290 TEXT-BOOK OF PHYSIOLOGY 



ing with the incoming of a new volume of blood, in consequence of which 

 the heart is pressing against the chest walls. The systolic plateau is charac- 

 terized by one or more elevations and depressions, the true cause of which 

 is unknown. 



From the correspondence of the curve of cardiac pressure against the 

 chest wall with the curve of intra-ventricular pressure it becomes possible 

 to indicate with approximate accuracy the time of the opening and closing 

 of the auriculo-ventricular valves and the semilunar valves and hence 

 the time of occurrence of the heart sounds and other features of the 

 cardiac cycle. Such a construction is shown in Fig. 128. 



The Pulse Volume. The pulse volume or the systolic output or the 

 amount of blood discharged by the ventricle at each systole has long been a 

 subject of investigation, but by reason of the inherent difficulties of the 

 problem the results that have been obtained have varied within wide limits, 

 viz.: from 180 c.c. to 50 c.c. The methods that have been employed for 

 the determination of this volume are complicated and need not be detailed 

 here. Suffice it to say that the results of the more recent experiments 

 would indicate that the volume varies from 80 c.c. to 100 c.c. If the pulse 

 volume be assumed to weigh 100 grams and the total volume of blood in a 

 man weighing 70 kilograms to weigh 3684 grams then the pulse volume will 

 be about one-thirty-eighth of the total weight of blood, or about 0.00142 of 

 the body weight. In 38 heart beats therefore the entire amount of blood 

 will have passed through the heart. The systolic output is conditioned by 

 the factors which increase or decrease the length of the diastole and hence 

 the filling of the ventricle. 



The Frequency of the Heart-beat. The frequency of the heart-beat 

 varies with a variety of conditions: e.g., age, sex, posture, exercise, etc. 



Age. The most important normal condition which modifies the activity 

 of the heart is age. Thus: 



Before birth, the number of beats a minute averages 140 



During the first year it diminishes to 128 



During the third year it diminishes to 95 



From the eighth to the fourteenth year it averages 84 



In adult males it averages 72 



Sex. The heart-beat is more rapid in females than in males. Thus 

 while the average beat in males is 72, in females it is usually 8 or 10 beats 

 more. 



Posture. Independent of muscle efforts the rate of the beat is influenced 

 by posture. It has been found that when the body is changed from the lying 

 to the sitting and to the standing position, the beat will vary as follows 

 from 66 to 71 to 8 1 on the average. 



Exercise and digestion also temporarily increase the number of beats. 



A rise in blood-pressure from any cause whatever is usually attended by 

 a decrease, while a fall in blood-pressure is attended by an increase in the 

 rate. 



The Electric Currents of the Heart. The wave of contraction of the 

 heart muscle, like the wave of contraction of the skeletal muscle (see page 

 79) is accompanied or perhaps preceded by the development of electric 

 currents the existence of which can be demonstrated by connecting the base 

 and apex, by means of non-polarizable electrodes, with a suitable galvan- 



