THE CIRCULATION OF THE BLOOD AND LYMPH 81 



the cardiogram is a record of a complex series of events. The very 

 rapid variation of pressure within the ventricles, the change of shape 

 of the heart, the slight change of position of its apex, if such occurs, 

 must all leave their mark upon the curve, which is besides distorted 

 by the resistance of the elastic chest-wall, the inertia of the recording 

 lever, and the compression of the air in the connecting tubes. It 

 is only by comparing in animals the cardiographic record with the 

 changes of blood-pressure in the heart and arteries that our present 

 degree of knowledge of the human cardiogram has been attained. 



Endocardiac Pressure. The function of the heart is to 

 maintain an excess of pressure in the aorta and pulmonary 

 artery sufficient to overcome the friction of the whole 

 vascular channel, and to keep up the flow of blood. So 

 long as the semilunar valves are closed, most of the work 



FIG. 20. CURVES OF ENDOCARDIAC PRESSURE TAKEN WITH CARDIAC SOUNDS. 



Aur., auricular curve ; Vent. t ventricular curve ; AS, period of auricular systole ; 

 VS, of ventricular systole ; D, diastole. 



of the contracting ventricles is expended in raising the 

 pressure of the blood within them. At the moment when 

 blood begins to pass into the arteries, nearly all the energy 

 of this blood is potential ; it is the energy of a liquid under 

 pressure. During a cardiac cycle the pressure in the cavities 

 of the heart, or the endocardiac pressure, varies from 

 moment to moment, and its variations afford important data 

 for the study of the mechanics of the circulation. 



For the study of the endocardiac pressure, the ordinary mercurial 

 manometer (p. 99) is unsuitable, since, owing to the relatively great 

 amount of work required to produce a given displacement of the 



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