522 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



and diameter of the tube only holds as long as the calibre of the tubes 

 does not fall below a certain diameter. 



In capillary tubes the conditions are so far similar in that the resist- 

 ance is proportional to the length of the tube. It has been found, how- 

 ever, that the discharge is not proportional to the calibre of the tube, but 

 to the fourth power of the diameter. This is evidently to be explained by 

 the greater prominence attributed to the adhesion of the fluid to the 

 walls of the tube, and will, therefore, differ greatly in different liquids. 



The Flow of Liquids Through Elastic Tubes. — When a constant 

 stream passes through an elastic tube the conditions are precisely the 

 same as have been described as governing the movement of liquids 

 through rigid tubes. When, however, the current is intermittent, the 

 elasticity of the tube then comes into play, and decidedly modifies the 

 conditions of movement. 



If a quantity of liquid be forcibly injected into an elastic tube 

 already distended with fluid, the first part of the tube suddenly dilates 

 to accommodate the quantity of fluid propelled into it. 



This impulse communicates a movement of undulation to the par- 

 ticles of fluid, which is rapidly transmitted to all the particles of fluid 

 within the tube. In other words, a wave movement is rapidly propa- 

 gated throughout the entire length of the tube. If the elastic tube 

 be imagined to be closed at its further end, the wave will be reflected 

 from the point of occlusion, and will be conducted to and fro in the tube, 

 gradually decreasing in intensity until it at length disappears. This 

 propagation of the wave should not be confounded with the forward move- 

 ment of the fluid. For when the fluid itself moves the movement of 

 each particle is in the line of the axis of the tube, but in a wave move- 

 ment the motion of the particles is simply one of undulation at right 

 angles to the line of movement, and not of forward movement. 



In a rigid tube, a movement of progression alone exists. In a 

 closed elastic tube, filled with liquid, into which more fluid is suddenly 

 injected, the wave movement alone exists. 



If the peripheral end of the elastic tube be open and more fluid be 

 injected, both movements co-exist ; that is, there is a forward progression 

 of the particles of the liquid added to the wave movement already de- 

 scribed. When the wave movement passes in the same direction as the 

 current, it is called a positive wave; when in the opposite direction, it is 

 called a negative wave. The speed of propagation of the wave is pro- 

 portional to the elastic force of the walls of the tube, while the height 

 of the wave depends upon their extensibility. 



It is evident from the above that the movement of liquids in open 

 tubes will vary according to whether their walls are rigid or elastic. If 

 in a rigid tube a definite amount of liquid be injected, no more or no less 



