THE CIRCULATION. 283 



The immediate cause of the cardiac impulse is, without question, the 

 shock of the heart against the walls of the chest, Its character, its 

 coincidence in time with the ventricular systole, its position, and its 

 variation with the changes of attitude and respiration, all indicate its 

 dependence on the muscular action of the heart's apex. As to the 

 exact manner in which it is produced there is a difference of opinion. 

 By some a large share is attributed to the direct protrusion of the apex 

 and its lateral movement toward the right ; both of which would bring 

 it in contact with the chest with sufficient force to lift the integuments 

 at the intercostal spaces. Others regard it as due to the sudden harden- 

 ing of the ventricle at the time of systole and the slight increase in its 

 antero-posterior thickness at the same time. It is certain that the heart 

 in contraction acquires a much firmer consistency, and this undoubtedly 

 adds to the effect of the protrusion and movement of the apex, as felt 

 externally. 



Rhythm of the Heart's Action. The succession of phenomena in a 

 cardiac pulsation consists of a double series of contractions and relaxa- 

 tions ; namely, those of the auricles and those of the ventricles. The 

 two auricles contract simultaneously with each other, and afterward 

 the two ventricles ; and in each case the contraction is followed by a 

 relaxation. The auricular contraction, which is short and compara- 

 tively feeble, occupies the first part of the time of a pulsation. The 

 ventricular contraction is longer and more powerful, and occupies the 

 latter part of the same period. Then comes a short interval of repose, 

 after which the auricular contraction again recurs. The auricular and 

 ventricular contractions, however, are not completely separated from 

 each other, like the alternate strokes of two pistons in a forcing-pump, 

 but are in some measure connected and continuous. The muscular 

 action, after beginning at the auricle, is at once propagated to the ven- 

 tricle and runs rapidly toward the apex. The entire ventricle contracts 

 vigorously, its walls harden, its point protrudes, impinges against the 

 walls of the chest and twists from left to right, the auriculo-ventricular 

 valves shut back, the first sound is produced, and the blood is driven 

 into the arterial system. These phenomena occupy about one-half the 

 time of pulsation. Then the ventricle is relaxed, and a period of repose 

 ensues. During this period the blood flows from the large veins into 

 the auricle, and through the auriculo-ventricular orifice into the ventri- 

 cle ; filling the ventricle, by a kind of passive dilatation, about two- 

 thirds or three-quarters full. Then the auricle contracts with a quick 

 motion, forcing the last drop of blood into the ventricle, and distending 

 it to its full capacity ; when the ventricular contraction again takes 

 place, driving the blood into the large arteries. These movements 

 alternate with each other, and form, by their recurrence, the successive 

 cardiac pulsations. 



The successive elements in a cardiac pulsation, and the corresponding 

 variations of pressure in the auricular and ventricular cavities, are most 

 distinctly shown by means of the double cardiograph, a registering 



