124 THE CIRCULATION. 



formed by an elastic membrane, on which rests a finely- 

 balanced lever, like that of the sphygmograph (fig. 42). 



When used, the cup at one end of the tube is placed 

 immediately over the part of the chest- wall at which the 

 apex of the heart beats ; while the lever on the drum is 

 placed in contact with a registering apparatus. (See de- 

 scription of sphygmograph, p. 147.) When the heart 

 beats, the shock communicates a series of impulses to the 

 column of air in the now closed tube, with the effect oi 

 raising the elastic wall of the drum, and of course the 

 lever which is attached to it. A tracing of the heart's 

 impulse is thus obtained in the same way as that of the 

 pulse, in the arteries (figs. 44 and 45). 



The tracing shows that besides the strong beat which 

 alone the finger recognises as the impulse of the heart, 

 and which is caused by the contraction of the ventricles, 

 there are other minor shocks which are imperceptible to 

 the touch. The latter, M. Mare} 7 , by experiments on the 

 lower animals, has proved to be the results, respectively, 

 of the contraction of the auricles, and of the closure of the 

 auriculo-ventricular and semilunar valves. 



Frequency and Force of the Heart's Action. 



The frequency with which the heart performs the actions 

 we have described, may be counted by the pulses at the 

 wrist, or in any other artery ; for these correspond with 

 the contractions of the ventricles. 



The heart of a healthy adult man in the middle period 

 of life, acts from seventy to seventy-five times in a minute. 

 The frequency of the heart's action gradually diminishes 

 from the commencement to near the end of life, but is said 

 to rise again somewhat in extreme old age, thus : 



