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MUSCLE-NERVE PHYSIOLOGY 



tions, but unless the stimuli become very rapid indeed, the muscle will still be 

 in a condition of vibratory contraction and not of unvarying contraction. 



If the shocks, however, be repeated at very short intervals, varying, in the 

 frog, from eighteen to thirty per second, the muscle contracts to its utmost 

 at once and continues at its maximum contraction for some time. The 

 lever rises almost perpendicularly and then describes a straight line, figure 

 331, c. The rate of stimulation required increases with the rapidity of the 

 simple contraction. If the stimuli are not so rapid, the line of maximum con- 

 traction becomes wavy, indicating a tendency of the muscle to relax during 



FIG. 331. a, Frog's gastrocnemius muscle stimulated with four induction shocks per second, 

 showing complete relaxation between stimuli ; b, same muscle stimulated eight times per 

 second, showing partial relaxation between stimuli (incomplete tetanus); c, same muscle 

 stimulated twelve times per second, showing development of an almost complete tetanus. 



the intervals between. the stimuli, figure 331, b. As the muscle becomes 

 fatigued, a less rapid rate of stimulation is required to produce a complete 

 tetanus, owing to the prolongation of the relaxation period in such a muscle. 

 The height of the contraction, however, is lessened. This condition of pro- 

 longed relaxation is known as contracture. 



Coordinated Muscular Contractions. In the human body the skel- 

 etal muscles contract only on stimulation through their motor nerves, i.e., 

 under the influence of nerve impulses that have their origin in the central 



