48 EXPERIMENTAL PHYSIOLOGY. 



the inductorium arranged for minimal tetanizing currents touch 

 the various sacral nerves with the needle and observe whether 

 the same or different muscles respond in the case of stimulation 

 of each nerve. 



With the aid of figure 4 make a table indicating which 

 muscles are supplied by each nerve. 



B. Cut the trunk of the sciatic nerve high in the thigh and 

 repeat the observations. 



What answer does this experiment give to the question 

 raised in the 1st Paragraph? 



Why is it necessary to do experiment B. before this answer 

 can be given? 



How can you explain excitation by the method of unipolar 

 induction? 



Summation of Inadequate Stimuli. 



An impulse passing along a nerve produces a change in the 

 nerve whether it is sufficient to cause a visible response in its 

 muscle or not. The nerve is left for a very brief period in a more 

 sensitive condition so that a stimulus which is insufficient when 

 applied once, may bring the end-organ into action if applied several 

 times in close succession. 



Experiment 20. Choose a stimulus which is just below the 

 threshold of that necessary to cause contraction of a muscle 

 through its nerve. Now apply a rapid series of these stimuli. 



Velocity of the Nervous Impulse. 



The time required for the passage of an impulse along a nerve 

 trunk can be determined by means of a sensitive galvanometer or 

 by the method described in the experiment below where the response 

 of a muscle is used. 



Helmholtz, by means of a myogram of the thenar muscle when 

 stimulated through the median nerve at the axilla and then at 

 the wrist, estimated that the rate in man was 30-35 meters per 

 second. 



Piper, by means of the string-galvanometer, has studied the 

 same nerve and found the rate to be as high as 125 meters per 

 second. 



