CH. XX.] PHYSIOLOGY OF THE HEART. 231 



division of the ventricle into two beginning, but it is not complete 

 till we reach the birds. The heart reaches its fullest development 

 in mammals, and we have already described the human as an 

 example of the mammalian heart. The sinus venosus is not 

 present as a distinct chamber in the mammalian heart, but is 

 represented by that portion of the right auricle at which the 

 large veins enter. 



CHAPTER XX. 



PHYSIOLOGY OF THE HEART. 

 The Cardiac Cycle. 



THE series of changes that occur in the heart constitutes the 

 cardiac cycle. This must be distinguished from the course of the 

 circulation. The term cycle indicates that if one observes the 

 heart at any particular moment, the heart from that moment 

 onwards undergoes certain changes until it once more assumes 

 the same condition that it had at the moment when the observa- 

 nt >n commenced, when the cycle is again repeated, and so on. 

 This series of changes consists of alternate contraction and 

 relaxation. Contraction is known as systole, and relaxation as 

 diastole. 



The contraction of the two auricles takes place simultaneously, 

 and constitutes the auricular systole; this is followed by the 

 simultaneous contraction of the two ventricles, ventricular systole, 

 and that by a period during which the whole of the heart is in 

 a state of diastole ; then the cycle again commences with the 

 auricular systole. 



Taking 72 as the average number of heartbeats per minute, 

 each cycle will occupy T Vr of a minute or a little more than 0*8 of 

 a second. This may be approximately distributed in the following 

 way : 



Auricular systole about OT -f- Auricular diastole 0*7 = o - 8 

 Ventricular systole about o'3 + Ventricular diastole 0*5 = o - 8 

 Total systole about 0*4 + Joint auricular and 



ventricular diastole o - 4 = o'8 



If the speed of the heart is quickened, the time occupied by 

 each cycle is diminished, but the diminution affects chiefly the 



