VELOCITY AND PRESSURE OF BLOOD-FLOW. 



447 



and diastolic pressure. In speaking of the pressure in the blood- 

 vessels we refer usually to what is called the mean pressure. It is 

 obvious that, so far as the larger arteries are concerned, the mean 

 pressure is only a convenient expression for the average pressure 

 during a certain period. If we know at any moment the systolic 

 and the diastolic pressure in an artery we can estimate the mean 

 pressure with approximate accuracy by taking the arithmetical mean 

 of the two figures. In physiological observations, as a rule, no 

 attempt is made to estimate the mean pressure for any given time 

 with mathematical accuracy. In the ordinary tracing as given by the 

 mercury manometer (Fig. 179) the mean pressure for any given period 

 during which the variations have been symmetrical and not ex- 

 treme is estimated from the arithmetical mean of the highest and 

 lowest points reached. When desirable, the mean pressure may be 



Fig. 181. Schema to indicate the general relations of systolic, mean, and diastolic 

 pressures throughout the arterial system: a, Systolic; m, mean; d, diastolic; c, pressure 

 at beginning of the capillaries. The distance from 8 to d represents the pulse pressure at 

 different parts of the arterial system. 



recorded by introducing a resistance (narrowing the tube) between 

 the artery and the manometer. The latter will then record mean 

 pressure and show no variations with the heart beat. A general 

 idea of the variations in systolic, diastolic, and mean pressure, 

 as also pulse pressure throughout the arterial system, may be ob- 

 tained from the schema given in Fig. 181. 



Method of Measuring Systolic and Diastolic Pressure in 

 Animals. In animals in which the manometer may be connected 

 directly with the artery the systolic and diastolic pressures may be 

 obtained in one of two general ways: *(1) By using some form of 

 pressure recorder or manometer sufficiently mobile to follow very 

 quick changes of pressure. (2) By using a mercury manometer 

 provided with maximum and minimum valves. Of the manometers 

 that have been devised to register accurately the quick changes in 

 pressure due to the heart beat, the two that are most frequently 



