THE CIRCULATION OF THE BLOOD AND LYMPH 115 



This consists of a plethysmograph connected with the tube of 

 a gas-burner. When the part enclosed in the plethysmograph 

 expands, air issues from the connecting tube, and causes an 

 increase in the height of the flame. When the part shrinks, air 

 is drawn in from the flame, which is depressed. Since the speed 



FIG. 45. SIMULTANEOUS TRACINGS OF THE VELOCITY (UPPER CURVE) AND 



PRESSURE (LOWER CURVE) (LORTET). 



The tracings were taken from the carotid artery of a horse. The curve of 

 velocity was obtained by the dromograph. The dicrotic wave is marked on it. 

 The slightly curved ordinates drawn through the curves indicate corresponding 

 points. 



of the blood in the veins may be considered constant during the time 

 of an experiment, the rate at which the volume of the part alters at 

 any moment is a measure of the pulsatory change of velocity in the 

 arteries of the part. And by photographing the movements of the 

 flame on a travelling sensitive surface, the velocity-pulse is directly 

 recorded. 



FIG. 46. PHOTOGRAPHIC RECORD OF THE VELOCITY-PULSE OBTAINED BY I-HE 

 GAS TACHOGRAPH (v. KRIES). 



The upper curve is the photographic representation of the movements of the 

 flame, and corresponds to the curve of velocity. 



The mean velocity, like the mean blood-pressure, is more 

 variable in the large arteries near the heart than in the smaller 

 and more distant arteries. Dogiel found in measurements taken 

 with the stromuhr (a good instrument for the estimation of mean 

 speed), within a period of two minutes, velocities ranging from 



82 



