ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 61 



that in both the frog and tortoise different muscles are distin- 

 guished by characteristic differences in the form and process of 

 the curve of contraction. The average duration of contraction in 

 different skeletal muscles of the frog is shown in the following 

 table : 



Yet more characteristic than the duration is the nature of the 

 process (form), as expressed in the myograms. Many of these have 

 such significant forms that they ought in a measure to indicate the 

 species of muscle. The accompanying figure (Fig. 35A) shows 

 how differently gastrocnemius behaves from the triceps and semi- 

 membranosus - gracilis group. These last muscles reach the 

 maximum of contraction soon after the half of their entire 

 contraction period, while gastrocnemius takes two-thirds of its 

 period for contraction, and only one-third for extension. If the 

 following group of the most sluggish frog muscles (Fig. 35s) is 

 compared with these curves, the difference is very striking. 



Fig. 35c of tortoise is even better qualified to show how 

 the contraction curves of different striated muscles may vary in 

 form in the same animal. M. omohyoideus contracts most rapidly 

 in correspondence with its function of withdrawing the head of 

 the animal quickly under the protecting carapace when in danger, 

 while the powerful pectoralis major, which serves the movements 

 of the heavy animal, " begins with an energetic lift, and delays 

 some time at the apex of contraction." Similar differences should 



