[Illustration: FIG. 40.--Left auricle and ventricle, opened and part of their walls removed to show their cavities: 1, Right pulmonary vein cut short; 1´, cavity of left auricle; 3, 3´, thick wall of left ventricle; 4, portion of same with papillary muscle attached; 5, the other papillary muscles; 6, 6´, the segments of the mitral valve; 7, in aorta is placed over the semilunar valves; 8, pulmonary artery; 10, aorta and its branches. (Allen Thomson.)] The heart itself is a hollow conical organ composed of cardiac muscle, a combination of smooth and striated fibers found nowhere else in the body. It lies obliquely, base up, between the lungs, suspended by the great blood-vessels and with the apex directed downward, forward, and to the left, the apex beat being normally felt in the fifth intercostal space, one inch inside and two inches below the left nipple. In size it varies in different people and is generally smaller in women than in men. On the average it is five inches long, three and a half inches broad, and two inches thick. A man’s heart usually weighs about eleven ounces and that of a woman nine ounces. It never leaks except from disease and such leakage is fatal. _The Cavities._--The heart contains four cavities, two _auricles_ above and two _ventricles_ below, with a longitudinal septum between the auricle and ventricle on the right and those on the left. The posterior surface is largely made up of the left ventricle and the anterior of the right ventricle. The right auricle, which receives the blood from the general circulation, has a capacity of about two fluid ounces and is larger than the left, which receives the blood returning from the lungs, though its walls are thinner. Of the ventricles the left is the larger and its walls are about three times as thick as those of the right, for it has to send the blood all over the body. All the cavities are lined with smooth, transparent, serous membrane, the _endocardium_, which is continuous with the intima of the great vessels. [Illustration: FIG. 41.--Orifices of the heart, seen from above, both the auricles and the great vessels being removed: _PA_, Pulmonary artery and its semilunar valves; _Ao_, aorta and its valves; _RAV_, tricuspid, and _LAV_, bicuspid valves; _mv_, segments of mitral valve; _lv_, segment of tricuspid valve. (Huxley.)]