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TEXT-BOOK OF PHYSIOLOGY 



sarcoplasm. As the contraction wave reaches its maximum the optic proper- 

 ties of the bright and dim bands change. The former now becomes darker 

 and less transparent until at the crest of the wave it assumes the appearance 

 of a distinct dark band; the latter now becomes clear and bright in compari- 

 son. -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 disappear, 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. 



IS 



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306' 



FIG. 22. EXTENSION CURVES: B B', of the resting; b B', of the contracting muscle. 



Change of 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 increased and the elas- 

 ticity 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, in passive and 

 active states respectively; the line B B', the extension curve qf the passive 

 muscle produced by successive weights, 5, 10, 15, 20, 2 5, 30 grams, differing by 

 a common 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 shortening. 

 It will be observed from these facts that while the muscle is extended in 

 both the passive and active states by corresponding weights, the extension 

 during the latter is progressively greater, until with a given weight the length 

 of the muscle is the same. Under such circumstances, there being no short- 

 ening of the muscle, the force of its contraction manifests itself physically 

 simply as tension. In the successive actions of the muscle represented in 

 the same figure there is to be observed also a combination of a change of 

 length and a change of tension, the ratio of the one to the other being deter- 

 mined by the amount of the supported weights. When the weight is slight 



