CIR CULA TION. 413 



woman the average is slightly higher, perhaps 80 to the minute. The heart 

 beats less frequently in tall people than in short ones. The difference between 

 men and women largely depends upon this, but careful observation shows that 

 in the case of men and women of the same stature the heart-beats are slightly 

 more frequent in the women. There is, therefore, a real difference as to the 

 pulse between the sexes. Shortly before and after birth the heart-beats are 

 very frequent, from 120 to 140 to the minute. During childhood and youth, 

 the frequency diminishes gradually, the average falling below 100 to the min- 

 ute at about the sixth year, and below 80 to the minute at about the eighteenth 

 year. In extreme old age the pulse becomes slightly increased in frequency. 

 It must, however, be borne in mind that there are very wide differences 

 between individuals as to the average frequency of the heart-beats. Pulses 

 of 40 and even fewer strokes to the minute, or, on the other hand, of more 

 than 100 to the minute, aje natural to some healthy people. 



In every individual the frequency of the pulse varies decidedly, and may 

 vary very greatly, during each twenty-four hours. It is least during sleep, 

 and less in the lying than in the sitting posture. Standing makes the heart 

 beat opener, the difference being greater between standing and sitting than 

 between sitting and lying. During muscular exercise the pulse-rate is much 

 increased, violent exercise carrying it possibly to 150 or even more. Thermal 

 influences have a marked effect, a hot bath, for instance, heightening the fre- 

 quency of the pulse and a cold bath diminishing it. The taking of a meal 

 also commonly puts up the frequency. The influence of emotion upon the 

 heart's contractions is well known. It may act either to heighten the rate or 

 to lower it. Finally, the practising physician soon learns that the heart's 

 rate is more easily affected by comparatively slight causes, emotional or other- 

 wise, in women, and especially in children, than in men a fact of some 

 importance in diagnosis. 



The causes of the differences referred to in this section are partly unknown, 

 and partly belong to the subject of the regulation of the circulation. 



L. THE RELATIONS IN TIME OF THE MAIN EVENTS OP THE CARDIAC 



CYCLE. 



We have now considered the effects produced by the cardiac pump ; its 

 general mode of working ; and the actual frequency of its strokes. We must 

 next study certain important details relating to the individual strokes or beats 

 of the ventricles and of the auricles. For this study the basis has already 

 been laid in the sections headed " Causes of the Blood-flow " (p. 369), " Mode 

 of Working of the Pumping Mechanism " (p. 370), " The Cardiac Cycle " 

 (p. 396), and "Use and Importance of the Valves" (p. 400). These sections 

 should now be read again in the order just given. Details can best be dealt 

 with if we use, instead of the more familiar word " beat," the more technical 

 one " cycle." 



The Auricular Cycle ; the Ventricular Cycle ; the Cardiac Cycle. 

 Each systole and succeeding diastole of the auricles constitute a regularly 



