THE EFFECT OF FATIGUE 497 



then move the induction coil toward the primary i cm. at a time and repeat 

 until the first slight contraction appears. Continue to slide the secondary coil 

 toward the primary, stimulate at each new position, moving the drum forward 



FIG. 353- Record of a Simple Contraction of the Gastrocnemius of the Frog. Time in .01 

 of a second. 52, Moment of stimulation. Record taken on a rapid drum that was provided with 

 an automatic key. 



for each stimulus as directed, until a series of contractions is obtained 

 through the range of variation of induction of which the apparatus is capa- 

 ble, usually twenty to thirty contractions. 



A typical tracing, figure 326, shows that as the strength of the stimulus is 

 increased the amplitude of the contractions quickly mounts from the minimal 

 to a maximal, after which all further increase in the strength of the stimulus 

 produces contractions of practically the same height. The first perceptible 

 contraction is called the minimal contraction, the strength of the current which 

 produced it a minimal stimulus for that preparation. The contractions of 

 the greatest amount are called maximal contractions. The weakest stimulus 

 which produces a maximal contraction is called the maximal stimulus, and 

 all stronger stimuli supramaximal. 



7. The Effect of Fatigue on the Amplitude of the Simple Muscle 

 Contraction. Prepare a gastrocnemius muscle for direct stimulation 

 and mount it in a moist chamber. Arrange the induction apparatus for single 

 stimuli. Adjust the recording lever of the muscle to a smoked-paper kymo- 

 graph and set the speed of the kymograph to revolve at the rate of i mm. per 

 second. Now stimulate the muscle with the make induction (short-circuiting 

 the break) once every two seconds. The contractions will be recorded as vertical 

 marks on the drum in regular order, at a distance of 2 mm. apart, hence very 

 slight changes in amplitude are readily detected. The contractions gradually 

 increase in height for the first ten or twenty, the phenomenon of treppe, then 

 run for from fifty to one hundred contractions of practically uniform ampli- 

 tude, after which there is a gradual but sharp decrease known as fatigue. 

 Repeat the experiment after ten minutes' rest. The former variations occur 

 now very rapidly, indicating that the fatigue effects are only partially recov- 

 ered from. 

 32 



