270 CIRCULATION OF THE BLOOD. 



millimetre of mercury, the height of the venous column of 

 serum must be about half an inch = 12 millimetres, the spe- 

 cific gravity of mercury being about twelve times that of serum. 

 In the distal column of the manometer is a glass piston, the 

 upper end of which bears a horizontal arm arranged in the 

 same way as that which bears the writing pencil in the ordi- 

 nary kymograph the main differences being that in this case 

 the manometer is much smaller, and that, in order to avoid 

 friction, the tracing is recorded, as in the sphygmograph, on 

 glazed paper, blackened by passing it over the flame of a 

 paraffin lamp. The record so obtained is shown ri fig. 234. 

 On account of the relative slowness of the movements and the 

 inconsiderable lumen of the manometer, the curve is very little 

 modified by the oscillation proper to the mercurial column, and 

 is therefore a true representation of the succession of changes 

 of pressure which take place in the ventricle. We learn from 

 it that in the frog the pressure exercised by the ventricle on 

 the blood it contains arrives at its acme somewhat gradually, 

 and persists for an appreciable period ; and that when the heart 

 relaxes, the subsidence of pressure is at first extremely rapid, 

 but subsequently somewhat more gradual. The rate of move- 

 ment of the paper being 40 centimetres per minute, the dura- 

 tion of each systole can be easily measured. 



65. b. Modifications of the Endocardia! Pressure Curve 

 under various Conditions. For the purpose of investigating 

 the influence of various mechanical conditions on the action of 

 the heart, and particularly of changes in the relation of the 

 pressure in the veins and that in the arteries, the apparatus 

 must be so modified that the ventricle, instead of communi- 

 cating exclusively with the manometer, pumps the liquid, con- 

 stant^ supplied to it from the venous reservoir, along a tube 

 or system of tubes representing the arterial system. To fulfil 

 these conditions, all that is necessary is, (1) to insert the arte- 

 rial canula, not in the bulb, but in the left aorta (the right 

 being tiecf), so as not to interfere with the play of the aortic 

 valve ; and (2) to join to the proximal limb of the gauge an 

 India-rubber tube, dilated near the junction into an elastic 

 bulb, and ending in a nearly capillary beak of glass, the pur- 

 pose of the latter being to furnish the required resistance, that 

 of the former to render the discharge as nearly equable as pos- 

 sible in short, to replace the elasticity of the arteries. 



The advantage of this arrangement does not lie in the cir- 

 cumstance that the mode of action of the heart is more natural, 

 for it makes little difference to that organ whether the liquid' 

 it discharges at one contraction returns to it during the next 

 relaxation or is pumped forwards, provided that the pressures 

 to which it is subjected are the same in systole as in diastole. 

 It is rather that when the heart is so arranged that liquid is 



