236 EQUALIZING ACTION OF ARTERIES :—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 af 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. 
