146 THE CIRCULATION OF THE BLOOD 



The left ventricle occupies the posterior and apical portion of the heart, 

 and is connected directly with the great aorta. It is separated from the 

 auricle by the bicuspid or mitral valves, and the opening into the great aorta 

 is guarded by the semilunar valves. The walls of the left ventricle are two 

 or three times as heavy as those of the right, and may be as much as half an 

 inch in total thickness. 



The left ventricle is capable of containing 90 to 120 c. c. of blood. The 

 capacity of the auricles is considerably less after death owing to their con- 

 tracted condition. The whole heart is about 12 cm. long by 8 cm. at its 

 greatest width, and 6 cm. in thickness. The average weight in the adult is 

 about 300 grams. 



The walls of the heart are constructed almost entirely of layers of muscu- 

 lar fibers; but a ring of connective tissue, to which some of the muscular 

 fibers are attached, is inserted between each auricle and ventricle and forms 



FIG. 137. Cross-section of a Completely Contracted Human Heart, at the Level of the Lower 

 and Middle Thirds. (According to Krehl.) 



the boundary of the auricula-ventricular opening. Fibrous tissue also exists 

 at the origins of the pulmonary artery and aorta. The muscular fibers of 

 each auricle are in part continuous with those of the other, and in part separate; 

 and the same holds true for the ventricles. The fibers of the auricles are, 

 however, quite separate from those of the ventricles, the bond of connection 

 between them being the fibrous and the embryonic muscular tissue of the 

 auriculo-ventricular rings and the bundle of His in the septum. 



The development of the heart shows that it is derived from an embryonic 

 tube, which in its growth becomes twisted upon itself and divided into the 



