20 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



appear bright, while the light bands remain dark, as is shown 

 in Fig. 3. From this standpoint the material of the light 

 bands in the normal fibrils is spoken of as isotropous, and that in 

 the dim bands as anisotropous. The anisotropic material of the 

 dim bands consists of doubly refracting positive uniaxial particles, 

 and Engelmann has claimed that such particles may be discovered 

 in all contractile tissues.* The inference made by him is that 

 this anisotropic substance is the contractile material in the pro- 

 toplasm, the machinery, so to speak, through which its shortening 

 is accomplished. Engelmann supports this conclusion by the 

 statement that during contraction the size of the dim bands in- 

 creases at the expense of the material in the light bands. f This 

 theory is indicated in the schema given in Fig. 3. The relative 

 changes in appearance of the anisotropic and isotropic bands 

 during the phase of contraction, which are shown in the figure, 

 may be explained on the assumption that the anisotropic sub- 

 stance absorbs or imbibes water from the isotropic layer. Engel- 

 mann has used such an assumption as the basis for a theory of the 

 shortening of the muscle (p. 72). Unfortunately, the histological 

 changes indicated in Fig. 3 have not been wholly corroborated by 

 later observers. Htirthle f states that during contraction the 

 anisotropic band may shrink to less than one-half its width, while 

 the isotropic layer shows no change. He finds in this appearance 

 a confirmation of the view that the anisotropic substance consti- 

 tutes the active contractile material of the muscle, but there is 

 no evidence, he thinks, to support the assumption that the change 

 in the anisotropic layer is due to imbibition of water from the 

 isotropic layer. 



The Extensibility and Elasticity of Muscular Tissue. Muscular 

 tissue, when acted upon by a weight, extends quite readily, and 

 when the weight is removed, it regains its original form by virtue 

 of its elasticity. In our bodies the muscles stretched from bone 

 to bone are, in fact, in a state of elastic tension. If a muscle is 

 severed by an incision across its belly the ends retract. The 

 extensibility and elasticity of the muscles add to the effective- 

 ness of the muscular-skeletal machinery. A muscle that is in a 

 state of elastic tension contracts more promptly and more effec- 

 tively for a given stimulus than one which is entirely relaxed. 

 Moreover, in our joints the arrangement of antagonists flexors 

 and extensors is such that the contraction of one moves the 

 bone against the pull of the extensible and elastic antagonist. 



* This claim has been denied, see Ellis, " American Journal of Physiology." 

 31, 370, 1913. 



f Biedermann, "Electro-physiology," vol. i, translated by Welby, and 

 Engelmann, "Archiv fur die gesammte Physiologic," 18, 1. 



I Hurthle, "Archiv f. d. ges. Physiologic," 126, 1, 1909. 



