62 ELEMENTARY PHYSIOLOGY 



is thrown into the arteries by the heart. The whole 

 arterial system, therefore, becomes over-distended with 

 blood. 



Now we know by experiment that under such conditions 

 as these, an elastic tube has the power, if long enough and 

 elastic enough, to change a jerked impulse into a continu- 

 ous flow. 



If an ordinary syringe or other convenient form of 

 pump be fastened to one end of a long glass tube, and 

 water be forced through the tube, it will flow from the far 

 end in jerks, corresponding to the jerks of the syringe. 

 This will be the case whether the tube be quite open at 

 the far end, or drawn out to a fine point so as to offer 

 great resistance to the outflow of the water. The glass 

 tube is a rigid tube, and there is no elasticity to be brought 

 into play. 



If now a long india-rubber tube be substituted for the 

 glass tube, it will be found to act difl"erently, according as 

 the opening at the far end is wide ou narrow. If it is 

 wide, the water flows out in jerks, nearly as distinct as 

 tliose from the glass tube. Tliere is little resistance to 

 the outflow, little distension of the india-rubber tul)e, little 

 elasticity brought into play. If, however, the opening be 

 narrowed, as by fastening to it a glass tube drawn out to 

 a fine point, or if a piece of sponge be thrust into the end 

 of the tube — if, in fact, in any way resistance be offered 

 to the outflow of the water, the tube becomes distended, 

 its elasticity is brought into play, and the water flows out 

 from the end, not in jerks l)ut in a stream, which is mox'e 

 and more comi)letely continuous the longer and more 

 elastic the tube, and the greater the resistance at its open 

 end. 



Substitute for the syringe the heart, for the finely 

 drawn glass tube or sponge the small arceries and capil- 

 laries, for the india-rubber tube the whole arterial system, 

 and you have exactly the .same result in the living body. 

 Througii the action of the elastic arterial walls the separate 



