FLOW OF BLOOD THROUGH THE ARTERIES 1035 



applied to the piston will raise the pressure simultaneously at all points 

 in the tube AB. The increased pressure applied at A is therefore 

 transmitted with practically no loss of time to all parts of the tube AB. 

 This immediate spread of the wave of pressure only applies to an 

 incompressible fluid within a rigid tube.' If the fluid were compressible, 

 if it consisted, e.g., of air, a sudden movement inwards of the piston at A 

 would not be felt immediately at B. The propagation of the wave of 

 pressure from A to B would take a finite period of time, its velocity 



B 



FIG. 408. 



being identical with that of the velocity of propagation of a wave of 

 sound in air, i.e. 1100 feet per second. The same retarding effect will be 

 produced if we have an incompressible fluid within a tube whose wall is 

 distensible and elastic. If we imagine (Fig. 409) an elastic tube BC filled 

 and distended with water and connected at B to a rigid tube, which is 

 provided with a piston, the first effect of a rapid movement of fluid 

 driven in by the piston will be a rise of pressure at the point immediately 

 in front of the piston, viz. at a. The wall being distensible, and pressure 



FIG. 409. 



being propagated along the fluid in every direction, the rise of pressure 

 at A will be spent partly on the particles of fluid in front of it, viz. 

 at 6, but also on the walls of the tube, so that this is stretched and 

 the cross-section of the tube enlarged. The distended segment at a 

 will then exert a pressure on the contained fluid, driving this backwards 

 and forwards. The fluid on its side towards the piston will tend to 

 come to a stop, while that towards the distal end of the tube will be 

 accelerated. The distended wall therefore returns to its original 

 diameter, and the next segment at 6 is stretched in its turn, so that 

 a wave of increased pressure is propagated along the tube in the direc- 

 tion of the arrow. 



The velocity with which this wave is propagated depends on the 

 density of the fluid, i.e. its inertia, and on the resistance of the walls 



