180 THE CIRCULATION OF THE BLOOD 



filled with the blood flowing into them from the veins, and a portion of this 

 blood passes at once through the auricles into the ventricles, the opening 

 between the cavity of each auricle and that of its corresponding ventricle 

 being free during the pause of the entire heart. The auricles, however, 

 receive more blood than at once passes through them to the ventricles. 

 Near the end of the pause they become passively distended. At this 

 moment a contraction wave begins at the bases of the venae cavae and, run- 

 ning down over the walls about the mouths of the veins, passes to the 

 muscular walls. The contraction of the auricles, the right and left 

 contracting at the same time, forces the blood into the ventricles. 



The contraction of the muscular walls at the mouths of the great veins 

 and of the sinus region maintains a condition of constriction during the 

 time of the auricular contraction. This hinders the reflux of blood from 

 the auricles into the veins during the auricular systole. Any slight re- 

 gurgitation from the right auricle is limited by the valves at the junction of 

 the subclavian and internal jugular veins beyond which the blood cannot 

 move backward, and by the coronary vein which is supplied with valve-like 

 fold at its mouth. The force of the blood propelled by the auricle into the 

 ventricle at each auricular systole is transmitted in all directions, but, 

 being insufficient to open the semilunar valves, it is expended in distend- 

 ing the walls of the ventricle. 



Action of the Ventricles. The dilatation of the ventricles which 

 occurs during the latter part of the diastole of the auricles, is completed by 

 the forcible injection of the contents of the latter. The ventricles, now 

 distended with blood, immediately begin to contract. The tricuspid and 

 mitral valves are closed by the initial reflux of blood, or possibly by the 

 currents of blood formed by the sudden injection of the ventricles by the 

 auricular contractions. The ventricular systole follows the auricular 

 systole so closely that it seems continuous with it. As a result of the ventri- 

 cular systole, sufficient pressure is produced on its contents to overcome the 

 pressure against the semilunar valves of the aorta and the pulmonary 

 artery, and the ventricles are then emptied completely. After the whole 

 of the blood has been expelled from the ventricles, the walls remain con- 

 tracted for a brief period. 



The form and position of the fleshy columns on the internal walls of the 

 ventricles no doubt help to produce the obliteration of the ventricular 

 cavities during contraction. The completeness of the closure may often 

 be observed on making a transverse section of a heart shortly after death 

 in any case in which rigor mortis is very marked, figure 136. In such a case 

 only a central fissure may be discernible to the eye in the place of the 

 cavity of each ventricle. The arrangement of the muscles of the heart, 

 as described on page 171, is such as to expend the whole force of the 

 contraction in diminishing the cavity of the ventricle, or, in other words, 

 in expelling the contents of blood. 



