GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 



69 



ually diminishes, however, until the muscle reaches a maximum of contrac- 



tion. The superposition of the second contraction on the first, the third on 



the second, and so on, is termed summation of contractions or effects. Experi- 



ment has shown that the greatest effect of a second stimulus thaHs, the 



highest contraction is produced when the stimulus is applied during the 



last third of the period of rising energy, when the sum of the two contractions 



is almost twice as great as the first contraction would have been. (Fig. 32.) 



The effects following both maximal and submaximal stimuli indicate that 



the muscle cannot attain its maximum 



of shortening except through a summa- 



tion of several stimuli. If a second max- 



imal stimulus enters a muscle during the 



latent period following the first, the effect 



produced will be no greater than that 



produced by a single stimulus. The 



muscle during this period is said to be 



refractory or non-responsive to a second 



stimulus. If, however, the stimuli are 



submaximal they add themselves to- 



gether, and though the effect is but a 



single contraction, it is larger than either 



would have produced separately. This 



is termed the summation of stimuli. 



Still further, if a series of submini- 

 mal stimuli, each of which is alone in- 

 sufficient to produce a contraction of 

 the muscle, be applied in rapid succes- 

 sion, a contraction frequently results. 

 This is termed the summation of submini- 

 mal stimuli. 



Tetanus. Tetanus may be defined 

 as a more or less continuous contraction 

 of a muscle which arises when the time 

 intervals between the stimuli are shorter 

 than the time of "the contraction proc- 



ess. Tetanus will be incomplete or FIG. 32. TRACING SHOWING THE EF- 

 complete according to. the number of -- gSSfifSSSSStt. 

 stimuli that reach the muscle in a^sec- VAL ON A MUSCLE CONTRACTION. To be 



Ond Of time. When a muscle is Stimu- read from below upward. 



lated directly or, better, indirectly 



through its related nerve by a series of induced currents at the rate of four 

 or six per second, it undergoes a corresponding number of contractions 

 of about equal extent. If the rate of stimulation is increased up to the 

 point when the interval between each stimulus is less than the duration of 

 the entire contraction process, the muscle does not have time to relax com- 

 pletely before the arrival of the succeeding stimulus, and hence remains in 

 a more or less contracted state, during which it exhibits a series of alternate 

 partial contractions and relaxations. To this condition of muscle activity 



