THE CIRCULATION OF THE BLOOD 



281 



though prolonged, is not of the nature of a tetanus and therefore not charac- 

 terized by rapid variations of tension. The apex element may be eliminated 

 by placing the individual in the recumbent position. 



The second sound is the product of the simultaneous closure and subse- 

 quent vibration of the aortic and pulmonic valves which occur at the 

 beginning of the ventricular diastole as the blood surges back against the 

 closed valves. This has been definitely proved by the fact that the sound 

 disappears when the valves are destroyed or held back by hooks introduced 

 into the aorta and pulmonary artery. It is also possible that the vibration 

 of the column of blood produces an additional tone which adds itself to that 

 produced by the valves. 



The foregoing events are shown in their order in the following diagram. 



/ Sound -?-* Sound 



FIG. 120. DIAGRAM SHOWING THE EVENTS IN THE CARDIAC CYCLE. 



Relative Functions of Auricles and Ventricles. Though both 

 auricles and ventricles are essential to the continuous movement of blood, 

 they possess unequal values in this respect. The passage of the blood 

 through the pulmonic and systemic vessels is accomplished by the driving 

 power of the right and left ventricles respectively, aided, however, by minor 

 extra-cardiac forces. They may be regarded therefore as force-pumps. 



If the heart consisted of ventricles only, the flow of blood from the venae 

 cavae and pulmonic veins would be temporarily arrested during their systole 

 and their subsequent refilling delayed. This is obviated, however, by the 

 addition of the auricles; for during the ventricular systole the blood continues 

 to flow into the auricles, in which it is temporarily stored until the ventricular 

 relaxation sets in. With this event the accumulated blood passes into the 

 ventricles, which are thus practically filled before the auricular systole occurs 

 by which the filling is completed. By this means there is no delay in the 

 filling of the ventricles, and hence their effective working as force-pumps is 

 more readily secured. The auricles may therefore be regarded as feed- 

 pumps. For this reason it is probable, notwithstanding the contraction of 

 the circular muscle-fibers at the terminations of the venous system, that the 

 flow of blood into the auricles is never entirely arrested, but merely retarded. 

 Regurgitation in these vessels does not occur for the reason that the re- 

 tardation develops a side pressure during the auricular contraction which 

 is equal to if it does not exceed the pressure in the auricle. 



Synchronism of the Two Sides of the Heart. If the balance of the 

 circulation is to be maintained, the two sides of the heart must act synchron- 

 ously. That they do so can be shown by attaching levers to their walls, and 

 thus recording their activities. The synchronism is so perfect that until 

 recently it was generally believed to be dependent on nerve connections; but 

 Porter has shown that if the ventricles are cut away from the auricles, in 

 which the nerve mechanism seems to lie, the synchronism of the former is 

 not interfered with; that the apical halves of the ventricles will beat syn- 

 chronously if perfused with blood through an artery; that a very small bridge 



