212 



CIRCULATION OF THE BLOOD 



In man the variations of velocity in the peripheral arteries can be esti- 

 mated, but no absolute value can be obtained. 



To understand the principle of the method employed, we must bear firmly 

 in mind that the blood in the peripheral veins flows continuously, exhibiting as 

 a rule no variation due to the heart beats or to the respiratory movements. 



If a part of the body for example, an arm be placed in an air-tight cylin- 

 der the cavity of which is connected with a suitable Tecording device, a Marey 

 tambour or manometer (plethysmograph, Fig. 83), a curve is traced in which 

 the separate heart beats appear clearly marked. The variations thus recorded 

 are caused by the variations in the volume of the arm, and the so-called plethys- 

 mographic curve (Fig. 84) is therefore a volumetric curve. 



Since the return flow in the veins is constant, the variations are produced 

 by fluctuations in the arterial flow. When the curve rises the arterial inflow is 



FIG. 84. Plethysmographic curve (the upper black line). Pulse curve (the lower black line), 

 and velocity curve (red) in man, after Fick. To be read from left to right. 



greater than the venous outflow; when it sinks, the inflow is less than the out- 

 flow, and when the curve runs horizontally inflow and outflow balance each other. 



It is clear however that the volume changes of the arm will follow more 

 quickly the more rapid is the flow of blood to the arm. Thus if we estimate 

 the steepness of the changes in different sections of the curve., we can construct 

 a velocity curve from the volume curve (Fick). In Fig. 84 the red line 

 represents the velocity curve derived from the volume curve (the upper line). 

 Its similarity to the curve recorded by means of the hydrometric pendulum, 

 and pictured in Fig. 82, is unmistakable. In both we have, after a sharp 

 rise, a fall, upon which follows an increase in velocity again. The latter 

 coincides in time with the so-called dicrotic wave of the pulse curve (cf. 

 below). 



4. THE ARTERIAL PULSE 



A. THE MOVEMENT OF WAVES IN ELASTIC TUBES 



Imagine an elastic tube, filled and distended with water, to be divided by 

 fixed lines into the segments a, ~b, c, d, e, f, g, h, i (Fig. 85). The piston, we 



