252. The blood, being necessary to nourish the tissues and to stimulate the organs, must be in motion in order to be borne to them. An apparatus is provided partly for the purpose of originating an impelling force to put the blood in motion, and partly for the purpose of conveying the blood when in motion to the different parts of the body. 253. The heart is the impelling organ; the great vessels in immediate connexion with it are the transmitting organs (fig. CXIV. 1, 2). The heart is divided into two sets of chambers (fig. CXIV. 3, 4, 10, 11), one for the reception of the blood from the different parts of the body (fig. CXIV. 3, 10); the other for the communication of the impulse which keeps the blood in motion (fig. CXIV. 4, 11). The chamber which receives the blood is termed an auricle (fig. CXIV. 3, 10), and is connected with a vessel termed a vein (fig. CXIV. 1, 2, 9); that which communicates impulse to the blood is termed a ventricle (fig. CXIV. 4, 11), and is connected with a vessel termed an artery (fig. CXIV. 7, 12). The vein carries blood to the auricle; the auricle transmits it to the ventricle; the ventricle propels it into the artery; the artery, carrying it out from the ventricle, ultimately sends it again into the vein, the vein returns it to the auricle, the auricle to the ventricle, the ventricle to the artery, and thus the blood is constantly moving in a circle; hence the name of the process, the circulation of the blood. [Illustration: Fig. CXIV. View of the heart with its several chambers exposed, and the great vessels in connection with them. 1. The superior vena cava; 2. the inferior vena cava; 3. the chamber called the right auricle; 4. the chamber called the right ventricle; 5. the line marking the passage between the two chambers, and the points of attachment of one margin of the valve; 6. the septum between the two ventricles; 7. the pulmonary artery arising from the right ventricle, and dividing at 8, into right and left for the corresponding lungs; 9. the four pulmonary veins bringing the blood from the lungs into 10, the left auricle; 11. the left ventricle; 12. the aorta arising from the left ventricle, and passing down behind the heart to distribute blood, by its divisions and subdivisions, to every part of the body.]