108 THE CIRCULATION. 



the auricles and ventricles during the dilatation of the latter, 

 but recover themselves when, at the beginning of the contrac- 

 tion of the ventricles, the flow through the auriculo-ventricular 

 orifices is stopped. But these last-mentioned conditions can 

 only be accessory in the perfect state of things ; for the same 

 tilting movement of the heart ensues when its apex is cut off, 

 and when, therefore, no tension or change of form can be pro- 

 duced by the blood. 



Although what we generally recognize .as the impulse of the 

 heart is produced in the way just mentioned, the beat is not 

 so simple a shock as it may seem when only felt by the finger. 

 By means of an instrument called a cardiograph, it may be 

 shown to be compounded of three or four shocks, of which the 

 finger can only feel the greatest. 



The cardiograph is a tube, dilated at one end into a cup or 

 funnel, either open-mouthed or closed by an elastic membrane, 

 while at the other it communicates with the interior of a small 

 metal drum, one side of which is formed by an elastic mem- 

 brane, 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 imme- 

 diately over the part of the chest-wall at which the apex of 

 the heart beats ; while the lever on the drum is placed in con- 

 tact with a registering apparatus. (See description of sphyg- 

 mograph, p. 125.) When the heart beats, the shock commu- 

 nicates a series of impulses to the column of air in the now 

 closed tube, with the effect of 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 recognizes 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. Marey, 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 

 seniilunar 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 contrac- 

 tions of the ventricles. 



The heart of a healthy adult man in the middle period of 



