CIECULATION 



273 



long glass tube in the femoral artery of a horse laid on its 

 back, and found that the pressure supported a column of blood 

 of 8 feet 3 inches, while, when the tube was placed in a vein, 

 only 1 foot was supported. The capillary pressure is, of course, 

 intermediate between these two. 



At the present time, instead of letting the blood pressure 

 act directly against the force of gravity, it is found more 

 convenient, in studying the pressure in an artery, to let it 



A B 



FIG. 127. A, The Mercurial Manometer with recording float, used in taking 

 records of the arterial blood pressure of lower animals. B, The Hill- 

 Barnard Sphygmometer, for measuring the arterial pressure without 

 opening a vessel. 



act through a column of mercury placed in a U tube (fig. 127, 

 A.). To record the changes in pressure a float is placed upon 

 the mercury in the distal limb of the tube, and this carries a 

 writing style which records the changes upon a moving surface. 

 With such an apparatus a record such as is shown in fig. 126 

 is given. The actual pressure is measured by taking the 

 difference between the level of the mercury in the two limbs 

 of the tube. To make the measurement it is customary to 

 describe an abscissa when the mercury is at the same level 

 in the two sides of the tube. The difference between this and 

 the level of the style at any moment multiplied by two, on 



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