66 



MUSCULAR TISSUE 



[CH. VI. 



ever much rarer than the separation into fibrils; indeed, indications 

 of the fibrils are seen in perfectly fresh muscle before any reagent 

 has been added, and this is markedly evident in the wing muscles of 

 many insects. It is now believed that cleavage into discs is a 

 purely artificial phenomenon. 



Haycraft, who has also investigated the question of muscular 

 structure, concludes that the cross-striation is entirely due to optical 



B 



FIG. 80. A. Portion of a human muscular fibre, x 800. 



magnified; o, a, larger, and b, b, smaller collections; c, still smaller; d, d, the smallest wliic 

 could be detached, possibly representing a single series of sarcous elements. (Sharpey.) 



1$. Separated bundles of fibrils equally 

 , which 



phenomena. The fibrils are varicose, and where they are en- 

 larged different refractive effects will be produced from those 

 caused by the intermediate narrow portions. This view he has 

 very ingeniously supported by taking negative casts of muscular 

 fibres by pressing them on to the surface of collodion films. The 

 collodion cast shows alternate dark and light bands Like the muscular 

 fibres. 



Most histologists have rejected this view, for the behaviour of the 

 dark stripes to various micro-chemical and staining reagents, and to 

 polarised light, is different from that of the light stripes. The 

 difference is therefore not merely one of diameter, but of chemical 

 composition. 



The rapidity of muscular contraction seems to be proportional 

 to the clearness of the cross-striation, and insects' muscles which are 



