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TEXT-BOOK OF PHYSIOLOGY. 



until at the crest of the wave it assumes the appearance of a distinct 

 dark band; the latter now becomes clear and bright in comparison. 

 This change in the appearance of the fiber is due to an increase in 

 refrangibility of the bright and a decrease in the refrangibility of the 

 dim band coincident with the passage of the fluid from the former 

 into the latter. There is at the height of the contraction a complete 

 reversal in the positions of the striations. At a certain stage between 

 the beginning and the crest of the wave the striae almost entirely dis- 

 appear, giving to the fiber an appearance of homogeneity. There is, 

 however, no change in refractive power as shown by the polarizing 

 apparatus. When the contraction wave has reached the stage of 

 greatest intensity, there is a reversal of the above phenomena as the 

 fiber returns to its former condition, that of relaxation. 



FIG. 21. SHOWING THE CHANGES IN A MUSCLE AND MUSCLE-FIBER DURING CON- 

 TRACTION. 



Elasticity. During the contraction of a muscle there is a greater 

 or less alteration in its elasticity, as shown by the fact that it is ex- 

 tended to a greater degree by the same weight in the active than in 

 the passive condition. The degree to which the extensibility is in- 

 creased and the elasticity decreased is dependent on the amount of 

 the resisting force. These facts, as determined experimentally, are 

 represented in Fig. 22. Let A B and A b represent the length of the 

 normal unweighted muscle, passive and active states respectively; the 

 line B B', the extension curve of the passive muscle produced by 

 successive weights, 5, 10, 15, 20, 25, 30 grams, differing by a com- 

 mon increment; the line b B', the extension curve of the active con- 

 tracted muscle when weighted with the same weights ; A' B' the length 

 of the muscle when the weight is sufficiently great to prevent shorten- 

 ing. It will be observed from these facts that while the muscle is 

 extended in both the passive and active states by corresponding 



