THE BLOOD-CURKENT. II7 



If the blood were uniformly distributed throughout the vascular system and under 

 the same pressure, it would remain in a position of equilibrium (as after death). 

 If, however, the pressure be raised in one section of the tube, the blood will move 

 from the part where the pressure is higher to where it is lower ; so that the blood- 

 current is a result of the difference of pressure within the vascular system. If 

 either the aorta or the venae cava? be suddenly ligatured in a living animal, the 

 blood continues to flow, but gradually more slowly, until the difference of pressure 

 is equalised throughout the entire vascular system. 



The velocity of the current will be greater the greater the difference of pressure, 

 and the less the resistance opposed to the blood-stream. 



The difference of pressure which causes the current is produced by the 

 heart. Both in the systemic and pulmonary circulation the point of greatest 

 pressure is in the root or beginning of the arterial system, while the point of lowest 

 pressure is in the terminal portion of the venous orifices at the heart. Hence the 

 blood flows continually from the arteries through the capillaries into the venous 

 trunks. The heart keeps up the difference of pressure required to produce this 

 result ; with each systole of the ventricles a certain quantity of blood is forced into 

 the beginning of the arteries, while at the same time an equal amount flows from 

 the venous orifices into the auricles during their diastole (E. H. Weber). 



Donders showed that the action of the heart not only causes the difference of 

 pressure necessary to establish a blood-current, but also raises the mean pressure 

 within the vascular system. The terminations of the veins at the heart are wider 

 and more extensible than the arteries where they arise from the heart (fig. 133), 

 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 ex- 

 tensible as the veins. Thus the total pressure must also increase. # 



Cause of Continuous Flow. The volume of blood expelled from the ventricles 

 at every systole would give rise to & 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 propelling 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 compensa- 

 tion 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 the difference of pressure becomes, 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 phenonema are due 

 to the same fundamental cause. Every pulse-beat causes a temporary rapid pro- 

 gressive 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: Rigid and Elastic Tubes. These facts are easily demonstrated. Tie a Higginson's 

 syringe to a piece of an ordinary gas-pipe. On forcing water through the tube, by compressing 



