74 EXPERIMENTAL PHARMACOLOGY' 



and thus let the signal magnet record the moment and du- 

 ration of stimulation rather than the time. After the 

 heart records have been secured then the time tracing (in 

 five second, etc., intervals) can be put on the drum. The 

 electrodes should be adjusted under the vagus nerve in 

 such a way that the} 7 need not be disturbed or moved in 

 any way when one desires to stimulate the nerve. Keep 

 the nerve moist with normal salt solution. The record 

 should be made in such a manner that it will read from 

 left to right and from bottom to top. The amplitude of 

 the beats should be about one inch. The drum should turn 

 at a moderately slow speed. When all adjustments of the 

 apparatus are completed record about one or two inches 

 of the normal tracing and then stimulate the vagus nerve. 

 The right nerve usually is more effective in stopping the 

 heart than is the left. Allow the heart to recover (the 

 drum should be kept going all the time) and when about 

 one or two inches more of normal record have been ob- 

 tained, stimulate the vagus (or the opposite vagus) again 

 and thus get two sets of records. If there are three stu- 

 dents in "the group it is often advisable to thus make three 

 sets of records so that each student can have one. The 

 cardiac inhibitory powers of the vagi in the turtle are not 

 nearly so easily exhausted by electrical stimulation as are 

 those of the frog. 



When enough normal tracings have been secured lower 

 the drum so that about one-half inch will be left between 

 the first round of the record and the second and start over 

 from the beginning (left hand side) of the record again. 

 When the second record reaches a point about one inch 

 to the left of the place where the vagus nerve was stimu- 

 lated in the first round then rapidly drop ten drops of 

 a solution of ether in salt solution on the heart as was 

 done with the frog's heart in Experiment II (1). When 

 the record comes directly over the place where the vagus 

 nerve was stimulated in the first round of the tracing stim- 



