THE PHENOMENON OF CONTRACTION. 



19 



animals, as is indicated in the accompanying illustrations. 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 the contraction ;3 proportional 



to 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, Fig. 3. TO show the 



so to speak, through which its shortening ? a Tsot?Sp C fc) of and he HgS 



is accomplished. In the striated fiber this 3 t SStSton f a JL Sen 



Conclusion is Supported by the fact, repre- by ordinary and by polar- 



J . ized light. The figure rep- 



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



-.. . , ,, ,, /T i j_\ i i (beetle) in which the lower 



liquid passes from the ISOtrOpOUS (light) band portion has been fixed in a 



< 



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 fiir die gesammte Physiologic," 18, 1. 



