THE HEART AND CIRCULATION. 103 



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 longi- 

 tudinal 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 



IVJ. 



lV3 \ 



RAV 



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, seg- 

 ments of mitral valve; Iv, segment of tricuspid valve. (Huxley.) 



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, trans- 

 parent, serous membrane, the endocardium, which is con- 

 tinuous with the intima of the great vessels. 



The Valves. The opening from the auricle into the 

 ventricle on either side is guarded on the ventral side by 

 a valve formed of folds of endocardium. The valve on 

 the right side has three flaps or cusps and is called the 



