MAXIMAL AND MINIMAL STIMULI 27 



exciting apparatus with a contact key in the primary and a Du Bois key 

 in the secondary. The muscle may he stimulated either directly or 

 indirectly. Arrange the electrodes accordingly. Remove the secondary coil 

 to some distance from the primary. Bring the writing-point to the drum 

 surface, and while the latter is at rest close and open the key in the primary. 

 No contraction residts either on make or on break. If one occur move the 

 secondary further from the primary. Gradually move the secondary up to 

 the primary, when a position will be found at which a slight twitch will 

 occur at break. This is recorded as a vertical line on the drum. Now turn 

 the drum by hand through about *5 cm. Move up the secondary coil 1 cm. 

 and stimidate as before. Repeat gradually, increasing the strength of the 

 stimulus and moving the drum after each contraction has been recorded. 

 At a certain position the make shock will be found to cause a contraction as 

 well as the break. After a time it will be found that a further increase of 

 the strength of the stimulus does not lead to an increase in the height of the 

 contraction. 



Fig. 33 records an experiment carried out in this way. It was 

 obtained from a gastrocnemius with indirect stimulation, and a 

 magnification of 5. The first indication of a contraction was on break 

 with the secondary coil at 17 cm. of the scale. This strength of 

 stimulus is called the MINIMAL STIMULUS, and the contraction is 



^^■l«JH£^BL : flilU<~BKEJ*U<>lMt9H Mltia— :»«T: 



Fig. 33. — Heights of Conteaction of a Muscle with Different Steengths of 

 Stimuli. The Numbers Eefer to the Distances of the Secondary Coil. 

 The Interrupted Line above Shows the Instants at which the Prima by 

 Cubeent was Made and Bboken. A Eise in this Line indicates Make, a 

 Fall Beeak. 



also termed minimal ; any strength of stimulus lower than that was 

 for this muscle sub-minimal. As the stimulus was increased it is seen 

 that the contractions on break increased at first rapidly, and then more 

 slowly, but that beyond 9 cm. the height did not increase. The 

 stimulus at 9 cm. was therefore a MAXIMAL STIMULUS. All 

 strengths of stimulus below this were SUB-MAXIMAL. A con- 

 traction on make was first obtained when the secondary coil stood 

 at 13 cm., and this rapidly increased in amount till it reached a maxi- 

 mum at 9 cm. 



