THE HEART 249 



of its edge, a small thorn-like projection, and when the 

 valves are closed these cartilaginous processes are locked 

 in the center of the vessel as shown in the diagram. 



The Heart-beat. By a heart-beat we mean a coordi- 

 nated contraction of the cardiac muscle resulting in the 

 expulsion of blood from both ventricles. The heart of a 

 man at rest may beat seventy-two times a minute. Some 

 individuals have a lower and some a higher average. 

 The resting rate may be more than doubled by violent 

 exercise. After each beat there is an interval during 

 which the heart is passive. The term heart cycle is used 

 to cover the combination of the active and the passive 

 phases, the round of events connected with a single 

 beat of the heart and including the preparation for a 

 succeeding beat. Two old words have survived in the 

 description of the heart action which are almost obsolete 

 elsewhere in physiology: the terms systole and diastole. 

 The former means contraction and the latter relaxation. 



A description of the sequence of events making up a 

 heart cycle may be begun with any phase that we choose 

 inasmuch as we have to do with recurring conditions. 

 It will be convenient to start with the period of general 

 relaxation which, as a matter of fact, lasts about half 

 the whole time. If we assume that the heart is beating 

 seventy-five times a minute we have to assign to one of its 

 cycles 0.8 second. During about 0.4 second in each 

 cycle the heart is not manifesting any active contraction. 

 No blood is being pressed out of it. On the contrary, 

 all its cavities are increasing in capacity and receiving 

 the inflow from the veins. 



While this passive condition continues, the mitral and 

 tricuspid valves hang apart and we do well to think of the 

 auricle and ventricle of either side as forming, for the 

 time, a single chamber. The incoming blood may 

 gather in the widening auricle or pass on to the ventricle. 

 If this is clearly grasped the student will be saved from 

 a common mistake: that of thinking that the blood all 

 accumulates in the auricle before any is forwarded to 



