PLETH Y SMOG RA PH IG EXPERIMENTS. 
455 
of an intensity of 100. In No. 17 A, B and C the intensity employed 
was 50. The rates per second employed were as follows : 
For 17 A 2 interruptions. 
17 B 21 
» x t JJ "2 » 
„ 17 G, a 5 „ 
» 17(7,6 7* 
The critical point is situated at 17 B ; every increase in the 
frequency of the shocks above that point is accompanied by an increase 
of the contracting and a lessening of the dilating effects, until con¬ 
traction alone is obtained. 
No. 14 exhibits the effects of variation of the third factor, the 
duration of the stimulus. The nerve had been cut nine days. Tetanic 
irritation of the same intensity was used for each stimulation. The 
duration of the stimulus varied from \ a second to SO seconds. The 
longer the duration, the greater the contraction. After the shorter 
durations (£", 1", 3",) a dilatation follows the contraction. 
No. 15 shows contrasted effects produced by different kinds of 
stimulation. The effect at a was due to two induction shocks in a 
second, having an intensity of 100 ; 6, c and d are the effects of tetanic 
irritation of variable intensity. The critical point for the last three is 
placed at d. 
Double tracings obtained simultaneously from both legs of the frog 
show that the effects of irritating the peripheral end of the cut sciatic 
of one leg are local and are not due to any effect upon the heart. 
This is also proved in a more conclusive manner by the experiments 
that we have performed in which tracings of the heart were obtained 
with those of the legs. 
The number of plethysmographic tracings that have been taken 
in this series of experiments is very great. Many frogs were used. 
Those which gave the best results were lai'ge specimens of our native 
bull-frog (rana pipiens). A large share of the observations were made 
in the winter ; some were made in the autumn and spring. 
Conclusion. 
The results of our experiments point to the existence of a vaso¬ 
dilator as well as a vaso-constrictor mechanism in the frog. The 
vaso-motor mechanism is not so active in cold blooded animals as 
in warm, and therefore we are not surprised that we obtain dilatation in