THE EFFECT OF TEMPERATURE 499 



to lower the external temperature very slowly and gradually, say about one 

 degree in two minutes. Stimulate 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 increase it until the muscle goes into heat rigor, which 

 begins at from 38 to 40 C. 



While the muscle is entering rigor, move the drum i cm. for each degree, 

 as before, so as to record the development of that process. 



11. The Effect of Temperature on the Time of the Simple Con- 

 traction. Repeat the preceding experiment, but record the con- 

 tractions by the method described in experiment 5 above, recording a con- 

 traction for every change of 5 C. Measure the time and amplitude of the 

 different contractions, and the phases of the simple contractions, and tabulate 

 them as shown in experiment 5. 



12. Effect of Load on the Height of the Contraction and on the 

 Work of Voluntary Muscle. 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. Re- 

 cord the contractions as a series of vertical lines on the kymograph, 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. Support the lever under a tension 

 of 20 grams. Use care that no mechanical changes of the apparatus are re- 

 corded on the smoked cylinder. Repeat the experiment on a fresh muscle, 

 but do not support the lever. 



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 length of the recording arm and power arm of the lever, in 

 which the length of the recording lever is to the length of the power lever 

 as the height of the record obtained is to the actual shortening of the muscle. 

 Compute the exact amount of work done by each contraction under varying 

 loads, and tabulate on co-ordinate paper. Compare the variation in work 

 done with the variation in amplitude of the contraction. 



13. Tetanus. A continued contraction of a voluntary muscle can 

 be shown to be a fusion of simple muscle contractions. This is called a tetanus. 

 Prepare a muscle -nerve in the moist chamber and arrange the induction coil 

 for stimulating with a series of rapidly repeated stimuli. The rate of the 

 stimulation is obtained from the tetanometer, a form of key for rapidly in- 

 terrupting the current, which should be connected with the primary coil in- 

 stead of the key, K, figure 351. Stimulate the muscle at a rate of 10 per 



