114 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



that the rate of excitation was 80 per minute, instead of 23, as in the preced- 

 ing experiment, and the record of every 50th contraction was recorded. 



FIG. 45. Effect of frequent excitation on the form of separate contractions. The method employed 

 to obtain this record is the same as in the preceding experiment, except that the drum is revolving more 

 rapidly, and every 50th contraction is recorded : 1 marks the first contraction ; 2, the 50th : 3, the 100th ; 

 4, the 150th ; 5, the 200th ; 6, the 250th ; 7, the 300th. 



A comparison of the first with the 50th contraction gives a number of 

 points of interest. The stimulating effect of action upon the contraction pro- 

 cess is shown by the fact that the latent period of the 50th (2 of Fig. 45) is 

 shorter than that of the first, the rise of the curve is somewhat steeper, and the 

 height is considerably greater. It is noticeable, however, that the crest is pro- 

 longed, and consequently the total length of the contraction is increased. In 

 considering the greater activity of the contraction process of this 50th con- 

 traction as compared with the first, we must recall that it represents one 

 of a series of staircase contractions, such as we noticed in Figure 43. If 

 we examine the 100th contraction (3 of Fig. 45) we see the evidences of the 

 beginning of fatigue ; although the latent period is nearly as quick as in the 

 first, the rise of the curve is less rapid, the height is less, and rate of relaxation 

 is very much slowed. These changes are to be seen in a more marked degree 

 in the 150th contraction (4 of Fig. 45), and the prolongation of the crest of 

 the contraction and the decreased rate of relaxation are particularly noticeable. 

 The same sort of differences are to be observed in the later contractions. By 

 still more rapid rates of excitation these alterations in the contraction curve 

 are not only exaggerated, but develop more quickly, and play a very important 

 part in producing the peculiar form of continued contraction known as tetanus. 



(3) Effect of Frequent Excitations to Produce Tetanus. As we have seen, the 

 normal muscle the first time that it is excited relaxes almost as quickly as it 

 contracts, but if it be excited rhythmically a number of times a minute, gradu- 

 ally loses its power of rapid relaxation. The tendency to remain contracted 

 begins to show itself in a prolongation of the crest of the contraction curve, 

 even before fatigue comes on, and increases for a considerable time in spite of 

 the effect of fatigue in lessening the height of the contractions. If a skeletal mus- 

 cle of a frog be excited many times, at a rate of about once every two seconds, 

 the gradual increase in the duration of the contractions will have the effect of 

 preventing the muscle from returning to its normal length in the intervals be- 



