VOLUNTARY MUSCLE. 37 



Summation. If two stimuli are sent into a muscle in close 

 succession so that the muscle has not completely relaxed before 

 the second stimulus is effective, the latter contraction will be 

 higher than the first. The second contraction adds its effect to 

 that of the first, therefore the greatest summation will occur when 

 the second stimulus is effective during the greatest height of the 

 first contraction (Fig. 11). A certain amount of summation 

 results even though a maximal stimulus is used. A series of rapidly 

 repeated stimuli on account of the summation effects, will cause a 

 much higher contraction than is obtained from a single stimulus. 

 Analysis of Tetanus. Voluntary muscle contraction is really 

 a series of twitches occurring so rapidly that relaxation is incom- 

 plete or fails to develop. This action can be analyzed best by a 

 study of two simple twitches occurring close together. 

 Experiment 14. Arrange a muscle in the moist chamber for 

 stimulation by the inductorium. Set an electro-magnetic 

 signal close to and in line with the writing point of the muscle 

 lever. Connect the signal and the kymograph in the primary 

 circuit. Adjust both contacts upon the disk of the kymograph 

 so that two stimuli are sent into the muscle with each revolu- 

 tion of the drum. With the contacts far apart obtain a record 

 of the twitches when the drum is released. Make a series of 

 records in which the stimuli are closer and closer together, by 

 adjusting the sliding contact. Continue this series to the 

 point of complete summation. 



In order to observe the effect of more than two successive 

 stimuli, replace the kymograph by the wheel interrupter in 

 the circuit. Make records on a slower drum, turning the inter- 

 rupter at different rates. With each new trial increase the rate 

 until complete fusion is obtained. 



Next replace the wheel interrupter by the vibrating spring 

 on the inductorium. In this way tetanus can be produced 

 automatically. 



Discuss summation and the development of tetanus. 

 In order to perform this experiment with the Harvard Kymo- 

 graph use the highest speed. A key is placed in the primary 

 circuit but one of the wires is connected to a dissecting needle. 

 Stimulation is obtained by quickly snapping the needle over one 



