84 A MANUAL OF PHYSIOLOGY 



cases in which the right ventricle is in contact with the chest- wall at 

 the position of the apex-beat the cardiogram is ' inverted 'that is 

 to say, the chest-wall is drawn in during systole and protruded during 

 diastole of the ventricles. Inversion of the cardiogram is, therefore, 

 not an infallible sign of the pathological condition known as adherent 

 pericardium (Mackenzie). 



d 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 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 



FIG. 24. CURVES OF ENDOCARDIAC PRESSURE TAKEN WITH CARDIAC SOUNDS. 



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

 including relaxation ; VS, of ventricular systole, including relaxation ; D, pause. 



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. 101) is unsuitable, since, owing to the relatively great 

 amount of work required to produce a given displacement of the 

 mercury, it does not readily follow rapid changes of pressure, and the 

 mercurial column, once displaced, continues for a time to execute 

 vibrations of its own, which are compounded with the true oscilla- 

 tions of blood-pressure. But by introducing in the connection 

 between the manometer and the heart a valve so arranged as to 

 oppose the passage of blood towards the heart, while it favours its 



