FLOW OF BLOOD THROUGH THE ARTERIES 921 



tracing be taken of the oscillations or variations of pressure in the tube, 

 two waves at least are seen, one of which is the primary wave due to the 

 movement of fluid caused by the piston ; the other is the secondary wave 

 reflected back from the periphery. The fact that the secondary wave is 

 a reflected one is shown by the fact that the nearer to the peripheral resist- 

 ance the pulse is recorded, the nearer is the secondary to the primary wave, 

 as is seen in Fig. 410. 



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 ah, with a tap 

 at c and a manometer m 

 (Fig. 411), the current of fluid 

 be suddenly checked by turn- 

 ing the tap c, the column in 

 front of the tap, having a 



certain momentum, will tend a Fia 



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 

 any 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 

 of 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 

 to 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 



