FLOW OF BLOOD THROUGH THE ARTERIES 1037 



where v is the velocity per second, 



g, the acceleration due to gravity, 

 e, the elastic coefficient of the wall, 

 a, the thickness of the wall, 

 d, the diameter of the tube, 

 D, the density of the fluid, 

 k, a constant. 



If the end c of the tube is closed, the wave of a positive pressure 

 on arriving at B will be reflected back as a positive reflected wave. If 

 a 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 



FIG. 411. 



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 resistance 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 ab, with a stop-cock at c and a 

 manometer m (Fig. 411), the current of fluid be suddenly checked by 

 turning the cock c, the column in front of the cock, having a certain 

 momentum, will tend 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. 



