the apex, ascends in the right longitudinal groove and joins the
coronary sinus, or opens separately into the atrium just in front of the
orifice of the left vein, so that a common trunk (coronary sinus) does
not then exist.
The =small cardiac veins= (Vv. cordis minores), three to five in number,
are small vessels which return some blood from the right ventricle and
atrium; they open into the latter near the coronary groove in spaces
between the musculi pectinati.