236 EQUALIZING ACTION OP ARTERIES I PULSE. 



jets ; but this is prevented, so that the current is reduced to 

 an equable stream by the time it reaches the capillaries, 

 through the elasticity of the walls of the arteries. In order 

 to comprehend how this acts, we may suppose a forcing-pump 

 ( 270) to propel its fluid, not into a hard unyielding tube of 

 iron or lead, but into an elastic tube of india-rubber. The 

 effect of each stroke of the pump will be partly expended in 

 distending the tube, so as to make it contain an additional 

 quantity of water ; and the suddenness of the jet at its oppo- 

 site extremity will be diminished. In the interval of the 

 stroke, the elasticity of the wall of the tube will cause it to con- 

 tract again, and to force-out the added portion of its contents ; 

 this it will not have completed by the time that the action of 

 the pump is renewed ; and in this manner, instead of an inter- 

 rupted jet at the mouth of the tube, we shall have a continuous 

 flow, which, if the tube be long enough, will become quite 

 equable.* It is precisely in this manner that the elasticity of 

 the arteries influences the flow of blood through them, by 

 converting the interrupted impulses which the heart com- 

 municates to it, into a continued force of movement. In the 

 large arteries, these impulses are very evident ; in the smaller 

 branches they are less so, but they still manifest themselves 

 by the jerking in the stream of blood proceeding from a 

 wound in one of these vessels ; whilst in the capillaries, the 

 influence of the heart's interrupted impulses cannot usually 

 be seen at all, the streams that pass through them being 

 perfectly equable. 



276. The phenomenon which we call the pulse, is nothing 

 else than the change in the condition of the artery occasioned 

 by the increased pressure of the fluid upon its walls, at the 

 moment when the heart's contraction forces an additional 

 quantity of blood into the arterial system. By the frequency 

 and force of this change, we can judge of the power with 

 which the blood is being propelled. But the pulse can only 

 be well distinguished, when we can compress the artery 

 against some resisting body, so that there is a partial obstruc- 

 tion to the flow of blood through it, which causes the disten- 

 sion to be more powerful ; the most convenient artery for this 



* The same effect is obtained in an ordinary fire- or garden-engine, 

 by the interposition of an air-vessel, in which the elasticity of com- 

 pressed air is substituted for that of the wall of the pipe. 



