IRRITABILITY. 453 



upon the frog's gastrocnemius, found that the latent period occupied 

 yj-g- second ; the rise of the curve, or the stage of contraction 

 proper, I ^- second; and the fall or stage of elongation, Y^-Q : or 



FIG. 261. Development of incomplete tetanus and contracture, by indirect 

 stimulation. A gastrocnemius muscle of a frog was indirectly stimulated by 

 breaking induction-shocks, of medium strength, applied to the sciatic nerve. 

 The rate was about 8 per second, as shown by comparison of the seconds traced 

 at the bottom of the figure with the oscillations caused by the separate contractions. 



-jig- second in all. A close inspection of the myogram will show 

 a slight rise after d ; this is due to the elasticity of the muscle, and 

 constitutes the stage of elastic after-vibration or contraction-remain- 

 der. Sometimes more than one of these curves are produced. 

 Since the time of Helmholtz's experiments other observers have 

 found that the true latent period is much shorter, certainly not 

 more than ^Vo" second, and probably even shorter than this, for 



FIG. 262. Effect of rapid excitations to produce tetanus. Experiment with a 

 gastrocnemius muscle of a frog, excited directly with breaking induction-shocks 

 of medium strength, at the rate of 33 per second. The weight was about 15 grams. 

 The time record gives fiftieths of a second (Lombard). 



it is highly probable that changes begin immediately in the muscle 

 upon the receipt of the stimulus, although these changes are not 

 at once apparent. The muscle-curve differs in the muscles of 

 different animals and also in those of the same animal. 



Summation of Stimuli. If a muscle which has been stimulated 

 is again stimulated before the effect of the first stimulation has 



