292 



THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



FIG. 294. Haemauto- 

 graph, to be read 

 from right to left. 



tion of a second ; the wave travels at the rate of from 5 to 10 metres 

 a second, that is twenty to thirty times the rate of the blood current. 



The Rate of Propagation of the Pulse-Wave. The method of ascertaining this 

 may be illustrated by the use of a long elastic tube into which fluid is forced by 

 the sudden stroke of a pump. If a series of levers are placed 

 along the tube at measured distances those nearest the pump 

 will rise first, those farthest from it last. If these are arranged 

 to write on a revolving cylinder under one another, this will 

 be shown graphically, and the time interval between their 

 movements can be measured by a time tracing. The same 

 principle is applied to the arteries of the body ; a series of 

 Marey's tambours are applied to the heart and to various 

 arteries at known distances from the heart ; their levers are 

 arranged to write immediately under one another, as in fig. 248. 

 The difference in time between the commencement of their up- 

 strokes is measured by a time tracing in the usual way. 



The tracing taken with a sphygmograph is that 

 of the pressure pulse ; we may regard it as a blood- 

 pressure tracing without a base line. The actual 

 measurement of the blood-pressure in the human 

 subject cannot obviously be effected by the appar- 

 atus employed on animals, and numerous instru- 

 ments have been invented for the purpose which 

 may be applied to the vessels without any dissec- 

 tion. One of the simplest of these sphygmometers, as they are termed, 

 has been introduced by Hill and Barnard. 



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. 295). 

 A small indiarubber 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 compresses the air in the 

 bulb ; the air acts as an elastic spring. The more 

 one presses the more the fluid rises ; at a certain 

 height the meniscus of the fluid exhibits more pulsa- 

 tion 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 como to the explanation why the maximal pulsation gives 



FIG. 295. Hill and 

 Barnard's Sphyg- 

 mometer. 



