542 



A TEXTBOOK OF PHYSIOLOGY 



The length of time taken by a single contraction or twitch varies 

 greatly with the kind of muscle employed. In general, the response 

 of the gastrocnemius of the frog occupies about one-tenth of a 

 second (Fig. 275). The following table gives examples of the time of 

 various muscles : 



Tortoise : Pectoralis major 

 Semi-membranosus 



Frog : gastrocnemius . . 

 Hyoglossus (tongue) 

 Rectus abdominis 



Wing muscle of wasp . . 

 Wing muscle of honey-bee 

 Wing muscle of bumble-bee 



Seconds. 

 1*8 

 0-6 



0-12 

 0-25 

 0-17 



0-009 

 0-005 

 0-004 



The muscle curve may b6 divided into three periods the latent 

 period, the period of contraction, and the period of relaxation. In 

 the ordinary graphic curve of the frog's gastrocnemius these periods 



occupy approximately 



TOO' 



and 



of a second respectively. 



In such a curve, the length of the latent period does not represent 

 the true latent period of the muscle. It is, in reality, much too long, 

 and is due largely to the inertia of the apparatus. This may be shown 

 by contrasting the graphic curve, in Avhich the muscle pulls upon 

 the writing lever, which is made to record upon a smoked surface, 

 with the curve obtained when a muscle is excited, and its. change 

 of shape recorded photographically. Under these latter conditions, 

 the latent period is but 0-001 to 0-005 of a second. This latent period 

 is probably due to the time taken by the impulse to reach sufficient 

 muscle fibres to bring about a change in the shape of the muscle. 



Although in the graphic curve the length of the latent period is 

 mainly due to the inertia of the apparatus, it is partly due to another 

 factor namely, that the contracting muscle is more extensile. 

 With delicate recording apparatus it can be shown that the latent 

 period, represented by a straight line in the ordinary muscle curve, 



should be (\j -shaped, owing to the fact 

 that, as the muscle starts to contract, 

 it becomes more extensile, and is 

 stretched somewhat by the recording 

 lever; thus the true beginning of the 

 period of contraction is as a rule not 

 recorded. 



When the muscle is " indirectly " 

 stimulated that is, when the impulse 

 causing the contraction is sent to the 

 muscle through the nerve supplying it 



the time taken for the impulse to pass down the nerve through the 

 nerve-endings to stimulate sufficient fibres to induce contraction adds 

 considerably to the latent period of contraction. For this reason, 

 the latent period of direct stimulation of a muscle is shorter than 

 that for indirect stimulation of the same muscle. In general, it may 



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FIG. 277. CURVES OF ARRESTED 

 CONTRACTIONS OF UNLOADED 

 MUSCLE. (Kaiser.) 



