GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 



59 



bright band is greater proportionally than the diminution in the length of the 

 dim band, a fact which gave rise to the supposition on the part of Englemann 

 that there is at the time of the contraction a passage of fluid material from 

 the bright into the dim band or from the sarcoplasm into the sarcostyles. 

 When the relaxation begins, a reverse change in the dim and bright bands 

 sets in and continues until they regain their former shape and volume. Coin- 

 cidently there is a passage of fluid material from the sarcostyles to the 

 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 



o 



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



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. 



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 of the passive 

 muscle produced by successive weights, 5, 10, 15, 20, 25, 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 



