280 



PHYSIOLOGY 



CHAP. 



the limb can only depend ou variations in the blood-stream, or the 

 velocity with which the blood Hows into the arteries at different 

 moments of the cardiac cycle. It is possible from the different 

 inclinations of plethysmographic curves to the axis of the abscissa 

 to construct the velocity curve (as Fick did), and thus to derive 

 the tachygram from the plethysmogram. 



To illustrate this conversion of volume curve into velocity 

 curve, we enlarged the first plethysmogram of Fig. 117 by means 

 of a projection apparatus, subsequently reducing the ordinates by 

 half. This gave the curve represented by the fine line in Fig. 118 ; 

 which has the same form as the plethysmogram of Fig. 117 would 



Fio. 118. The line curve is the first plethysmogram of Fig. 117 enlarged and drawn out. 

 The black curve is the tachygram, constructed graphically by. measuring the degree of inclina- 

 tion of the different sections of the plethysmogram. (Luciani.) 



have assumed had it been recorded on a drum rotating at double 

 speed. From the plethysmogram thus transformed we have 

 graphically constructed the tachygram represented by the black 

 line of Fig. 118, which has a very similar course to that of the 

 tachygram recorded by Chauveau and Lortet with the haemo- 

 dromograph (Fig. 114). It must be noted, however, that the 

 tachygram thus derived from the plethysmogram does not give 

 absolute values of oscillations in velocity, and that the abscissa oo 

 corresponds not to zero velocity but to the moments in which the 

 volume of the forearm was unvaried, because the arterial inflow at 

 that moment balanced the venous outflow. 



In order graphically to transform plethysmograms into tachygrams, the 

 ordinates of which represent the inclination of the different sections of the 

 first curve, the following method must be adopted. At any point P of the 

 plethysmogram draw the tangent to the curve ; measure the trigonometric 



