454 



MUSCLE PHENOMENA. 



passed, a second curve will be produced, which will be higher 

 than the first (Fig. 260), due to the addition of the second stimu- 

 lation to the first. This is the summation of stimulation, summa- 

 tion of effects, or superposition of contraction. 



Tetanus. When a series of stimuli are added one to another 

 before the effects of the preceding ones have passed, so that the 

 muscle at no time becomes completely relaxed, the condition of 

 tetanus is produced : incomplete tetanus, if the effect of each stimu- 

 lation can be seen in the separate curves (Fig. 261), and complete 

 tetanus, where these have disappeared and their place is taken by a 



TOO 



800 



900 



1000 



1100 



1200 



1300 



1400 



1500 



1600 



1700 



FIG. 263. Effect of fatigue on the height of muscular contractions. The figure 

 is a reproduction of parts of a record of over 1700 contractions made by an isolated 

 gastrocnemius muscle of a frog. The contractions were isotonic, the weight being 

 about 20 grams. The stimuli were maximal breaking induction-shocks, and were 

 applied directly to the muscle at the rate of 25 per minute. Between the first group 

 of 66 contractions and the following groups a rest of five minutes was given ; after 

 this rest the work was continued without interruption for about one and a half 

 hours. The second group of contractions, that immediately following the period of 

 rest, contains the first 20 contractions of the new series: the next group the 100th 

 to the 110th ; the next the 200th to the 210th, and so on (Lombard). 



continuous line (Fig. 262). Voluntary tetanus is the term applied 

 to the normal voluntary contraction of a muscle. The impulses 

 sent out by the nerve-cells are generated and emitted with such 

 frequency as to produce tetanus, and not a simple contraction. 

 The character of the simple muscle-curve is modified by : 



