MOVEMENT OF FLUIDS IN ELASTIC TUBES. 119 



(2.) Wave-Motion. If however, more fluid be forced in jerks into an elastic 

 tube, i.e., interruptedly the first part of the tube dilates suddenly, corresponding 

 to the quantity of fluid propelled into it. The jerk communicates an oscillatory 

 movement to the particles of the fluid, which is communicated to all the fluid 

 particles from the beginning to the end of the tube ; a positive wave is thus rapidly 

 propagated throughout the whole length of the tube. If we imagine the elastic tube 

 to be closed at its peripheral end, the positive wave will be reflected from the 

 point of occlusion, and it may be propagated to and fro through the tube until 

 it finally disappears. In such a closed tube a sudden jet of fluid produces only a 

 wave-movement, i.e., only a vibratory movement, or an alteration in the shape 

 of the liquid, there being no actual translation of the particles along the tube. 



(3.) If, however, fluid be pumped interruptedly or by jerks into an elastic tube 

 filled with fluid, in which there is already a continuous current, the movement 

 of the current is combined with the wave-movement. We must carefully dis- 

 tinguish the movement of the current of the fluid, i.e., the translation of a mass of 

 fluid through the tube, from the wave-movement, the oscillatory movement, or 

 movement of change of form in the column of fluid. In the former, the 

 particles are actually translated, while in the latter they merely vibrate. The 

 current in elastic tubes is slower than the wave-movement, which is propagated 

 with great rapidity. 



This last case obtains in the arterial system. The blood in the arteries is 

 already in a state of continual movement, directed from the aorta to the capillaries 

 (movement of the current of blood) ; by means of the systole of the left ventricle 

 a quantity of fluid is suddenly pumped into the aorta, and causes a positive wave 

 (pulse-wave) which is propagated with great rapidity to the terminations of the 

 arteries, while the current of the blood itself moves much more slowly. 



Rigid and Elastic Tubes. It is of importance to contrast the movement 

 of fluids in rigid and in elastic tubes. If a certain quantity of fluid be forced 

 into a rigid tube under a certain pressure, the same quantity of fluid will flow 

 out at once at the other end of the tube, provided there be no special re- 

 sistance. In an elastic, tube, immediately after the forcing in of a certain 

 quantity of fluid, at first only a small quantity flows out, and the remainder 

 flows out only after the propelling force has ceased to act. 



If an equal quantity of fluid be periodically injected into a rigid tube, with 

 each jerk an equal quantity is forced out at the other end of the tube, and the 

 outflow lasts exactly as long as the jerk or the contraction, and the pause between 

 two periods of outflow is exactly the same as between the two jerks or contractions. 

 In an elastic tube it is different, as the outflow continues for a time after the 

 jerk; hence, it follows that a continuous outflow current will be produced in 

 elastic tubes, when the time between two jerks is made shorter than the duration 

 of the outflow after the jerk has been completed. When fluid is pumped 

 periodically into rigid tubes, it causes a sharp abrupt outflow isochronous with 

 the inflow, and the outflow becomes continuous only when the inflow is 

 continuous and uninterrupted. In elastic tubes, an intermittent current under 

 the above conditions causes a continuous outflow which is increased with the 

 systole or contraction. 



65. Structure and Properties of the Blood- Vessels. 



In the body the large vessels carry the blood to and from the various 

 tissues and organs, while the thin-walled capillaries bring the blood into 



