THE BLOODVESSELS 65 



of the tube as enters it at the other. Between the strokes of 

 the pump no fluid issues from the pipe ; the jet is only produced 

 at the moment the pump is in action. No more water can enter 

 this rigid tube from the pump end than can leave it at the outlet. 

 If now water be pumped through a short elastic tube, the outlet 

 of which is in no way obstructed, the current of water through 

 it behaves just as if it were a rigid tube — viz., a stream of water 

 issues from the outlet during the action of the pump, and nothing 

 more happens until the next stroke. An important alteration 

 can, however, be made to the current through the elastic tube, 

 by offering an obstruction at the outlet to the free passage of 

 the water. The effect of this obstruction is that the elastic 

 tube expands to accommodate the contents, while a stream pours 

 from the partly obstructed outlet which no longer corresponds 

 to the stroke of the pump, but is a continuous stream which 

 issues so long as the pumping is continued. This continuous 

 stream is produced by the elastic recoil of the tube keeping up 

 the pressure which the pump imparted to the fluid, and the 

 reason why the elastic recoil of the tube is now brought into play 

 is owing to the partly obstructed outlet, or, as we have already 

 termed it, the peripheral resistance. If the elastic tube be of 

 sufficient length, a continuous stream will issue in spite of the 

 absence of an obstruction ; this is brought about by the internal 

 fluid friction against the walls of the tube, which of course causes 

 a peripheral resistance. In elastic tubes, therefore, the recoil of 

 the tube converts an intermittent into a continuous flow, and the 

 distension of the tube which produces the recoil is caused by the 

 peripheral resistance. 



Whether in a living tube like a bloodvessel, or in a dead tube 

 like a pipe, fluid flowing under a head of pressure meets with 

 resistance ; this is due to friction of the fluid particles against 

 the wall of the tube. The amount of resistance is not the same 

 throughout the length of the pipe : it steadily decreases to the 

 exit ; but this assumes that the pipe maintains an even bore 

 throughout. If the pipe were to narrow at any point in its 

 length, the friction would in consequence increase at that par- 

 ticular place. The resistance or pressure due to friction may 

 be estimated at any part of the tube by the introduction of a 

 gauge — viz., a vertical tube placed at right angles to the flow. 

 In this tube the fluid will rise to a certain height, and the weight 

 of the fluid column indicates the pressure exerted at that par- 

 ticular spot. Such a tube is known as a manometer, and the 

 pressure it gauges is the side or lateral pressure due to frictional 

 resistance. 



Of the total force or head of pressure engaged in forcing fluid 

 along a tube, the bulk is used up in overcoming the frictional 



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