Fig. 17—*Diagram of the circulation*, showing in general the work done by each part of the heart. The right ventricle forces the blood through the lungs and into the left auricle. The left ventricle forces blood through all parts of the body and back to the auricle. The auricles force blood into the ventricles. *Connection with Arteries and Veins.*—Though the heart is in communication with all parts of the circulatory system, it makes actual connection with only a few of the blood tubes. These enter the heart at its upper portion (Fig. 15), but connect with its different cavities as follows: 1. _With the right auricle_, the superior and the inferior venæ cavæ and the coronary veins. The superior vena cava receives blood from the head and the upper extremities; the inferior vena cava, from the trunk and the lower extremities; and the coronary veins, from the heart itself. 2. _With the left auricle_, the four pulmonary veins. These receive blood from the lungs and empty it into the left auricle. 3. _With the right ventricle_, the pulmonary artery. This receives blood from the heart and by its branches distributes it to all parts of the lungs. 4. _With the left ventricle_, the aorta. The aorta receives blood from the heart and through its branches delivers it to all parts of the body. *How the Heart does its Work.*—The heart is a muscular pump(18) and does its work through the contracting and relaxing of its walls. During contraction the cavities are closed and the blood is forced out of them. During relaxation the cavities open and are refilled. The valves direct the flow of the blood, being so arranged as to keep it moving always in the same direction (Fig. 17). The heart, however, is not a single or a simple pump, but consists in reality of _four_ pumps which correspond to its different cavities. These connect with each other and with the blood vessels over the body in such a manner that each aids in the general movement of the blood. [Fig. 18] Fig. 18—Diagram illustrating the "cardiac cycle."