ACTION OF THE VENTRICLES 175 



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

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

 receiving more blood than at once passes through them to the ventricles, 

 become, near the end of the pause, passively distended. At this moment a 

 contraction wave begins on the bases of the venae cavae and, running down 

 from the walls of the veins, passes to the muscular walls of the auricles. 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 of the great veins maintains a 

 condition of constriction of these veins during the time of the auricular con- 

 traction. This hinders the reflux of blood from the auricles into the veins 

 during the auricular systole. Any slight regurgitation 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 a valve at its mouth. The force of the blood 

 propelled by the auricle into the ventricle at each auricular systole is trans- 

 mitted in all directions, but, being insufficient to open the semilunar valve, 

 it is expended in distending 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 dis- 

 tended with blood, immediately begin to contract. The tricuspid valve 

 is 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 contraction. 

 The ventricular systole follows the auricular systole so closely that it seems 

 continuous with it. As a result of the ventricular 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 thus 

 emptied completely. After the whole of the blood has been expelled from 

 the ventricles, the walls remain contracted 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 cavity 

 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 147, is 

 such as to expend the whole force of the contraction in diminishing the cavity 

 of the ventricle, or, in other words, in expelling its contents. 



On the conclusion of the systole the ventricular diastole begins. The 

 muscular walls relax and, by virtue of their elasticity, a slight negative press- 

 ure is set up. This negative or suctional pressure on the left side of the 

 heart may be of importance in helping the pulmonary circulation. It is 



