THE BLOOD-PRESSURE 



877 



tioiis of pressure in the bag suddenly became maximal, and so to determine approxi- 

 mately the diastolic pressure in the artery. 



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

 some modification of von Rscklinghausen's method. A circular, disc-shaped, incom- 

 plete rubber bag (Fig. 330) is made by cementing 

 together at the circumference 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. 381). 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 

 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. 



FIG. 380. 



FIG. 381. 



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

 of the mean blood- pressure (not systolic) 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 the individual and 

 the physiological activity of the various parts and organs of the body : 



90 mm. mercury (65-110). 



85 mm. 



15 to 40 mm. mercury. 



9 mm. mercury. 

 10 mm. ,, 



3 mm. 



to -8 mm. mercury. 



Large arteries (e.g. carotid) . 



Medium arteries (e.g. radial)' . 



Capillaries . . about 



Small veins of arm ..... 



Portal vein . . 



Inferior vena cava . . . . , 



Large veins of neck . . . from 



The cause of these peculiarities in the circulation in different parts of the vascular 

 system will be rendered clearer by a study of a flow of fluid through a tube of uniform 

 bore (Fig. 382). If the tube AG be connected with the reservoir n, fluid will flow from 

 A to G under the influence of the pressure difference between the fluid in the reservoir 

 and that at G. The pressure on the fluid at each part of the tube can be measured by 

 attaching at a series of points e.g. at B, c, D, E, r- vertical tubes in which the fluid 

 will rise to a height corresponding to the lateral pressure existing at these several 

 points. When fluid is flowing from A to G it will be found that the heights of the fluid 



