CHAPTER XXI 

 THE ARTERIAL PRESSURE 



THE term " blood-pressure " is somewhat loosely used. Generally, 

 it signifies the arterial pressure, but it can be equally well applied to 

 the pressure of blood in the capillaries or in the veins. For the sake 

 of accuracy, it is better to speak of the arterial blood-pressure or arterial 

 pressure, the capillary pressure, and the venous pressure. 



The Blood-Pressure. It has long been known that the blood is 

 under different pressure in the various parts of the system. From 

 a divided artery the blood flows out in forcible spurts, from a vein it 

 flows out continuously and with little force. It takes very little 

 pressure of the fingers to blanch the capillaries of the skin or nail-bed, 

 to stop the blood-flow in the superficial veins, but an appreciable 

 amount of pressure to obliterate the radial artery. 



FIG. 91. ARTERIAL CANNULA. 



Measurement of Arterial Pressure. Stephen Hales (1733) was the 

 first to measure the blood-pressure. He fastened a long glass tube 

 held vertically -to the femoral artery of a horse, using a brass cannula 

 and a goose's trachea as a flexible tube for making connection. He 

 saw the arterial blood rise some 6 feet high in the tube, and oscillate 

 there- with each pulse-beat and respiration. 



Later the mercurial manometer was adapted to the same 

 purpose. This consists of a U-shaped tube containing mercury, 

 which, being 13-5 times heavier than blood, allows the manometer 

 to be brought to a convenient height. On the top of the mercury 

 rides a float provided with a writing style (see Fig. 92). The 



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