562 MUSCLE-NERVE PHYSIOLOGY 



This experiment does not demonstrate complete exhaustion, but merely 

 fatigue down to a certain level. If an apparatus is previously arranged for 

 direct stimulation of the muscles by electric currents it will be found that the 

 contractions of the muscles still occur after the voluntary power is lost, show- 

 ing that at least a part of the phenomenon of fatigue, possibly the chief part, 

 is located in the nervous tissue rather than in the muscle substance. 



9. Effect of Load on the Height of the Contraction and on the Work 

 of Voluntary Muscle. a. Make a muscle-nerve preparation and arrange 

 it for stimulation, as in Experiment 6 above. Set the induction coil of the 

 stimulating apparatus for an effective supramaximal stimulus. Record the 

 contractions as pairs of vertical lines on the kymograph, the pairs separated 

 by a distance of i cm. Begin with the load of the lever only for the first 

 contraction, then increase the load by steps of 20 grams each until the muscle 

 is no longer able to lift the weight used. Record two contractions for each 

 weight. Use care that no mechanical changes of the apparatus are recorded 

 so as to confuse the record of the contraction on the smoked cylinder. 



b. Repeat the experiment on a fresh muscle, supporting the lever with 

 a tension of 20 grams on the muscle. 



The amount of work done by the muscle at each contraction is the prod- 

 uct of the load in grams times the height in centimeters. The height of 

 the lift can be obtained in this experiment from the height of the record on 

 the drum and the lengths of the recording and power arms of the lever, in 

 which r ./., the length of the recording lever, is to p.l., the length of the power 

 lever, as h., the height of the record obtained, is to c., the actual shortening of 

 the muscle. Compute the exact amount of work done by each contraction 

 (c the contraction times w the weight gives the work), and tabulate on co- 

 ordinate paper. Compare the variation in work done with the variation in 

 amplitude of the contraction. 



10. The Effect of Temperature on the Amplitude of the Muscle Con- 

 tractions. Prepare a muscle-nerve and mount it in Porter's latest form of 

 temperature apparatus. Insert the thermometer bulb beside the muscle. 

 Adjust the levers for vertical records on the smoked paper of the kymo- 

 graph. Begin with a temperature of tap water and gradually lower the 

 temperature of the preparation by adding small amounts of crushed ice at 

 first; later, add ice and salt crystals to the external chamber, siphoning off 

 excess of fluid into a cup. Take care to lower the external temperature 

 very slowly and gradually, say about one degree in two minutes. Stimu- 

 late the muscle with a supramaximal stimulus twice in rapid succession, 

 for i C. of change. Record these contractions as pairs of vertical marks 

 on the drum i mm. apart, separating each pair by a space of i cm. 

 When o C. is reached, or before if the muscle fails to contract at a 

 higher temperature, reverse the direction of the temperature change, 

 gradually but slowly increasing it until the muscle goes into heat rigor, 

 which begins at from 38 to 40 C. 



