Chap. XX.] RIGID AND ELASTIC TUBES. 221 



tube presses forward the fluid which distended them, 

 and a succeeding portion of the tube then experiences 

 the pressure and proceeds to undergo the same pro- 

 cess. So that, while the fluid in the neighbourhood of 

 the point of atflux experiences discontinuous pressure 

 owing to the intermittent action of the force, fluid at 

 some distance . from the point experiences a less and 

 less degree of intermittence, owing to the elastic 

 reaction of the walls following up the intermittent 

 force. For this elastic reaction acts in the intervals 

 between the action of the intermittent force. The 

 farther one passes from the point of afflux the more 

 nearly does the fluid exhibit a continuousness of move- 

 ment, though showing still periodic variations in the 

 speed of progression, till at length, when the full 

 effect of the elastic reaction has developed, the fluid 

 has acquired a uniform continuous flow. 



Thus elastic tubes have the power of transforming 

 an intermittent into a continuous flow. 



Thus the fluid may be said to experience two 

 forces, one the intermittent force, the pressure com- 

 municated to the fluid, and the other the force ex- 

 erted by the elastic walls, due to their distension ; in 

 other words, the tension of the walls. It is well to 

 distinguish now between these two, so that there may 

 be no difficulty in understanding the difference between 

 the phrases BLOOD PRESSURE, the force exerted by the 

 blood upon the walls of the vessels, and due to the 

 heart's action, and ARTERIAL TENSION, the force exerted 

 by the walls of the arteries upon the blood, and due to 

 the elastic recoil of these vessels. 



The effect of the action of elastic tubes on the 

 rate of movement of fluid through them is obviously 

 to slow it, for at the same instant that there enters the 

 tube a quantity of fluid, an equal quantity does not 

 issue from it, as in rigid tubes, owing to the distension 

 of the tubes. At the same time, experiment has 



