THE PHENOMENON OF CONTRACTION. 



19 



Disregarding the nuclei, the muscle plasma consists of two 

 different structures : the fibrils, which are long and thread-like and 

 run the length of the fiber, and the inter- 

 vening sarcoplasm. The fibrils consist of 

 alternating dim and light discs or segments, 

 which, falling together in the different fibrils, 

 give the cross-striation that is character- 

 istic. In mammalian muscles the fibrils are 

 grouped more or less distinctly into bundles 

 or columns (sarcostyles), between which lies 

 the scanty sarcoplasm. The relative amount 

 of sarcoplasm to fibrillar substance varies 

 greatly in the striped muscles of different 

 animals, as is indicated in the accompanying 

 illustrations. The evidence from compara- 

 tive physiology indicates that the fibrils are 

 the contractile element of the fiber, while 

 the sarcoplasm, it may be assumed, pos- 

 sesses a general nutritive function. Among 

 mammals there are certain muscles in which 

 the amount of sarcoplasm within each fiber 

 is relatively large, and this sarcoplasm, 

 having the granular structure common to 

 undifferentiated protoplasm, interferes with 

 the clearness of striation of the fibers. 

 Fibers of this latter sort are usually of a 

 deeper color than those in which the sarco- 

 plasm is less abundant, and the two varie- 

 ties have been designated as the red (more 

 abundant sarcoplasm) and the pale fibers. 

 Muscles containing chiefly the less clearly 

 striated red fibers, for example, the dia- 

 phragm and the heart, are characterized 

 physiologically by a slower rate of contrac- 

 tion and by a relatively small susceptibility 

 to fatigue. The so-called red and pale fibers 

 may occur in the same muscle. The sepa- 

 rate fibrils, like the entire fiber, show two 

 kinds of substance, the alternating dim and 

 light bands, 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 



Fig. 3. To show the 

 appearance of the dim 

 (anisotrppic) and light 

 (isotropic) bands at rest 

 and in contraction, as seen 

 by ordinary and by polar- 

 ized light. The figure rep- 

 resents a muscle fibril 

 (beetle) in which the lower 

 portion has been fixed in a 

 condition of contraction. 

 On the left the relations of 

 the dim and light bands are 

 shown as they appear in 

 ordinary light, in the re- 

 laxed (upper part) , and the 

 contracted (lower part) 

 state. On the right the re- 

 lations of the bands are 

 shown as they appear when 

 placed between crossed 

 Nicol prisms. The white 

 spaces represent the dim 

 bands. 



