CH. XXI.] MEASUREMENT OF BLOOD-PRESSURE IN MAN. 2Q5 



other nerves cannot be understood until we have studied the 

 vaso-motor nervous system, to the consideration of which we shall 

 immediately pass. 



Measurement of Blood-pressure in Man. 



The measurement of the blood-pressure in the human subject 

 cannot obviously be effected by the apparatus employed on 

 animals, and numerous instruments have been invented for the 

 purpose which may be applied to the vessels without any dis- 

 section. One of the simplest of these sphygmometers, as they 

 are termed, has been introduced by Hill and Barnard, and I 

 am indebted to Mr. L. Hill for the following 

 description of their methods and results. 



The instrument consists of a vertical glass tube 

 about five inches in length, which expands above 

 into a small bulb, and is closed at the top by a 

 glass tap (see fig. 283). 



A small india-rubber bag is fixed to the tube 

 below ; this is surrounded by a metal cup, attached 

 in such a way that only the base of the bag is 

 exposed. The bag is filled with coloured fluid. On 

 pressing the instrument down over the radial or 

 other artery, the fluid rises in the tube and com- 

 presses the air in the bulb ; the air acts as an 

 elastic spring. The more one presses the more the Fig. 

 fluid rises ; at a certain height the meniscus of the 

 fluid exhibits more pulsation than it does at any 

 other height (maximal pulsation). The tube is empirically 

 graduated in divisions that correspond to millimetres of mercury 

 pressure. The point of maximal pulsation gives the arterial 

 pressure. Before each observation the tap is opened, and by 

 gentle pressure on the bag the fluid is set at the zero mark on 

 the scale. Thus errors due to changes in barometric pressure or 

 temperature are avoided. 



We now come to the explanation why the maximal pulsation 

 gives us a reading of arterial pressure. If the mean pressure 

 inside and outside an artery be made equal, then the wall of the 

 vessel is able to vibrate at each pulse with the greatest freedom. 

 The mean pressure is less than the systolic, but greater than the 

 diastolic pressure ; thus during the heart's systole the artery is 

 opened out to its fullest extent, while during the heart's diastole 

 its lumen is obliterated ; hence the vessel wall swings with the 

 greatest amplitude. If the pressure exerted by the sphygmometer 



