The heart (Fig. 164) consists of four chambers. Two of these, the auricles, are receiving-chambers, and are placed at the thick, anterior end of the heart. Into the right auricle open the great veins (SVC, IVC,) which bring blood from all parts of the body except the lungs; the left auricle receives only blood from the lungs. Each auricle opens into a more posterior chamber called a ventricle, the right auricle opening into the right ventricle and the left auricle [Pg 238] into the left ventricle. The ventricles pump blood into the arteries. The blood from the right ventricle is sent into the artery (P.A.) which supplies the capillaries of the lungs; while the blood of the left ventricle is forced into the aorta (Ao.), a great artery which branches and supplies with blood all parts of the body except the lungs. The two auricles contract simultaneously, forcing their contents into the flaccid ventricles. Then both the filled ventricles contract at once, and pump blood into the great arteries, flaps of membrane between the auricles and ventricles preventing a backward flow into the auricles. Similarly the bases of the great arteries are provided with membranous pockets which readily admit the blood from the ventricles when these contract, but entirely prevent a return of blood to the ventricles. The appearance of these four sets of valves, as seen from above, is shown in Fig. 165. After the contraction of the two ventricles there is a short rest, then the auricles contract again and the whole process is repeated. Fig. 164.—Heart of Sheep, in position between the lungs. R.A., appendage of right auricle; L.A., appendage of left auricle; R.V., right ventricle; L.V., left ventricle; S.V.C., I.V.C., great veins from the system generally; P.A., artery to lungs; Ao., aorta; A´o´., branch of aorta; L., lung; Tr., windpipe, leading to lungs; 2, 3, 4, fat. [Pg 239] Fig. 165.—The orifices of the heart seen from above, the auricles and great vessels being cut away. PA, valves at base of artery to lungs; Ao, valves at base of aorta; R.A.V., orifice between right auricle and ventricle, with its valves l.v. 1, l.v. 2, and l.v. 3; L.A.V., orifice between left auricle and ventricle, with its valves m.v. 1 and m.v. 2. Fig. 166.—Diagrammatic sections of veins with valves. In the upper figure the blood is supposed to be flowing in the direction of the arrow, towards the heart; in the lower, back towards the capillaries. Fig. 167.—A vein laid open to show a pair of pouch-shaped valves.