MECHANICS OF THE CIRCULATION IN THE VESSELS 115 



pressure in the armlet is raised considerably (about 50 mm. Hg) above 

 what it is expected to be. While the lever is writing on the drum 

 the small oscillations due to the impact on the bag of the pulse-waves 

 in the central portion of the obliterated artery, the pressure is gradually 

 diminished by allowing air to escape. At the moment when the 

 pressure upon the 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 maxi- 

 mum, begin abruptly to diminish, corresponds to the minimum blood- 

 pressure. 



In using the sphygmometer of Hill and Barnard (Fig. 46), the bag 

 is inflated with air till the pulsation indicated by the index of the 

 pressure gauge 

 reaches a maxi- 

 mum. The mean 

 pressure shown 

 by the gauge at 



this point is rap ^ *H fl ^ 



approximately!^ ^^ /J/ /hr**^N=<t 

 equal to, or some- 

 what greater 

 than, the mini- 

 mum arterial 

 pressure. With 

 this instrument 

 it has been found 

 that in the bra- 

 chial artery the 

 normal arterial 

 pressure in most 

 healthy young 

 men is no to 

 130 mm. of mer- 

 cury in the sitt in g 



posture. When the person is resting in the recumbent 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. 



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, 

 1 80 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 dilatation of the cutaneous 



Fig. 46. 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 T-tube, B, with a pressure gauge, 

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



