. Kirkes' handbook of physiology . amen between the 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 theauricle by the bicuspid or mitral valves, and the opening into the great aortais guarded by the semilunar valves. The walls of the left ventricle are twoor three times as heavy as those of the right, and may be as much as half aninch in total thickness. The left ventricle is capable of containing 90 to 120 c. c. of blood. Thecapacity of the auricles is co
. Kirkes' handbook of physiology . amen between the 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 theauricle by the bicuspid or mitral valves, and the opening into the great aortais guarded by the semilunar valves. The walls of the left ventricle are twoor three times as heavy as those of the right, and may be as much as half aninch in total thickness. The left ventricle is capable of containing 90 to 120 c. c. of blood. Thecapacity 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 itsgreatest width, and 6 cm. in thickness. The average weight in the adult isabout 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 muscularfibers 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 Lowerand Middle Thirds. (According to Krehl.) the boundary of the auricnlo-ventricidar opening. Fibrous tissue also existsat the origins of the pulmonary artery and aorta. The muscular fibers ofeach 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 connectionbetween them being the fibrous and the embryonic muscular tissue of theauriculo-ventricular rings and the bundle of His in the septum. The development of the heart shows that it is derived from an embryonictube, which in its growth becomes twisted upon itself and divided into the THE HEART 147 two main divisions that we know in the adult. Anatomical dissections haveshown that the muscles of the ventricles form spiral sheaths extending from
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