THE CIRCULATION OF TUB BLOOD 



367 



The Measurement of Blood-Pressure. Direct determinations 

 of arterial and venous pressures are made in living, anesthetized 

 animals by inserting into a large artery or vein a glass tube con- 

 nected with a pressure-gauge. The usual form of gauge for such 

 work is the mercury manometer represented in Fig. 111. This 



a . c 



FIG. 111. Mercury manometer for recording blood-pressure, d g, glass U-tube 

 partly filled with mercury. In one limb is borne a float, e, bearing a recording de- 

 vice /; the other limb is filled with a suitable liquid and connected water-tight with 

 the heart end of a divided artery b, by means of glass connection a. Changes in 

 the mercury level indicate changes of arterial pressure. 



instrument, on account of the great inertia of mercury, follows 

 only slightly the rapid fluctuations of pressure due to the beats 

 of the heart. It therefore gives mean or average pressures. Re- 

 sults obtained with mercury manometers are expressed in terms 

 of the height of the mercury column sustained by the blood- 

 pressure. To reduce them to columns of blood they must be 

 multiplied by 13.6, the number of times mercury is heavier than 

 blood. The mean aortic pressure in average-sized dogs is ordi- 



