CIRCULATION OF THE BLOOD. 523 



can escape from the open end. If the calibre of a rigid tube be dimin- 

 ished, the increased resistance will react on the propelling power, and 

 will likewise prevent injection of more than can escape by the open end. 



Thus, suppose a rigid tube be connected with a pump which throws 

 any definite quantity, say one ounce, of water at each stroke, the rigid 

 tube being supposed to be already distended with liquid. At each 

 stroke of the pump, therefore, one ounce of liquid will escape from the 

 free end. Suppose, now, the free end of the rigid tube be so decreased 

 in calibre as to allow only one-half the previous quantity to escape, this 

 resistance will, therefore, react on the pump and prevent its throwing 

 more than one-half the quantity into the tube. 



If, on the other hand, the walls of the tube be elastic, the conditions 

 will vary according as the resistance is increased or diminished. 



If an elastic tube of the same length and diameter as the rigid 

 tube already experimented with be connected with a pump throwing 

 the same quantity of liquid at each stroke, it is evident that the con- 

 ditions will be the same as in the rigid tube ; that is, the same amount 

 of fluid will escape from the free end as enters at the opposite end 

 from the pump, and the time of injection and escape of liquid will 

 coincide. 



If, now, the distal end of the elastic tube be contracted so as to 

 diminish the outflow, the pump still throwing the same amount of 

 fluid, it is evident that if we say only one-half of the amount injected 

 can escape from the free end of the tube the other half will collect 

 in the tube and overdistend its walls. 



In the intervals of action of the pump, the elasticity of the walls 

 of the tube will lead to their contraction, and this recoil will act as a 

 propelling power on the contents of the tube, and lead to its escape 

 from the end of the tube. The stream, now, instead of being inter- 

 rupted and in jerks, will tend to become continuous. Thus, elastic tubes 

 have the power of transforming an intermittent into a continuous flow. 

 The fluid thus contained in a series of elastic tubes is subjected to two 

 pressures, one derived from the propelling force and the other exerted 

 by the elastic walls, due to the over distention of the tubes. In tubes 

 with elastic walls, the velocity of the current is diminished before the 

 quantity of fluid discharges is increased. 



In the mechanics of the circulation the former of these forces is 

 spoken of as blood pressure upon the walls of the vessels, and is due to 

 the propelling power of the heart; while the second, the force exerted 

 by the walls of the arteries upon the blood, due to the recoil of these 

 vessels, is spoken of as arterial tension. 



4. The Circulation in the Arteries. — The principal cause of 

 the movement of the blood in the arterial system is the intermittent 



