896 PHYSIOLOGY 



similarly on the right side of the heart. As soon as this happens the valves 

 open and the outflow of blood commences, and continues so long as the 

 pressure in the ventricles is higher than that in the great arteries. Directly 

 however, the ventricular pressure falls below the arterial pressure the valves 

 must close and the output of blood come to an end. 



In order to obtain an accurate idea of the exact duration of each of these 

 events in the cardiac cycle it is necessary to study the changes occurring in the 

 pressure within the auricles and ventricles during the various phases of the 

 heart-beat. 



THE ENDOCARDIAC PRESSURE 



A manometer which shall register accurately the changes in the pressure 

 within the heart must be capable of responding to very rapid changes. Thus 

 in the left ventricle at the beginning of the systole there may be a rise of 

 130 mm. Hg in -06 sec., i.e. 2170 mm. Hg per second. In a heart beating 

 rapidly and forcibly under the action of adrenalin, the rise may be still more 



FIG. 394. Diagram of Marey's cardiac ' sound,' consisting of a long tube ab, 

 terminating at one end in the ampulla m, which is covered with an elastic 

 membrane. The side-piece c serves to indicate the position of the ampulla 

 after it has been introduced into the vessels. 



rapid, e.g. 150 mm. Hg in -025 sec. A mercurial manometer with its great 

 inertia would be quite unequal to registering such rapid changes of pressure 

 and would moreover tend to enter into oscillations which would quite deform 

 the curve. We require an instrument with very small weight of moving 

 parts, so as to possess small inertia and be capable of registering a rapid rise 

 of pressure without entering into oscillations of its own. 



Several methods have been adopted for this purpose. In one (Chauveau and Marey) 

 a cardiac ' sound ' (Fig. 394) is passed down the jugular vein into the right auricle or 

 ventricle, or down the carotid artery into the left ventricle. The cardiac sound is a 



FIG. 395. Marey's tambour. 



a, axis of lever ; 6, metal tray covered with rubber membrane, and communi- 

 cating by tube / with free end of cardiac sound. 



stiff tube having an elastic bulb or ampulla at the end which is to be inserted into the 

 heart. The bulb is supported by a steel frame, so that it is not completely compressible 

 by external pressure. The free end of the tube is connected with a writing tambour 

 (Fig. 395), a small round metal tray covered with a delicate elastic membrane. To 

 the top of the membrane a lever is attached by which any change of pressure on the 



