io 4 



A MANUAL OF PHYSIOLOGY 



beats, on the average, very much faster than the heart of a dog, and 

 yet the arterial pressure in the dog is certainly at least as great as 

 in the rabbit. Nor does the size of the body seem to have any definite 

 relation to the mean pressure, even in animals of the same species ; 

 and there is no reason to suppose that the pressure is materially less 

 in the radial artery of a dwarf than in the radial artery of a giant. 



In man the blood-pressure has been estimated by adjust- 

 ing over an artery an instrument known as a sphygmo- 

 manometer or sphygmometer, which, in its most modern 

 form, consists essentially of a hollow rubber pad or bag con- 

 taining liquid or air, and connected with a metallic pressure 

 gauge, graduated beforehand by comparison with a mercurial 

 manometer. In using the sphygmometer of Hill and Barnard 



(Fig. 300), 

 the bag is 

 inflated with 

 air till the 

 pulsation in- 

 dicated by 

 the index of 

 the pres- 

 sure gauge 

 reaches a 

 maximum. 

 The mean 

 pressure 



FIG. 



SPHYGMOMETER OF HILL AND BARNARD. 



It consists of a broad armlet, A, which is strapped round the upper 



arm. On the inside of the armlet is a thin rubber bag containing * 11 



air, and connected by a T-tube, B, with a pressure gauge C, and SnOWnby the 

 a small compressing air-pump, D, fitted with a valve. ^ t th" 



point is approximately equal to the mean arterial pressure. 



With this instrument it has been found that in the brachial artery 

 the normal arterial pressure in most healthy young men is no to 

 130 mm. of mercury in the sitting posture. When the person is 

 resting in the iccumbent posture, the pressure may be as low as 

 95 mm. of mercury. Hard work and nervous strain may raise the 

 pressure to 140 or 145 mm. of mercury. In the anterior tibial artery 

 of a boy whose leg was to be amputated, the blood-pressure, measured 

 by means of a manometer connected directly with the artery, was 

 found to vary from 100 to 160 mm., according to the position of the 

 body and other circumstances. In a woman sixty years old, in good 

 health, the following readings were obtained with a sphygmomano- 

 meter : 



