124 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 c-d, Figure 56. 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 excitation 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. 



The experiment, the record of which is reproduced in Figure 57, was made 

 on the gastrocnemius muscle of a frog during the latter part of the winter, 

 and when the muscle had begun to show the effects of spring irritability. A 

 light weight was used. The muscle was first tested with four separate break- 

 ing induction shocks given at intervals of two seconds ; it was then subjected 

 for nine seconds to a tetanizing current; and in order that the condition of the 

 muscle during this period might be ascertained, the tetanizing current was 

 shut off from the muscle by a short-circuiting mechanism for a brief period 

 everv two seconds. Finally, at the close of the tetanus, the condition of the 

 muscle was again tested by single-breaking shocks of the same intensity as 

 those used before the tetanus. The curve reveals many points of interest. 



a. The first four single contractions show the " introductory " effect and 

 the beginning of a "staircase" effect such as is usually observed by serial 

 excitations. 



/;. Each of the short tetani starts with a sharp rise of the curve, making 

 what has been called the "introductory peak." These introductory peaks, 

 which are caused by the throw of the recording lever, give an evidence of the 

 intensity of the summation effects at these times. It is interesting to observe 

 that the first is high, the second low, and the third, fourth, and fifth show a 

 staircase-like growth, which is indication of the fact that excitation increases 

 the activity of the contraction processes. 



c. The amount that the curve falls in the short interval separating the 

 succeeding periods of tetanus reveals the extent of the contracture present at 

 these times. 



