MECHANICS OF THE CIRCULATION IN THE VESSELS 



115 



stopcock air can be allowed to escape more or less rapidly from the 

 armlet. 



To determine the maximum or systolic blood-pressure, the air- 

 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. 



According to Brooks and Luckhardt, the criteria which are supposed 

 to fix the systolic and diastolic pressures in these and similar methods 

 yield results which are too high. The personal equation also seems to 

 introduce a considerable error in the selection of the points on the trac- 

 ings at which the characteristic changes are supposed to occur (Kilgore). 



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

 is inflated till the pulsation indicated by the gauge reaches a maxi- 

 mum. The mean 

 pressure shown 

 at this point is 

 assumed to be 

 equal to.or some- 

 what greater 

 than, the dia- 

 stolic pressure. 



The effect of 

 muscular exer- 

 cise upon the 

 pressure is in- 

 fluenced by the 

 nature of the 

 work. Such an 

 effort as the lift- 

 ing 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, 



Fig. 46. Sphygmometer of Hill and Barnard. It consists of 

 a broad armlet, A, connected by a T-tube, B, with a pressure 

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

 a valve. 



