THE BLOODFLOW IN THE ARTERIES 199 



in front. In other words, a pulsatile acceleration of velocity is produced 

 by a pulsatile change in pressure between each two balls. The existence 

 of a pulse wave going in the same direction but quicker than a moving 

 column of fluid can also be illustrated by observing the waves traveling 

 down a stream when a stone is thrown into it. 



The length of the pulse wave is such that the beginning of it has ar- 

 rived at the periphery of the arterial system before the end has disap- 

 peared from the beginning of the aorta. This is important to remem- 

 ber, for it is a common mistake to think of the wave as being a local 

 one. The determination of the length of the pulse wave depends upon 

 the following equation: L = VT, where L equals the length of the pulse 

 wave, V its velocity of transmission, and T its duration at a given point 

 in the artery. Under ordinary circumstances L would usually work out 

 from 3.25 to 4.5 meters. 



The rate of transmission of the pulse wave varies according to the 

 rigidity of the walls of the arteries. To understand why this should be 

 so, it will be well for a moment to consider the physical conditions 

 upon which the pulse wave depends. If we connect a piece of rigid 

 tube with the nozzle of a large syringe, with each movement of the pis- 

 ton a wave of pressure will be transmitted to the fluid in the tube, along 

 which it will travel at such a high velocity that it will arrive at the 

 free end of the tube almost instantaneously, and incidentally the out- 

 flow of fluid from the end of the tube with each compression of the 

 pump will be exactly equal to that represented by the movement of the 

 piston. If, on the other hand, an elastic tube is employed, it will be 

 found that the sudden increase of pressure produced by each stroke of 

 the pump causes a distention of the walls, which travels along the tube 

 as a wave at a readily measurable velocity, which is slower the more 

 extensible the tube. Moreover, the outflow of fluid from the free end 

 of the tube will continue for some time after the cessation of the move- 

 ment of the pump. What happens in the tube with each discharge of 

 the fluid is that the portion which is immediately adjacent to the pump 

 undergoes distention and, being elastic, tends immediately afterward to 

 recoil and thus exert a recoil pressure- on the fluid contained in the tube. 

 As a result, pressure waves are set up in the fluid in all directions. Those 

 that travel back come to a stop because of the piston, while those that 

 travel distally act on the fluid in front of them so as to accelerate it 

 and by temporarily raising its pressure distend the next segment of the 

 vessel wall, until the end of the tube is reached. From this considera- 

 tion it is clear that the more extensible and elastic the wall of the tube 

 is, the longer will it take for the wave of pressure to travel from one 

 end to the other. 



