36 THE MECHANISM OF THE CIRCULATION. 



the tambour, and the instrument, therefore, yields the same form of curve 

 as does a tambour placed between the heart and the chest wall in an animal. 



The curve of the impulse, or cardiogram, taken when the heart is 

 beating normally, is entirely different from a cardiogram taken when 

 the vessels are ligatured and the heart bloodless. 1 



The cardiogram is not, therefore, a true muscle curve, but is a 

 combined curve of pressure and volume. The form of the curve is, 

 moreover, endlessly varied by the application of the receiving tambour 

 to different parts of the heart, and by alteration of the pressure 

 with which the instrument is applied. Thus the instrument is not 

 by itself a perfectly sure or trustworthy guide, and the main physio- 

 logical interest which it possesses is that, by its means, certain time 

 relations in the cardiac cycle can be decided in man. By simultaneous 

 record of the impulse curve, the time of the sounds of the heart, and 

 the carotid or radial pulse, interesting results can be arrived at, such, 

 for example, as the period of rising intraventricular tension. 



The occurrence of the sounds of the heart can be marked below the 

 impulse curve by means of an electric signal worked by the hand of 

 the observer who is charged with the duty of auscultating the heart. 

 The reaction time of a practised experimenter for the making of 

 signals in answer to sounds is known to be from 015 to 0'20 seconds. 

 With practice the reaction time for regular periodic excitations is 



FIG. 24. Impulse curve and sounds of the heart. Edgren, 



found to become very constant, and the error due to variations in 

 this may only be O'Ol sec. As the constancy of the reaction time 

 depends on the regular periodicity of the excitation, this method of 

 marking the time of the heart sounds can only be employed on healthy 

 hearts beating with a regular rhythm. In such cases the constancy of 

 the period of each pulse varies by no more than 0*03 to 0'04 sec. The 

 total error of observation is therefore equal to about 0*04 sec. With 

 the use of Edgren's apparatus the observer can discard the method of 

 signalling, and by watching the tracing of the impulse can see, at the 

 same time as he listens to the heart, the points on the curve at 

 which the sounds are heard. Since the heart sounds have been 

 successfully recorded by the combined use of the microphone and the 

 capillary electrometer, it will be possible in the future to improve on 

 the above method, and simultaneously to obtain curves of the impulse 

 and the sounds of the heart. 



Marey, 2 Edgren, 3 and others find that the second sound of the 

 heart corresponds in time with a small nick or elevation at the 



1 v. Frey, "Untersuch. des Pulses," S. 112. 



2 "La circulation du sang," Paris, 1881, p. 127. Cf. Fredericq, " Trav. du labor, 

 de Liege," 1888, tome ii. p. 84. 



3 Skandin. Arch. f. Physiol., Leipzig, 1889-91, Bd. i. S. 88. 



