160 THE BLOOD-CURRENT. 



system. The terminations of the veins at the heart are wider and 

 more extensible than the arteries where they arise from the heart. 

 As the heart propels a volume of blood into the arteries equal to that 

 which it receives from the veins, it follows that the arterial pressure 

 must rise more rapidly than the venous pressure diminishes, since the 

 arteries are not so wide nor so extensible as the veins. Thus the total 

 pressure must also increase. 



The volume of blood expelled from the ventricles at every systole 

 would give rise to a jerky or intermittent movement of the blood 

 stream 1. if the tubes had rigid walls, as in such tubes any pressure 

 exerted upon their contents is propagated momentarily throughout the 

 length of the tube, and the motion of the fluid ceases when the pro- 

 pelling force ceases. 2. The flow would also be intermittent in 

 character in elastic tubes if the time between two successive systoles 

 were longer than the duration of the current necessary for the compen- 

 sation of the difference of pressure caused by the systole. If the time 

 between two successive systoles be shorter than the time necessary to 

 equilibrate the pressure, the current will become continuous, provided 

 the resistance at the periphery of the tube be sufficiently great to bring 

 the elasticity of the tube into action. The more rapidly systole follows 

 systole, the greater becomes the difference of pressure, and the more 

 distended the elastic walls. Although the current thus produced is con- 

 tinuous, a sudden rise of pressure is caused by the forcing in of a mass 

 of blood at every systole, so that with every systole there is a sudden 

 jerk and acceleration of the blood-stream corresponding to the pulse 

 (compare 64). 



This sudden jerk-like acceleration of the blood-current is propagated 

 throughout the arterial system with the velocity of the pulse-wave : 

 both phenomena are due to the same fundamental cause. Every 

 pulse-beat causes a temporary rapid progressive acceleration of the 

 particles of the fluid. But just as the form-movement of the pulse is 

 not a simple movement, neither is the pulsatile acceleration a simple- 

 acceleration. It follows the course of the development of the pulse- 

 wave. The pulse-curve is the graphic representation of the pulsatory 

 acceleration of the blood-stream. Every rise in the curve corresponds 

 to an acceleration, every depression to a retardation of the current. 



Method. These facts are capable of demonstration by means of very simple 

 physical experiments. [Tie a Higginson's syringe to a piece of an ordinary gas- 

 pipe. On forcing water through the tube by compressing the elastic pump, the 

 water will flow out at the other end of the tube in jets, while during the intervals 

 of pulsation no water will flow out. As the walls of the tube are rigid, just as 

 much fluid flows out as is forced into the tube. If a similar arrangement be made, 

 and a long elastic tube be used, a continuous outflow is obtained, provided the 

 pulsations occur with sufficient rapidity and the length of the tube, or the resist- 



