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THE FLOW OF BLOOD THROUGH THE ARTERIES 965 



If the tube BC be widely opened a reflected wave is also observed, but 

 this time of reversed sign, i. e. the wave is one of negative pressure. The 

 production of this wave is 

 dependent on the momentum 

 of the moving column of fluid. 

 If in the tube ab, with a tap 

 at c and a manometer m 

 (Fig. 421), the current of fluid 

 be suddenly checked by turn- "Tj^ 



ing the tap c, the column in "^ 



front of the tap, having a U : 



certain momentum, will tend p IG 421. 



to go on moving and therefore 



produce a suction or negative pressure behind it. When a wave of positive 

 pressure arrives at the open end of a tube, there is a sudden increase in 

 the velocity of output, and the momentum of the mass of fluid which is 

 thrown out causes a similar suction or negative pressure, which travels back 

 the whole length of the tube. If the end of the tube is only partially 

 closed, every primary positive wave will be transformed into a reflected 

 one which is partly positive and partly negative. Since both these reflected 

 waves travel through the tube with the same velocity and will mutually 

 interfere, the result may be either a positive or a negative wave or nothing 

 at all, according to the degree of constriction. 



In a branching system of tubes, such as the arterial system, reflection 

 of waves must take place at every dividing place. All the conditions for 

 the origin of reflected waves and interference of such waves are present 

 in the arterial system. It is impossible a priori however to say whether 



,ny reflected wave will form a marked feature on the pulse tracing. It is 

 possible that the multitudinous reflections which must occur in every part 

 of the arterial system may interfere with one another to such an extent that 

 they mutually annul each other. The origin of any secondary wave in 

 the pulse tracing must therefore be determined by experiment. 



To study the pulse more fully it is necessary to obtain a graphic record 



f the expansion of the arteries or, what comes to the same thing, of the 

 exact changes in pressure which produce this expansion. The curve 

 obtained with the mercurial manometer shows elevations corresponding 

 the pulse; but the instrument is far too sluggish to record the finer 

 variations of pressure. For this purpose a manometer which has very 

 little inertia, such as Hiirthle's or Piper's, must be used. The expansion 

 of the artery is registered by means of a lever, which may be made to rest 

 more or less heavily upon the artery, and the movements of which are 

 recorded on a blackened surface. Such an instrument is called a sphygmor 

 graph. Of the many forms of sphymographs, Marey's or Dudgeon's is 

 perhaps the most convenient for clinical purposes. 



The principle of Marey's sphygmograph is shown in Fig. 422. The button b is 

 adjusted so as to press on the radial artery. Its movements are transmitted to a lever 







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