116 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of the muscular contractions, but in the length of the Intent period, in the rate 

 at which the muscle shortens, and in the rate at which the muscle relaxes. 

 The etfect of a large number of separate contractions, made in quick succes- 

 sion, upon the rate at which the muscle changes its form during contraction, 

 is illustrated in the myograms reproduced in Figure 47. 



In Figure 47 only the 1st, 100th, 200th, and 300th contractions were re- 

 corded. The perpendicular line marks the point at which the stimulus was 

 given. In this experiment the latent period for each of the succeeding con- 

 tractions is seen to be longer ; the height is lessened ; the rise of the curve of 

 contraction is slowed and the curve of relaxation is even more prolonged. These 

 and certain other changes are to be observed in the records of Figure 48, which 

 were taken in an experiment made under the same conditions as the last, except 

 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. 48.— 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. 48) 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 prolonged, and consequently the total length of the contraction is increased. 

 Such a prolongation of the contraction is known as "Contracture." In con- 

 sidering the greater activity of the contraction process of this 50th contraction 

 as compared with the first, we must recall that it represents one of a series 

 of staircase contractions, such as we noticed in Figure 46. If we examine 

 the 100th contraction (3 of Fig. 48), 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 

 L50th contraction (4 of Fig. 48), and the prolongation of the crest of the* 

 contraction and the decreased rate of relaxation are particularly noticeable. 

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



