STRUCTURE OF MUSCLE 



65 



dark, an apparent reversal being thereby produced in the strias. This 

 reversal is due to the enlargement of tlie rows of dark dots, and the 

 formation by their juxtaposition and blending of dark disks, whilst the 

 muscular substance between these disks has by contrast a bright 

 appearance. 



The wing-muscles of insects are easily broken up into very fine 

 fibres or fibrils, which also show alternate dark and light striae. The 

 number and relative thickness of these differ, however, considerably, 

 according to the amount of stretching of the fibres (fig. 75). Muscle- 

 rods are not seen in these fibres. 



D 



Tn I 



>be 



' h 



FIG. 75. FIBRES OF THE WING-MUSCLES OK AN INSECT. 



The fibres are in different conditions of extension, from A least extended, to D most ex- 

 tended, e, e, chief substance of the fibre ; m, m, intermediate lines or disks ; the light 

 bands, 5c, on either side of these only come to view when the fibre is sufficiently stretched 

 (C) ; with further extension (D), the middle of the dark baud appears lighter, A. 



In muscular tissue which has been hardened in alcohol and certain 

 other reagents, the structural appearances are a good deal altered from 

 those of tlie living muscle, although the cross-striae are still very obvious. 

 There is also a considerable tendency for the fibres to split up longi- 

 tudinally into fibrils, and by some authorities the finest of such fibrils 

 are regarded as the ultimate elements of the fibre. Certain other 

 reagents, such as dilute hydrochloric acid, cause a transverse splitting 

 of the fibres into disks, and these effects of reagents led Bowman to form 

 the opinion that the muscular substance may be in reality composed of 

 minute prismatic particles set side by side in rows or planes to form 

 the disks, and adhering end to end longitudinally to form the fibrils. 

 To these constituent particles of the muscular substance he gave the 

 name of ' sarcous elements.' 



When living muscular fibres are examined by polarised light, the 

 whole of the muscular substance except the muscle-rods is seen to be 

 doubly refracting, looking bright in the dark field produced by crossing 

 the axes of the Nichol's prisms. Contracted muscle and dead muscle 

 show, however, alternate bands of dark and light under those circum- 

 stances. 



Ending of muscle in tendon. A small tendon-bundle passes to 



