50 



EXPERIMENTAL PHYSIOLOGY 



the muscle be made to contract more frequently, and has not received 

 too large a dose of veratrine, an almost normal contraction may be 

 produced after a few stimulations. 



Fig. 



53.— Two Twitches given by a Muscle Poisoned with Veratrine. 

 Time in Seconds. 



THE WORK PERFORMED DURING A TWITCH 



We have already pointed out how the amount of work performed 

 during a twitch can be determined, and have studied some variations 

 which occur on altering the conditions under which the contraction 

 is carried out. It now remains to examine more completely the 

 variations in the amount of work as the load is increased. The 

 amount of work is represented by the product of the load lifted into 

 the height of lift, so that if we simply require the total work per- 

 formed we need only record the heights of the series of twitches. 



Experiment 7. — Arrange the primary coil with a spring key for 

 making and breaking the circuit by hand. Place a Du Bois key in the 

 secondary with two fine wires for electrodes. Prepare the myograph lever and 

 measure the distances of the point of attachment of the muscle and the end 

 of the writing point from the axis. The writing point should be cut so that 

 the ratio of the two is some simple multiple, e.g. 5. Next prepare a gastro- 

 cnemius and attach one electrode to the fixed end, the other to the movable 

 end of the muscle. Adjust the position of the secondary coil until it just 

 gives maximal stimuli. Load the muscle with a tension of 40 grams, apply- 

 ing the weight at a point nearer the axis than the point of attachment of the 

 muscle. The weight required to produce a tension of 40 grams will be 

 found by multiplying 40 by the ratio between distance of muscle attach- 

 ment to axis to distance of weight attachment to axis. This tension will 

 elongate the muscle. The lever must therefore be brought back to the 

 horizontal by altering the position of the fixed end of the muscle. Rotate 

 the same a little to mark the level of the lever. Stimulate with a make 

 shock recording the height of twitch on a stationary blackened surface. 

 Increase the tension to 80 grams and repeat the process. Take a series of 

 heights in this way, each time increasing the load by a fresh 40 grams. 1 



1 The load chosen must depend on the size of the muscle. That of 40 grrns. 

 as here given is the most convenient for a medium-sized gastrocnemius. 



