io6 



A MANUAL OF PHYSIOLOGY 



arm falls below the maximum blood-pressure, and the pulse-wave is 

 first able to break through the brachial artery, the oscillations of the 

 lever will more or less abruptly increase in amplitude. The pressure 

 shown by the manometer at this point is the systolic blood-pressure. 

 To obtain the minimum or diastolic pressure, the air-pressure in 

 the armlet is raised somewhat (10 to 15 mm. Hg) above the pressure 

 expected. The pressure is diminished by 5 mm. Hg at a time, 

 records of the oscillations being taken on the drum. The manometer 

 reading at the point at which the oscillations, after reaching the 

 maximum, begin abruptly to diminish, corresponds to the minimum 

 blood-pressure . 



In using the sphygmometer of Hill and Barnard (Fig. 38), 

 the bag is inflated with air till the pulsation indicated by the 

 index of the pressure gauge reaches a maximum. The mean 

 pressure shown by the gauge at this point is approximately 

 equal to, or somewhat greater than, the minimum 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 

 recumbent 

 posture, the 

 pressure may 

 be as low as 

 95 mm. of mer- 

 cury. Hard 

 work and ner- 

 vous strain 

 may raise the 

 pressure to 

 140 or 145 

 mm. of mer- 

 cury. 



The effect 

 of muscular 

 exercise upon 

 the pressure 

 is influenced 

 by the nature 

 of the work. 

 Such an effort 

 as the lifting 



of a heavy weight causes a sudden and great increase, which is 

 very transient. Thus, the average arterial pressure in a number 

 of men was in before, 180 during, and no two to three minutes 

 after the lift (McCurdy). The rise of pressure in this case is due 

 largely to the marked diminution of the calibre of the bloodvessels 

 mechanically produced by the strong and sustained contraction 

 of the muscles. This increases the resistance to the passage of 

 the blood along the arteries, while the veins are emptied by the 

 pressure, and more blood thus reaches the right side of the heart. 

 It is obvious that the heart and vessels may easily be exposed to 

 an injurious strain during such efforts. In such an exercise as 

 running, while the pressure mounts to some extent at first, as 

 already mentioned, the rise is not maintained, owing to the dilata- 

 tion of the cutaneous vessels. In the anterior tibial artery of a 



FIG. 38. 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 air, and connected by a ~]~*tube, B, with a pressure 

 gauge, C, and a small compressing air-pump, D, fitted with a valve. 



