336 THE OUTGO OF ENERGY 



muscle, which is rich in sarcoplasm, is about 

 1.0 sec., while in the white gastrocnemius a 

 "clear" muscle it is 0.25 sec. In the frog, the 

 contraction period of the hyoglossus is 0.205, 

 the gastrocnemius 0.120, and the gracilis 0.108 

 sec. (Cash). The latent period is longer in the 

 red muscles. The amplitude of contraction is 

 less in the red than in the white. 



The mixture of quickly and slowly contracting 

 fibres in the same muscle is sometimes obviously 

 an advantage. Thus in certain bivalves the quick 

 fibres in the shell-closing muscle close the shell 

 rapidly, and the slow fibres keep it closed after 

 the contraction of the quick fibres has ceased. 



The form of the contraction is influenced by 

 the mixture of fibres. The clear fibres reach 

 their maximum shortening sooner than those 

 rich in sarcoplasm. In some instances, indeed, 

 the contraction curve may show two summits. 

 These differences may perhaps explain the char- 

 acteristic differences in the form of the contrac- 

 tion wave of different muscles, observed by Cash 

 and others. The white fibres are more easily 

 fatigued than the red. Thus the triceps humeri 

 of the rabbit contracts at the beginning of stimu- 

 lation like an unmixed white muscle (quickly), 

 but later like a red muscle (slowly). 



The Excitation Wave. Prepare a plate oi 



