within which a thin fluid is secreted that serves to reduce friction during the movements of the heart, the two inner surfaces sliding over each other with every beat.
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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
