OIL XXII.] 



BLOOD-PRESSURE IN MAN 



295 



FIG. 263. Hsemauto 

 to be read 

 m right to left. 



use of any instrument at all, by allowing the blood to spurt from a 

 cut artery on to the surface of a large sheet of white paper travel- 

 ling past it. We thus obtain what is very appropriately called a 

 hcemautograph (fig. 263). 



A distinction must be drawn between the pulse 

 as felt at any one spot in the course of an artery, 

 and the pulse-wave which is propagated through- 

 out the arterial system. This wave of expansion 

 travels along the arteries, and is started by the pro- 

 pulsion of the contents of the left ventricle into the 

 already full arterial system. The more distant the 

 artery from the heart, the longer the interval that 

 elapses between the ventricular beat and the arrival 

 of the pulse-wave. Thus it is felt in the carotid 

 earlier than in the radial artery, and is still later 

 in the dorsal artery of the foot. The difference 

 of time is, however, very slight; it is only a small 

 fraction 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. 220, p. 241. The difference in time 

 between the commencement of their upstrokes 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 is effected by instruments which may be applied to the vessels 

 without any dissection. 



These instruments are termed sphygmometers, and the best of 

 them are modifications of one originally introduced by Eiva Eocci. 

 C. J. Martin's pattern consists of a four-sided elastic bag about four 

 and a half inches wide, and long enough to encircle the arm. It is 

 wrapped round the arm, and outside of it a cuff of strong canvas is 

 firmly strapped. Air is forced into the bag by a tube leading from 

 a ball syringe; this tube is also connected by a side branch to a 

 mercury manometer. As one continues to pump and distend the 

 bag, the pulse-beats are transmitted to the mercury which is seen to 

 rise in the manometer and oscillate with the pulse-beats. As the 



