ABSTRACT
INCORPORATIO N OF SINUS VENOSUS IN TO R A TR IU M The dorsal opening of the sinus into the R atrium is bounded by R and L venous valves (Figure 2). As atrial septation advances, the sinus is splayed out, forming the venous part of the R atrium. The L venous valve disappears. The R one survives as the incomplete valves of the superior and inferior venae cavae and coronary sinus (Figure 2d). Venous blood from the head and heart enters the R atrium and exits via the tricuspid orifice into the R ventricle. The two blood currents in the fetal R atrium, one venous and the other arterial, are deflect ed by the incomplete valves of the great veins entering the R atrium (Figure 2d), and hardly mix at all. The smooth venous part of the R atrium is bounded on the right by the crista terminalis, another survival of the R venous valve. The musculi pectinati and R auricle rep resent the right half of the primordial atrium.
