182 
IOWA ACADEMY OF SCIENCE Voi,. XXVIII, 1921 
made an admirable support for the tissue, and in turn served as 
contact terminals for the electrical connections, one through the 
muscle lever at the top, the other through the stopper in the glass 
chamber at the bottom. Thus the preparation when mounted 
had a mitotic spindle-like appearance. 
EXPERIMENTAL 
It was at first necessary to procure what may be called typical 
records under the conditions mentioned. To this end the strength 
of stimulus was the only variable allowed to enter, and we were 
able after considerable experimentation to predict just what effect 
this factor would have on the form of resulting curves in the 
various tissues explored. In general the height of the contraction, 
its duration and the phase of relaxation were prolonged in relative 
proportions (within limits) to the number and rate of stimuli. 
This fact is very clearly illustrated by comparing two typical 
curves of the anterior portion of the frog stomach where in one 
case the stimulus was a tetanizing current * of 2^ seconds duration 
(Fig. 1, PI. V) and in the other of about 5 seconds duration (Fig. 
2). The form of the curves differs only slightly in the relaxation 
phase of the second. Other tissues tried gave similar differences 
when this variable entered although the types of the curves were 
uniform according to the tissue used. This is similar to the re- 
sults obtained by Budington^ in his work on responses of earth 
worm muscles. On the other hand by keeping the strength and 
rate of stimulus constant, uniform repetitions could be made 'at 
will provided, of course, a standard interval of time was allowed 
to elapse between them to compensate for the fatigue factor. This 
is very characteristically shown in the curves of figure 6, which 
were obtained by using the middle of the frog esophagus, a ten 
minute interval being allowed between the two curves here shown. 
In experiments with frog muscles from regions of the alimentary 
tract where rhythmical contractions are frequently encountered, 
it was found best to allow an interval of ten to twenty minutes 
after mounting before Stimulation, so that these contractions 
could gradually pass off. During this interval the rhythmical 
responses gradually become less and less marked as shown in 
figure 5. If the muscle be stimulated before quiescence is reached, 
and sometimes even after this state, the rhythm appears again on 
■ * The rate of tetanizing current used was arbitrarily set at about thirty-eight to forty 
double vibrations per second in this and subsequent experiments. 
