CH. VI.] VOLUNTARY MUSCLE. 89 



running across the fibre in the middle of each dark band. It is 

 called Hensen's line. 



A muscular fibre may not only be broken up into fibrils or 

 muscle-columns, but under the influence of some reagents like 

 dilute hydrochloric acid, it can be broken up into discs, the 

 cleavage occurring in the centre of each light stripe. Bowman, 

 the earliest to study muscular fibres with profitable results, con- 

 cluded that the subdivision of a fibre into fibrils was a phenomenon 

 of the same kind as the cross cleavage into discs. He considered 

 that both were artificially produced by a separation in one or the 

 other direction of particles of the fibre he called " sarcous ele- 

 ments." The cleavage into discs is however 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 a muscular fibre is built up of contiguous 

 fibrils or sarcostyles, while cleavage into discs is a purely artificial 

 phenomenon. 



Haycraft, who has also investigated the question of muscular 

 structure, has arrived at the conclusion that the cross striation 

 is entirely due to optical phenomena. The sarcostyles are vari- 

 cose, and where they are enlarged different refractive effects will 

 be produced from those caused by the intermediate narrow por- 

 tions. This view he has very ingeniously supported by taking 

 negative casts of muscular fibres by pressing them on to the sur- 

 face of collodion films. The collodion cast shows alternate dark 

 and light bands like the muscular fibres. 



Schafer is unable to accept this view ; he regards the substance 

 of the sarcostyle in its dark stripes as being of different composi 

 tion, and not merely of different diameter, from the sarcostyle in 

 the region of the light stripes ; it certainly stains very differently 

 with many reagents, especially chloride of gold. His views regard- 

 ing the intimate structure of a sarcostyle have been worked out 

 chiefly in the wing muscles of insects, where the sarcostyles are 

 separated by a considerable quantity of interstitial sarcoplasm, 

 and a brief summary of his conclusions is as follows : 



Each sarcostyle is subdivided in the middle of each light stripe 

 by transverse lines (membranes of Krause) into successive por- 

 tions, which may be termed sarcomeres. Each sarcomere is occu- 

 pied by a portion of the dark stripe of the whole fibre ; this 

 portion of the dark stripe may be called a sarcous element.* The 



* Notice that this expression has a different meauing from what it 

 originally had when used oy Bowman. 



