EFFECT OF TWO STIMULI 57 



from the first, so that the contraction caused by the first shall have had time 

 to be completed before the second stimulus is sent in. The necessary 

 angular distance will of course depend upon the rate of rotation of the 

 drum. This need not be very rapid — about 10 to 15 cm. per second 

 at the circumference. The contraction will then extend over about 1 cm. 



Pig. 58. — Arrangement of Upper Part of Drum for applying Two Stimuli to 



a Muscle. 



of the paper. The double contraction may now be recorded with the drum 

 in motion. Next move the second contact a little nearer the first and lower 

 the myograph, so that it will record at a fresh level of the smoked surface. 

 Mark the two points of stimulation as before, and then record the two con- 

 tractions. Repeat this several times, on each occasion moving the second 

 contact a little nearer to the first. 



In this manner a series of curves will be recorded similar to those 

 of fig. 59. In 1 and 2 the first contraction was finished before the 

 second stimulation reached the muscle. The effect of the second stimu- 

 lation is therefore a repetition of the first. A closer examination, how- 

 ever, shows certain differences. In the first place the second contrac- 

 tion is higher than the first, 9 - 75 mm. and 9 mm. for 1 ; 10 mm. and 

 9 mm. for 2. Secondly, the total duration of the twitch is longer in 

 the second than the first, 13 - 5 mm. and 11 mm. for 1 ; 15 mm. and 

 13 mm. for 2. In tracing 3 the second excitation fell at about the com- 

 mencement of the last third of the relaxation period of the first contrac- 

 tion and the commencement of the second contraction just before the 

 lever reached the abscissa line. In 4 and 5 excitation occurred during 

 the relaxation period of the first contraction ; in 6 at about the apex 

 of the first, and in 7 during the early part of the period of con- 



