As mentioned earlier, the right sinus horn enlarges considerably and receives venous blood through the superior and inferior vena cava. It opens into the posterior wall of the developing right atrium through the sinuatrial orifice. This orifice is a vertically oriented slit-like opening which is flanked on each side by a tissue flap;
lar tissue flaps are called right and left venous valves. Superior to the sinuatrial orifice the right and left venous valves join to form a ridge, which is called septum spurium.
Between the 6th and 8th week the right atrium increases rapidly in size and the right sinus horn, failing to keep pace, is gradually incorporated into the wall of the right atrium to from the smooth-walled part of the right atrium (this part of the right atrium is called sinus venarum, indicating that it is derived from sinus venosus). The sinus venarum forms the right posterior part of the atrium. Rest of the right atrium and the right auricle have a rough trabeculated appearance.
As the right sinus horn is taken
up into the right atrium, the right and left venous valves move away from each other. The left venous valve (along with the septum spurium) fuses with the septum secundum and is incorporated into the interatrial septum. The cranial part of the right venous valve disappears; however, the caudal part 'of this valve forms valve of the inferior vena cava and valve of the coronary sinus. Superior to the valve of the inferior vena cava a vertical ridge of tissue, called crista terminalis, delimits the sinus venarum from the trabeculated part of the right atrium (some authorities believe that the crista terminalis represents the cranial part of the right venous valve). i