958 PHYSIOLOGY 



connected with some form of piston recorder or a tambour with a slack membrane. 

 The disadvantage of this method is that the graphic record of rapid and ample changes 

 in volume is one of the most difficult problems in experimental physiology, the inertia 

 and friction of the moving piston tending to deform the shape of the curve obtained. 

 Straub has therefore used a soap bubble as the volume measurer, photographing its 

 edge and using the record as an index to the change in volume. It is possible how- 

 ever to obtain a piston recorder moving sufficiently freely to give a fairly correct 

 reproduction of the volume changes of the heart, provided that these do not occur with 

 too great rapidity. It has been suggested by Piper to convert the volume changes 

 into small pressure changes, and to record these latter by one of the methods described 

 above. 



The factors which determine the output of the left ventricle are best 

 studied in the heart-lung preparation. In this it can be shown that, pro- 

 vided the venous inflow remains constant, the output is also constant and 

 is unaffected by considerable alterations of arterial resistance and of the 



FIG. 417. Henderson's glass cardiometer. 



rate of the heart. Thus with a moderate venous inflow the output remains 

 constant whether we maintain the average arterial pressure at 60 mm. Hg. 

 or at 160 mm. Hg. It is also unaffected by altering the rate of the heart 

 from 80 beats per minute up to 160, or even 200, beats per minute. On 

 the other hand, the output is at once altered by alterations in the venous 

 inflow end, as already stated, can be altered in a heart weighing 50 gms. 

 from a few c.c. up to 3000 c.c. per minute. The only essential in this 

 preparation is that the output from the left ventricle shall be sufficient 

 to maintain a circulation through the coronary vessels and so keep the 

 active muscle properly supplied with blood. 



With increasing inflow of blood into the heart the large veins, auricles, 

 and ventricles naturally become more filled during diastole, and during 

 systole of the ventricles, when the auriculo-ventricular valves are closed, 

 the blood rushing in from the venous system must accumulate in the big 

 veins and auricles to a still greater extent. The venous pressure therefore 

 rises with increased venous inflow. In so far as venous pressure is an 

 index of venous inflow, we may say that the output of the heart increases 

 with the venous pressure so lon^ as the heart is functionally capable of 

 dealing with the blood it receives during diastole. But although the 

 ventricular output is practically independent of the frequency of the heart 

 beat and a constant , venous inflow, the venous pressure tends to fall as 



