CHAP, iv.] THE VASCULAR MECHANISM. 159 



Variations in the Heart's beat. 



These are for the most part in reality vital phenomena, i.e. 

 brought about by events depending on changes in the vital 

 properties of some or other of the tissues of the body. It will 

 be convenient, however, briefly to review them here, though the 

 discussion of their causation must be deferred to its appropriate 

 place. 



The frequency of the heart, i.e. the number of beats in any 

 given time, may vary. The average rate of the human pulse or 

 heart-beat is 72 a minute. It is quicker in children than in adults, 

 but quickens again a little in advanced age. It is quicker in the 

 adult female than in the adult male, in persons of short stature 

 than in tall people. It is increased by exertion, and thus is 

 quicker in a standing than in a sitting, and in a sitting than 

 in a lying posture. It is quickened by meals, and while varying 

 thus from time to time during the day, is on the whole quicker 

 in the evening than in early morning. It is said to be on the 

 whole quicker in summer than in winter. Even independently of 

 muscular exertion it seems to be quickened by great altitudes. It 

 is profoundly influenced by mental conditions. 



The length of the systole may vary, indeed we have reason 

 to think that it does vary considerably, though as a general and 

 broad rule it may be stated that a frequent differs from an 

 infrequent pulse chiefly by the length of the diastole. Donders 

 found the length of the systole as measured by the interval 

 between the first and second sounds to be for ordinary pulses 

 remarkably constant in different persons, varying not more than 

 from '327 to '301 sec., and being therefore relatively to the whole 

 cardiac period less in slow than in quick pulses. 



The force of the beat may vary ; the ventricular systole may be 

 weak or strong. When the rate of beat is suddenly increased 

 there is a tendency for the individual beats to be diminished in 

 force, and on the other hand to be increased in force when the 

 rate is diminished. But there is no necessary connection between 

 rate and strength ; both a frequent and an infrequent pulse may 

 be either weak or strong. 



The character of the beat may vary; the systole may be sudden 

 and sharp, rapidly reaching a maximum and rapidly declining, or 

 slow and lengthened, reaching its maximum only after some time 

 and declining very gradually; the latter being the slow pulse 

 (pulsus tardus) as distinguished from the infrequent pulse (pulsus 

 rams). The pulse is also sometimes spoken of as being slapping, 

 and sometimes as heaving. But, as we shall see immediately, the 

 features of the pulse are dependent not only on the heart beat but 

 also on the condition of the arteries. 



