414 AN AMERICAN. TEXT-BOOK OF PHYSIOLOGY. 



recurring pair of events which may truly be spoken of as an " auricular 

 cycle ;" and so also it is exact to say that the ventricles have their cycle, con- 

 sisting of systole and succeeding diastole. As soon, however, as we strive for 

 clearness, we find that the useful phrase " cardiac cycle " is necessarily arbi- 

 trary and imperfect. A perusal of the account given on p. 370 of the " Mode 

 of Working of the Pumping Mechanism " shows at once that each auricular 

 cycle, consisting of systole followed by diastole, must begin shortly before 

 the corresponding ventricular cycle begins, and must end shortly before the 

 corresponding ventricular cycle ends. The pumping mechanism is such that 

 the auricular systole is completed just before the ventricular systole begins. 

 The phrase " cardiac cycle " implies a reference to both auricular and ven- 

 tricular events ; if now we assume that the beginning of the auricular sys- 

 tole marks the beginning of the cardiac cycle, this must end either with the 

 end of the auricular diastole or with the end of the ventricular diastole. In 

 the former case the cardiac cycle would coincide with the auricular cycle, but 

 would begin before the end of one ventricular diastole and would end before 

 the end of another, thus containing no one complete ventricular diastole. In 

 the second case, the cardiac cycle would contain one complete ventricular dias- 

 tole and a fraction of another, and would also contain two auricular sys- 

 toles. The second case is clearly even more objectionable than the first. The 

 cardiac cycle had best be defined as consisting of all the events both auricular 

 and ventricular which occur during one complete auricular cycle. The above 

 discussion deals with a phrase which is a constant stumbling-block to stu- 

 dents ; and the question may well be asked, Why should the expression 

 "cardiac cycle" not be abolished? The answer is, that this phrase is indis- 

 pensable in order to accentuate certain important relations of the auricular 

 cycle to the ventricular. During a heart-beat there is a period when the 

 auricles and ventricles are in diastole at the same time. During this period, 

 as M r e have seen, blood is passing from the veins directly through the auricles 

 into the ventricles, and all the muscular fibres of the heart are resting. This 

 period is therefore called that of "the repose of the whole heart," or the 

 " pause." Whenever the heart is not wholly at rest, either auricles or ven- 

 tricles must be in systole. We see, therefore, that each cardiac cycle must 

 coincide with an auricular systole, the instantly succeeding ventricular systole, 

 and a period of repose of the whole heart ; and it is precisely these two 

 systoles and the succeeding universal rest which most engage the attention 

 when the beating heart is looked at in the opened chest. These three 

 phenomena, it will be noted, exactly coincide with one complete auricular 

 cycle, and so do not confuse the definition of the cardiac cycle which has been 

 given already. We see, therefore, that the phrase which seemed at first so 

 misleading has a real value, and will cease to confuse if its limitations be care- 

 fully noted. 



The Brevity and Variability of Each Cycle. From the frequency with 

 which the cycles recur, it follows at once that each one, with its complex 

 changes in the walls, chambers, and valves, is very rapidly performed. If, for 



