The right atrium (in older texts termed the "right auricle", which now means the right atrial appendage) is one of four chambers (two atria and two ventricles) in the hearts of mammals (including humans) and archosaurs (which include birds and crocodilians). It receives deoxygenated blood from the superior and inferior vena cava and the coronary sinus, and pumps it into the right ventricle through the tricuspid valve. Attached to the right atrium is the right auricular appendix. Atria facilitate circulation primarily by allowing uninterrupted venous flow to the heart, preventing the inertia of interrupted venous flow that would otherwise occur at each ventricular systole. The Right Atrium is composed of a smooth intercaval part and a muscular part (formed by pectinate muscles) separated by an internal ridge called the crista terminalis and an external groove (sulcus terminalis). The muscular part is derived from the primitive atrium while the smooth part is formed from the embryological sinus venosus.