18 



EXPERIMENTAL PHYSIOLOGY. 



results. This is called the minimal stimulus. In other words it is 

 the least stimulus which will cause a contraction. On the other 

 hand the maximal stimulus is the least stimulus which will produce 

 maximal contraction. When a series of stimuli of increasing strength, 

 beginning with the minimal, is sent into a nerve or muscle, one 

 obtains a graded response (Fig. 3). This is true no matter how 

 finely graded is the increase of the stimulus. (See all-or-none 

 principle, p. 61). 



Experiment 4. Response to Stimuli of Different Intensities. 

 For this experiment employ a muscle which has been cut 

 away from its nerve. Instead of using ordinary stimulating 

 electrodes connect one terminal of the secondary coil to the 



i 



FIG. 3. Contractions produced by graded stimulation. Secondary coil moved 5 mm. 

 at each step. Both make and break shocks employed. Make contraction begins at m. 



femur end of the muscle by wire and in a similar way connect 

 the other terminal to ihe binding post on the muscle lever. 

 Then a bit of fine wire is wound around the tendon and finally 

 fastened to the coarser wire at the binding post on the muscle 

 lever. In this manner a current can be passed through the 

 muscle. 



Starting with the secondary as far as possible from the 

 primary, cautiously move it forward by stages, each time 

 stimulating the muscle with a make and break shock. When 

 the first perceptible contraction is produced you have reached 

 the threshold. Between each stimulation move the drum a 



