THE CIRCULATION OF THE BLOOD. 



335 



max^valve 



mm valve 



in the distal limb of the manometer and hence the column of mercury sup- 

 ported is that observed between the upper and lower levels of the mercury 

 in the distal and proximal limbs of the manometer. 



The blood-pressure as revealed by the tracing may be resolved into two 

 components: viz., (i) a more or less constant element represented by the 

 pressure in the arteries during the period of the cardiac diastole, which is 

 termed the diastolic or minimum pressure; and (2) a variable element 

 represented with certain limitations by that additional pressure occurring 

 at the time of the cardiac systole, which is termed the systolic or maximum 

 pressure. The diastolic pressure is represented by the distance between the 

 base-line and the points of the curve corresponding to the diastolic pause; the 

 systolic pressure, by the distance between the base-line and the apices of 

 the curves following the cardiac systole. The relation of these two com- 

 ponents varies in different animals and in the same animal at different times. 

 If the diastolic pressure is low, the systolic , to manometer 



increase may be considerable; if the former 

 is high, the latter may be slight in extent. 



There are good reasons for believing, 

 however, that this record does not represent 

 either the true diastolic or the true systolic 

 pressure but that the limits between the two 

 are far more widely apart than here repre- 

 sented. For, owing to the inertia of the 

 mercury, it is not capable of following the 

 rapid variations of the pressure throughout 

 their extent, that occur with each heart-beat. 

 The employment of one of the various forms 

 of the quickly responsive spring manometers 

 such as are used in determining the rapid 

 variations of intra-cardiac pressure will show 

 a much greater difference between the dia- 

 stolic and systolic pressures, often amount- 

 ing to as much as 40 millimeters. 



For the purpose of obtaining the maximum systolic and the minimum 

 diastolic pressures, it is best, however, to insert between the cannula and the 

 manometer a maximum and a minimum valve similar in principle to that 

 shown in Fig. 156. By permitting the blood to exert its pressure first through 

 the maximum valve and then permitting the mercurial column to exert its 

 pressure through the minimum valve in the reverse direction for a certain 

 length of time, or by permitting each to exert its pressure with alternate 

 heart-beats, the maximum systolic and the minimum diastolic pressures 

 will be recorded. By this method Dawson found an average maximum 

 pressure in the carotid artery of the dog of 162, and a minimum pressure of 

 103 mm. of mercury, a difference of 59 mm. Hg. The difference between 

 these two pressures is known as the pulse pressure. (A diagram showing 

 the relation of these different pressures one to another will be found on 

 page 338). 



In a series of experiments it will be found that the blood-pressure in the 

 arteries, recorded with the mercurial manometer, though rising and falling a 



to heart 



FIG. 156. v. FRANK'S VALVE. 

 This is placed in the course of the 

 tube between heart and manometer, 

 so that the latter may be used as a 

 maximum, minimum, or ordinary 

 manometer according to the tap which 

 is left open. (Starling.} 



