viii BLOOD-STREAM : MOVEMENT IN VESSELS 239 



blood -flow and making it continuous. A large Mariotte flask 

 raised to a certain height by a wooden block is employed, having 

 an orifice at its base opening into a flexible lead tube, divided into 

 two branches. One of these is attached by a short rubber junction 

 to a long narrow glass tube, somewhat pointed at the end ; the other 

 branch is continued as a fine tube of elastic rubber of the same 

 diameter as the glass tube, ending in a short glass mouthpiece 

 with the same aperture of outflow as the other (Fig. 90). When 

 the tap is opened and the water contained in the Mariotte flask is 

 allowed to flow out through the two tubes, one having rigid, the 

 other elastic walls, the amount of fluid escaping simultaneously 



Fi<;. 90. Mosso's apparatus for demonstrating the effect of an intermittent How on two tubes, 

 one having rigid, the other elastic, walls. 



from the two tubes will be equal, since the two orifices are equal in 

 diameter. This proves that when hydrostatic pressure is continuous 

 and uniform, elastic tubes act like rigid tubes. But if the action 

 of hydrostatic pressure is rendered intermittent by rhythmically 

 opening and closing the compression lever carried by the apparatus, 

 as shown in the figure, it will be seen that the glass tube expels 

 the water intermittently from its mouth, while the elastic tube 

 yields a continuous and regular flow. Uniformity of current is 

 thus shown to be due to the elasticity of the tube. 



The impulse imparted to the blood by the elastic reaction of the 

 vessels is not a new force added to that developed by the heart 

 during its systolic output ; it is only the restitution of that part of 

 the impulsive force of the heart which was applied to throwing the 

 arteries into elastic tension (Berard). Yet even if the elasticity of 

 the arteries adds nothing to the sum of the driving force of the 

 heart, it still diminishes the sum of the resistance opposed to the 



