84 TEXT-BOOK OF PHYSIOLOGY. 



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 (Fig. 32). The effects following both 

 maximal and submaximal stimuli indicate that the muscle cannot 

 attain its maximum of shortening except through a summation of 

 several stimuli. If a second maximal stimulus enter a muscle dur- 

 ing 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 together, and though the effect is but a single contrac- 

 tion, it is larger than either would have produced separately. This 

 is termed the summation of stimuli. 



Still further, if a series of subminimal stimuli, each of which is 

 alone insufficient to produce a contraction of the muscle, be applied 

 in rapid succession, a contraction frequently results. This is termed 

 the summation of subminimal stimuli. 



Tetanus. When a muscle is stimulated by a series of induction 

 shocks at the rate of four or six per second, it undergoes a corre- 

 sponding number of contractions of about equal extent. If the rate 

 of stimulation is increased up to the point when the interval before 

 each stimulus is less than the duration of the entire contraction pro- 

 cess, the muscle does not have time to completely relax between each 

 successive stimulus, and hence remains in a more or less contracted 

 state, during which it exhibits a series of alternate partial contTac- 

 tions and relaxations. To this condition of muscle activity the term 

 incomplete tetanus or clonus is applied. A graphic record of an in- 

 complete tetanus is given in Fig. 33. 



In such a tracing it is observed that the second stimulation, oc- 

 curring before the muscle had time to relax, gave rise to a second 

 contraction, which was superposed on the first; the same result fol- 

 lowed the third stimulus, the fourth, the fifth, and so on. Owing 

 largely to this summation of the contractions there is a gradual rise 

 in the height of the contraction curve. This condition of the muscle, 

 viz., continued contraction, combined with diminished power of 

 relaxation, is termed contracture. The tracing also shows that as 

 the stimulus continues, the base line, that connecting the lowest 

 points of the contractions, gradually rises and takes the form of a 

 curve which increases in height with the stimulation. The apex 

 line, that connecting the highest points of the contractions, also rises 

 at the same time, indicating a continuous increase in the height of 

 the contractions. The duration of incomplete tetanus depends on 

 a variety of circumstances, e. g., character of muscle, rate and strength 

 of stimulation, etc., but mainly on the rapidity with which the muscle 

 becomes fatigued. With the oncoming of fatigue the muscle begins 



