84 



PEACTICAL PHYSIOLOGY 



venae comites which surround the radial artery. While taking a third 

 tracing compress the upper arm of the subject with your hands, 

 excluding the radial artery. The venous congestion thus produced 

 will raise the line of the tracing and cut off the descent of the pulse 

 curve (Fig. 84, 1). The swollen venae comites raise the instrument 

 above the artery. It is important to remember that venous congestion 

 may alter the character of the radial pulse. 



FIG. 84. Effect of compressing femoral vein on sphygmogram. 1, Sphygmogram 

 taken with instrument resting on femoral artery and vein of dog. 2, On femoral 

 vein only. 3, On femoral artery only. (Hill, Barnard, and Sequeira.) 



Blood Pressure in Man. The pressure may be measured by .the 

 Hill-Barnard sphygmometer. This consists of a graduated glass tube, 

 which expands into a capsule below and a small air chamber, above. 

 The capsule and the tube, almost to the zero mark, are filled with 

 dilute glycerine acidulated with chromic acid ; the capsule is 

 covered by a. rubber membrane. The air chamber is closed by a 

 tap. In using the, instrument the fluid is first set at zero. To effect 

 this the tap is opened and the rubber membrane pressed until the 

 fluid reaches the zero mark on the scale. The tap is then closed. 

 The instrument is now pressed upon the artery until the position is 

 found at which the fluid meniscus gives the maximal pulsation. The 

 scale is read at this point, and the reading gives the mean arterial 

 pressure in mm. Hg. While taking the reading the hand of the 

 subject must be placed on the same level as the heart, so as to avoid 

 the influence of gravity. 



The air-chamber acts as a spring, and the instrument is a spring 

 manometer. The zero is set by opening the tap before each reading, 

 so as to avoid errors due to alterations of temperature and barometric 

 pressure. The instrument is graduated empirically. The maximal 

 pulsation is obtained when the mean pressure within and without the 



