VOLUNTARY MUSCLE. 31 



strength of stimulus in each case and superimpose the records, 

 taking care always to start at the same point on the drum. 

 A tuning fork record is made and the latent period is also 

 shown by the signal magnet. 



Discuss the variations in rate and magnitude of the phases 

 of the single contractions at the different temperatures. 



Since the temperature of cold-blooded animals depends 

 upon the temperature of the surrounding medium, it is much 

 more variable than that of the warm-blooded animal. What- 

 ever can be said regarding an optimum temperature for frog's 

 muscle can be applied in a modified form-to mammalian muscle. 

 The latter, however, is scarcely ever subjected to as great a range 

 of temperature and the optimum is higher. 



If one goes beyond the temperature at which the muscle 

 remains irritable certain changes will take place due to irre- 

 versible modifications in the muscle cells. The shortening 

 of the muscle fibre due to coagulation of the muscle proteins 

 can be shown by securing a record, upon a very slowly moving 

 drum, of the effect of a gradual increase of temperature. De- 

 scribe the appearance of a muscle so treated. 



Chemical changes can also be shown by comparing the 

 reaction to litmus paper of the cut surface of the muscle with 

 that of a fresh muscle. 



LOAD AND WORK. 



The amount of work which a muscle can do depends upon the 

 load which it carries. If a load is light the muscle does not accom- 

 plish a great deal, even though it contracts to its full extent. On 

 the other hand too great a load overtaxes the muscle so that the 

 load can scarcely be lifted and thus little work performed. There 

 is then for every muscle an optimum load. With this the muscle 

 is able to do the greatest amount of work at each contraction. 

 Experiment 11. Arrange for direct stimulation of a gastro- 

 cnemius muscle fastened to the lever of the tripod. Adjust 

 the after-loading screw so that the muscle picks up the load in 

 the large scale pan as it starts to contract. Choose a stimulus 

 which is approximately maximal. The records are to be super- 



