TETANUS. 465 



mum curve is produced, arising from the point at which the lever 

 is when the second stimulus is applied (Fig. 190). 



During the latent period a second stimulation produces less 

 marked effect, and is difficult to demonstrate, but if the second 

 stimulus come after an interval of more than g-J-g- sec., summation 

 can be appreciated. 



This summation of effect also takes place when the stimulus is 

 insufficient to produce a maximum contraction, the succeeding 

 weak stimuli give rise to the same extent of contraction of the 

 already partially contracted muscle, as if it were at ite normal 

 length at the time of the second stimulation. The following 

 tracings (Figs. 191-193) show the effects of repeated stimulations 

 applied at the various periods indicated by the numbers on the 

 abscissa line. 



TETANUS. 



If a series of stimuli be applied one after the other, at intervals 

 equal to about half the duration of a single contraction, a sum- 

 mation of contractions occurs, which results in the accumulation 

 of effect until the muscle has shortened to about one-half of the 

 length it attains during a single contraction, or about one-fourth 

 the normal length of the relaxed muscle ; it can then shorten no 

 more no matter how the stimulus be increased in rate or strength. 

 Not having sufficient time between the stimuli it cannot relax, so 

 it remains contracted permanently as long as the stimulus is con- 

 tinued, the various single contractions caused by the repeated 

 shock all being fused together (Fig. 191) ; but if the interval be 

 more than half the time occupied by a single contraction, then 

 the line drawn by the lever will show notches indicating the 

 apices of the fused single contractions (Figs. 192 and 193). 



This condition of continuous summation of contractions is called 

 tetanus, and in all probability is not only the commonest but the 

 only kind of muscular motion that can be produced by the action 

 of the nerves in obedience to the will. All the actions of our 

 skeletal muscles are then made up of the fusion of many single 

 contractions into tetanus, and such motions as appear too quick 



39 



