LABORATORY MANUAL OF PHYSIOLOGY. 



before the relaxation of the muscle is complete, send in a second 

 stimulus. Repeat, decreasing the interval between stimuli until the 

 two contractions merge to form one. This is known as the sum- 

 mation of contractions. How does the height of the two summed- 

 up contractions compare with that of the single twitch? Where 

 the second contraction begins during the relaxation phase of the 

 first, what determines the height of the second ? 



3. Place the primary of the inductorium in circuit with a metro- 

 nome or vibrating rod interrupter. Stimulate the muscle four 

 times per second; five times per second; six times per second; 

 eight times per second; ten times per second; twelve times per 

 second; fifteen times per second ; twenty times per second. Allow 

 the muscle sufficient rest between the series of stimuli in order to 

 avoid fatigue. 



How do the contraction curves change as the frequency of stim- 

 ulation is increased ? How many stimuli per second are needed to 

 cause the individual curves to merge into one apparently contin- 

 uous curve ? The latter condition is called tetanus. Where the in- 

 dividual twitches are still visible, the condition is one of incomplete 

 tetanus. 



4. That this condition of complete contraction is apparent rather 

 than real may be shown by stimulating the nerve of a muscle and 

 connecting the muscle itself with a capillary electrometer which 

 will show a series of action currents corresponding to a series of 

 single twitches. The capillary electrometer and the action cur- 

 rent of muscle will be described later. 



5. Influence of Temperature on the Production of Tetanus. 

 Repeat the previous experiments with a muscle cooled to 5 C.; 

 with a muscle warmed to 35 C. How does the frequency of stim- 

 ulation necessary to produce tetanus in the cold muscle and in the 

 warm muscle compare with that required for the muscle at room 

 temperature ? 



6. Influence of Fatigue on the Production of Tetanus. 

 After the muscle has become tired from repeated stimulation, re- 

 peat the foregoing experiments. Result ? Explain. 



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