GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 121 



traction process in raising the weight. The new aid to high contraction is 

 the support afforded by the developing condition of contracture. 



5. Effect of Gradually Increasing the Rate of Excitation. One of the 

 most instructive methods of exciting tetanus is to send into the muscle a series 

 of breaking induction shocks of medium intensity, at a gradually increasing 

 rate. The record of such an experiment has been reproduced in Figure 53. 



At the beginning of the experiment, a, one complete contraction with a 

 wave of elastic after-vibration was recorded ; this was followed by two con- 

 tractions of less height, " introductory contractions ;" then came three contrac- 

 tions each of which was higher than the preceding, " staircase contractions ;" 

 these were followed by three contractions, which, in spite of the developing 

 contracture, were of less height, " fatigue effect." The rate of excitation at 

 this place was about 17 per second. From this point on, the developing con- 

 tracture supported the muscle more and more and the contraction waves became 

 less and less, until finally, when the rate had become 36 a second, the effect 

 of the separate stimuli could scarcely be detected, although the curve continued 

 to rise. This is as far as the record shows, but the rate was increased still 

 further, and the contraction curve continued to rise, although less and less, 

 until finally an almost straight, unbroken line was drawn. After a little time 

 this was seen to begin to fall, the contracture yielding to the effect of fatigue. 



As the drum had nearly revolved to the place at which the experiment had 

 been begun, the rate of excitation was then slowly decreased. With the lessen- 

 ing rate, the curve fell more and more rapidly, and oscillations began to show 

 themselves. The character of the record during the rest of the experiment is 

 shown in the curve cd, Figure 53. At c the rate was about 17, and at d it 

 was so slow that separate contractions were recorded, nevertheless the curve as 

 a whole kept up. Indeed, even after the excitation had altogether ceased, the 

 muscle maintained a partially contracted state for a considerable time, on 

 account of the contracture effect, which only gradually passed off. 



6. Summary of the Effects of Rapid Excitation which produce Tetanus. 

 Muscle- tetanus is the result of the combined action of a great many different 

 factors, but the essential condition is that the muscle shall be excited at short 

 intervals, so that the effect of each contraction shall have an influence on the 

 one to follow it. This influence is exerted in several different ways : 1. In- 

 crease of irritability resulting from action, and leading to the production of 

 staircase contractions ; 2. Summation of excitation effects, as when each of the 

 succeeding stimuli begins to act, before the contraction process excited by its 

 predecessor has ceased ; 3. Support given by the contracting muscle to itself, 

 especially the support offered by contracture. 



7. Number of Excitations required to Tetanize. The number of stimuli per 

 second required to tetanize a muscle depends largely on the nature of the 

 muscle, for this decides the character of the separate contractions, and, through 

 them, the effect of their combined action. 



The duration of the separate contractions, and the tendency of the muscle 

 to enter into contracture, are the predominant factors in determining the result. 



