152 THE CIRCULATION OP THE BLOOD 



In the case of the right ventricle the contour of the curve also depends 

 on the degree* of resistance to the bloodflow through the pulmonary 

 circuit. The top of the curve becomes broader when the initial tension 

 is high, and more rounded when there is a high pulmonary resistance. 

 Another point of interest in connection with the ventricular curve is 

 that early in diastole it descends below the line of zero pressure, indicating 

 that a negative or suction pressure must exist in the ventricle at this 

 time. It will be further observed, however, that this subatmospheric 

 pressure exists for only a very short time. The auriculoventricular 

 valves being opened, a similar negative pressure is also present in the 

 auricular tracing. Were we to depend on such records alone for evidence 

 of the actual existence of this negative pressure in the heart, objection 

 might be taken to the conclusion on the ground that it was due to the 



to manometer 



max valve 



to heart 



Fig. 35. Von Frank's maximal and minimal valve, which is placed in the course of the 

 tube between heart and mercury manometer. By turning the stopcocks, it may be used as a 

 maximum, minimum, or ordinary manometer (central tubes open). (From Starling.) 



sudden recoil to which the instrument is subjected at the beginning of 

 diastole. It is necessary therefore to control these observations by the 

 use of an entirely different method. This consists in connecting the 

 heart with a valved mercury manometer (see Fig. 35). This instru- 

 ment does not of course record any sudden changes of pressure in the 

 cardiac cavity, but in obedience to changes in pressure the mercury slowly 

 moves in the direction in which the valve permits it to move. Such an 

 instrument, with the valve opening towards the heart, is called a minimal 

 manometer, and after it has been connected with the ventricle, it will te 

 found that a negative pressure of perhaps 40 or 60 mm. Hg is recorded. 

 Evidently, then, the negative pressure does actually exist in the ventricle 

 during some phase of the cycle, and the question arises as to whether it 

 is of importance in connection with the pumping action of the heart. At 

 first sight, considering the heart as an elastic structure, we might con- 



