72 TEXT-BOOK OF PHYSIOLOGY 



summation of several stimuli. If a second maximal 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 together, 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- 

 jmal 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 

 complete according to the number of 

 stimuli that reach the muscle in a sec- 

 ond of time. When a muscle is stimu- 

 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 corre- 

 sponding 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 completely before the 

 FIG. 32. TRACING SHOWING THE EF- arrival of the succeeding stimulus, and 

 FECTS OF Two SUCCESSIVE STIMULI, a. a' hence remains in a more or less con- 

 WITH GRADUALLY DIMINISHING INTER- trarfpf i ~ fatp Hnrino- whiVh it AYhiHitc * 

 VAL ON A MUSCLE CONTRACTION. To be I] Sta * e > a m S wm cn it exhibits a 



read from below upward. series of alternate partial contractions 



and relaxations. To this condition of 



muscle activity the term incomplete tetanus or clonus is applied. A 

 graphic record of an incomplete tetanus is given in Fig. 33. 



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

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

 which was superposed on the first; the same result followed the third stimu- 

 lus, 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, com- 

 bined with diminished power of relaxation, is termed \.contracture. The 

 tracing also shows that as the stimulus continues, the base line, that con- 

 necting the lowest points of the contractions, gradually rises and takes the form 



