TH-3 HP ART. .AND CIRCULATION. 



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. 



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, 



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.) 



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 peo- 

 ple and is generally smaller in women than in men. On 

 the average it is five inches long, three and a half 



