THE HEART 



39 



The ventricles are at this stage momentarily isolated, the auriculo- 

 ventricular valves are closed, so also are the semilunar. This is 

 the second time in the cardiac cycle that the ventricles have 

 been shown to be cut off from the other part of the heart. We 

 shall examine the question in greater detail presently. The 

 great arteries now contract and shorten, the heart rotates back- 

 wards to the left, the auriculo- ventricular valves open, the 

 auricles and ventricles, neither contracting nor dilating, assume 

 a passive condition during a period known as the pause, the blood 

 flows into the auricles, and from the auricles into the ventricles ; 

 the auricles now contract, and the whole process is repeated. 



We have thus the contraction of the auricles, the contraction 

 of the ventricles, and the pause. The time that each of these 

 occupies has not been determined with accuracy; the results 

 obtained by Chauveau and Marey on the horse show that the 

 auricular systole is brief, the ventricular systole twice as long, 

 and the pause equals in length the ventricular systole ; but the 

 time values as given by them would cause the horse to have a 

 pulse-rate of 60, which is abnormal. From 36 to 40 beats per 

 minute is the normal rate, and this gives a period of 1-5 seconds 

 for a complete cycle of the heart. 



There is a well-marked interval between the contraction of the 

 auricles and that of the ventricles, during which not only is the 

 ventricle getting up pressure, but the papillary muscles are 

 contracting to prevent the valves being pressed up further into 

 the auricles. Chauveau draws especial attention to the interval 

 in the horse which has been named the intersystolic period. It 

 must not, of course, be confused with the pause of the heart which 

 follows ventricular systole. The auricles have a longer period of 

 rest than the ventricles, but, as we shall see later, they are not 

 entirely idle between each systole. One important point may 

 here be conveniently stated, that no matter how fast the heart is 

 beating, the frequency depends, not on the duration of the 

 ventricular systole, but on the length of the subsequent pause. 



Summary of Events occurring during a Cardiac Cycle. — 

 Dividing the events into three periods, and starting with the con- 

 traction of the auricles, the following is a summary of the 

 changes occurring in the heart : 



First Period. — The contraction of the auricles completes the 

 filling of the ventricles. 



Second Period. — The auriculo-ventricular valves are closed, the 

 ventricles contract, the aortic and pulmonary valves open, blood 

 is pumped into the aorta and pulmonary artery, the impulse of 

 the heart is made against the wall of the chest, the first sound is 

 produced, the auricles fill with blood, and the whole is followed 

 by a short pause. 



