290 



MUSCULAR TISSUE. 



Although such a network as the one which is shown in fig. 330, B, with polygonal meshes, 

 is characteristic of the transverse section of the muscular fibres of vertebrates and of those 

 of some insects, the fibres of many insects have the appearance in transverse section which 

 is shown in fig. 330, C, in which the lines of the apparent network, i.e., of the sarcoplasm, 

 are disposed radially, and the muscle-columns also therefore have a radial disposition and a 

 flattened shape. They are generally, however, subdivided by econdary septa of sarcoplasm 

 (not shown in the figure). 



When a muscular fibre is examined in the fresh condition in serum, fine longi- 

 tudinal lines are seen, as before mentioned, running through the cross strise (see 

 figs. 327, 331). Under favourable conditions, and especially after the action of 

 weak acid, which swells the muscular substance and renders it clearer and more 



Fig. 331. LIVING LEG-MUSCLE OF WATER-BEETLE (DYTISCUS MAR- 

 GINALIS). HIGHLY MAGNIFIED. (E. A. S. ) 



s, sarcolemma ; a, dim stripe ; b, bright stripe ; c, row of 

 lots in bright stripe, which are enlargements or thickenings 

 on the longitudinal septa of sarcoplasm. These septa are repre- 

 sented by the longitudinal lines, d. The continuity of these 

 lines through the bright stripe is difficult to see in the fresh 

 fibre, but after treatment with acid it becomes quite distinct. 



transparent, these lines can be traced from end to 

 end of the fibre between the muscle-columns (fig. 

 332). By careful focussing it can be made out 

 that the lines are really the optical section of the 

 planes of separation between the sarcostyles, that 

 is to say, they are the optical effect of the intro- 

 columnar substance or sarcoplasm. The sarcoplasm, 



then, has in transverse section of the fibre, the appearance of a network ; in longi- 

 tudinal optical section the appearance of fine parallel lines ; both these appearances 

 are exhibited in the disk shown in fig. 330, B. It may easily 

 be understood how these two effects would be produced by 

 the presence of a small amount of interstitial substance 

 lying between closely packed prismatic columns. 



In most muscular fibres the sarcoplasm further exhibits 

 a peculiarity of arrangement which has a very characteristic 

 influence upon the optical appearance of the fibre. As is 

 shown in the longitudinal view of the fresh muscle (figs. 327, 

 331), and still more strikingly in the longitudinal view of the 

 muscle which has been treated with acid (figs. 329, 332), the 

 lines which represent the intercolumnar sarcoplasm exhibit 

 enlargements at regular intervals upon their course. These 

 enlargements lie in the bright cross striae, either near its junc- 

 tion with the dim cross-striae as shown in figs. 327, 331, or 

 in its middle : in the former case the enlargements form a 

 double row in each bright stria, in the latter case they may 

 be blended into a single row (as in fig. 340). In the longi- 

 tudinal optical section these enlargements of sarcoplasm have the appearance of dots 

 upon the fine longitudinal lines which run through the muscle : in the more extended 

 fibres or parts of a fibre, these dots are in double rows, in less extended parts they 

 are thicker and blend together in the middle of the bright stria ; this difference is 

 well exhibited in fig. 339 at the parts marked R and I respectively. 



The dots of the longitudinal view correspond to the coarser transverse networks 

 which are seen near the surface of the separated disks (fig. 330, B), while the fine lines 



Fig. 332. MUSCULAR 



FIBRE OF AN INSECT'S 

 LEO AFTER SHORT TREAT- 

 MENT WITH DILUTE FOR- 

 MIC ACID. (E. A. S. ). 



S, Sarcolemma ; D, dot- 

 like enlargement of sarco- 

 plasm ; K, Krause's mem- 

 brane. 



