THE BLOOD PRESSURE 



989 



still shows small oscillations, due to the impact of the unoccluded stump of the 

 artery on the upper border of the india-rubber bag. 



Many different methods have been introduced for the purpose of recording 

 the pressure oscillations in the bag. In Erlanger's apparatus the rubber bag 

 is put into connection with a thick- walled 

 rubber ball PS contained in a glass chamber. 

 The chamber (Fig. 375) communicates with a 

 sensitive tambour and also by means of a 

 capillary opening provided with a stop -cock 

 with the external air. By this means the 

 slow expansion of the bag E is not recorded 

 by the tambour, which only moves with the 

 sudden oscillations of pressure due to each 

 heart-beat. With this instrument it is easy 

 to read on the accompanying mercurial 

 manometer the point at which the oscil- 

 lations of pressure in the bag suddenly 

 become maximal, and so to determine ap- 

 proximately the diastolic pressure in the 

 artery. 



VENOUS PRESSURE. To determine the venous pressure in man we may 

 use some modification of von Recklinghausen's method. A circular, disc-shaped, 

 incomplete rubber bag (Fig. 376) is made by cementing together at the circum- 

 ference two rubber discs, each of which has a hole in the centre. This, is placed 

 over a peripheral vein and a glass plate laid on the top (Fig. 377). A tube 

 leads from the interior of the annular rubber bag to a water manometer and to a 

 bicycle pump or bellows for the injection of air. On blowing air into the bag 



FIG. 376. 



FIG. 377. 



the pressure in its interior rapidly increases. If the skin and glass plate have 

 been previously smeared with glycerine, the air does not escape, but distends 

 the bag, pressing it against the skin on the one hand and the glass plate on the 

 other. Through the hole in the rubber bag it is easy to see the pressure at which 

 the vein collapses that is to say, the point at which the pressure in the bag is 

 equal to the pressure within the vein. By a similar method, using a smaller bag, 

 we may determine the pressure which is just sufficient to obliterate the capillaries 

 in any given area of the skin, so causing a blanching of the skin lying under 

 the bag. 



The following Table may serve to give an idea of the average 

 height of the mean blood pressure at different parts of the vascular 

 system in man, in the horizontal position. The pressures are all 

 subject to considerable variations according to the activity of 



