VOLUNTARY MUSCLE. 



29 



reached in muscle, increase beyond 

 that temperature reduces more 

 and more the response of the 

 muscle until at about 38 C. in 

 the frog the irritability of the 

 muscle is lost. Muscle is not alone 

 in possessing an optimum temper- 

 ature for its activities. All tissues 

 have an optimum temperature. 

 This optimum varies iri different 

 individual animals, especially cold- 

 blooded animals like the frog. 

 Winter frogs possess a lower 

 optimum than summer frogs. 



Not only is the height of con- 

 traction changed by temperature 

 rise or fall, but the duration of its 

 phases is changed as well. At low 

 temperatures the latent period and 

 duration of the contraction may 

 be several times the corresponding 

 stages at higher temperatures 

 (Fig. 7). 



Mammals differ from frogs in 

 maintaining a more or less 

 optimum temperature for their 

 tissues independent of external 

 changes. 



Experiment 10.* Fasten, by 

 means of thread or fine wire, 

 the femur of a gastrocnemius 

 preparation to the short arm 

 of the L-shaped glass rod which 

 has previously been fixed by 

 its long arm in the flat-jawed 

 clamp (Fig. 8). Connect the 

 tendon by means of a fine wire 

 to the pulley of the muscle 

 lever, suitably weighted. Con- 



*This experiment may be done with the Har- 

 vard kymograph using the highest speed of the 

 clockwork. 



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