THE VELOCITY OF BLOOD FLOW. 



87 



Fig. 56. — Tachogram ; arm elevated. — v. Kries. 



If the resistance increases, and the output of the heart remains 

 constant, then the arterial tension rises and the velocity becomes less. 



If the output of the heart increases, and the resistance remains 

 constant, then both the tension and the velocity become greater. 



All experiments point to the fact that mere records of arterial tension 

 give us no insight into the velocity of flow. The latter depends, not on 

 the amount of the tension, but on the difference in tension between any 

 two parts of the vascular system. 



Certain attempts have been made to arrive at a method of recording 

 variations in the velocity of the arterial blood in man. If a volume curve 

 of the human arm is taken by the ordinary plethysmographic method, and 

 the outflow of venous blood is constant, then the variations in volume 

 of the passive arm can only be produced by the systolic expansion of the 

 arterial system. Thus, when the volume curve rises the arterial inflow is 

 greater than the venous outflow, and when the curve falls the venous outflow 

 is greater than the arterial inflow. It follows that the more rapid the change 

 of volume, the more rapid 

 must be the variation in 

 blood-flow. If, then, the 

 steepness of the volume 

 curve is estimated at dif- 

 ferent points, an approxi- 

 mate idea of the variations 

 in velocity can be reached, 

 and a velocity curve can 

 be plotted out. The abso- 

 lute worth of the measure- 

 ments cannot of course be 

 obtained, and since the 

 venous outflow is not con- 

 stant the results must be 

 considered as only ap- 

 proximative. It is, how- 

 ever, noteworthy that the 

 velocity curve thus ob- 

 tained bears a striking 

 resemblance to the curves 

 recorded by the dromo- 

 graph. 



v. Kries has con- 

 trived an ingenious and 

 delicate method of thus 

 obtaining the velocity 

 curve in man. The arm 

 is placed in a plethys- 

 mographic chamber in 

 the usual manner, and 

 this is connected by a 



tube with a sensitive gas-burner. Every expansion of the arm drives 

 gas out of the plethysmographic chamber, and causes the flame _ to 

 leap up. The movements of the flame are photographed on a moving- 

 sensitised plate. 



The records, or tachograms, thus obtained are entirely different from 

 the pressure curves obtained with the sphygmograph (see Figs. 56-59). 



Fig. 57. — Sphygmogram ; arm elevated. — v. Kries. 



58. — Taehogram ; arm dependent. — v. Kries. 



Fig. 59. — Sphygmogram ; arm dependent. — v. Kries. 



