THE PHENOMENON OF CONTRACTION. 



19 



animals, as is indicated in the accompanying illustration. The 

 evidence from comparative physiology indicates that the fibrils 

 are the contractile element of the fiber, while the sarcoplasm, it 

 may be assumed, possesses a general nutritive function. Com- 

 parative histology suggests that, in the 

 fibrils, we possess, so to speak, a mechanism 

 adapted to rapid contraction, and that the 

 perfection of the mechanism that is, the 

 rapidity of its contraction is indicated by 

 the clearness of the cross-striation. The 

 fibril, moreover, shows two kinds of sub- 

 stance, the alternating dim and light sub- 

 stance, and these two materials are obviously 

 different in physical structure as seen by 

 ordinary light. When examined by polarized 

 light this difference becomes more evident, 

 for the dim substance possesses the property 

 of double refraction. When the muscle fiber 

 is placed between crossed Nicol prisms 

 the dim bands 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, while the dim bands 

 are anisotropous. The anisotropic material 

 of the dim bands is composed of doubly 

 refracting positive uniaxial particles, and 

 Engelmann has shown that such particles 

 may be discovered in all contractile tissues. 

 The inference made by him is that this 

 anisotropic substance is the contractile p 

 material in the protoplasm, the machinery, 

 so to speak, through which its shortening 



Fig. 3. To show the 

 appearance of the dim 

 (anisotropic) and light 



is accomplished. In the striated fiber this SdYnTontrSn,^ ien 

 conclusion is supported by the fact, repre- 



by ordinary and by polar- 



. ized light. The figure rep- 



sented in Ing. 3, that during contraction resents a muscle fiber 



(beetle) in which the lower 

 portion has been fixed in a 

 condition of contraction. 

 (Engelmann.) 



liquid passes from the isotropous (light) band 

 into the anisotropous (dim) band.* 



The Extensibility and Elasticity of 



Muscular Tissue. The 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 



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

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



