The heart is divided by a muscular septum (separation wall) into two lateral halves, which are named respectively the right or venous side and the left or arterial side. The septum is called the longitudinal septum. Each side of the heart is further sub-divided into an upper and lower compartment, the upper on each side is called the auricle and the lower the ventricle. The upper and lower compartments of the heart (auricles and ventricles) are separated by the auricular-ventricular septums (meaning a separation between the auricle and ventricle). The superior and inferior venae cavae empty into the right auricle of the heart, also the blood from the coronary sinus. In fact, this compartment receives all the venous or impure blood from all parts of the body, and sends it through what is known as the tricuspid valve into the right ventricle or lower compartment. After getting into the right ventricle, the blood is sent forth into the lungs by first passing through the pulmonary semi-lunar valve into the pulmonary artery, which enters the lungs at the root of the same. This would then finish the circulation through the right side of the heart, and after the purification has been accomplished by the lungs, we find the blood being returned to the left side of the heart through the four pulmonary veins. The pulmonary veins extend from the lungs (two on each side) to the left auricle (upper compartment of the heart) and deliver the purified blood to the left or arterial side. The course of the blood from the left auricle is downward into the left ventricle (or lower compartment) through what is known as the bicuspid or mitral valve. The blood is then sent out into the body to nourish all the tissues, by being forced through the aortic semi-lunar valve into the great aorta artery. The circulation is then completed by the blood running into the branch arteries and from them into the smaller branches and into the capillaries from which the course of the blood is into the smaller veins and into the larger veins, finally terminating into the two large trunk veins, the ascending (or inferior) and descending (or superior) venae cavae. Of these two large trunk veins the ascending vena cava is the only one to have a valve at its termination (eustachian). The functions of this valve are to prevent a backward flow of blood into the vein from the auricle. The heart has three walls, the inner wall is called the endocardium, the middle wall is called the myocardium, and the outer wall is called the epicardium. The heart is surrounded by a serous sac called the pericardium. The heart receives its blood supply from the coronary arteries, which are branches of the ascending aorta, just after it leaves the aortic semi-lunar valve. The coronary veins bring the venous blood back from the tissues of the heart and empty into the coronary sinus, back of the right auricle of the heart.