TISSUES OF THE BODY. 



289 



Later on, its nature was made the subject of more extended research by 

 Krause, so that we may name it the " transverse plate of the transparent 

 zone" of Krause. This cross-line (fig. 279, a) maybe recognised without 

 great difficulty in the living muscles of mammals and the naked amphibia. 

 It is to be seen very distinctly in the muscle fibres of insects, after pre- 

 vious stretching, attaining a thickness at times of 0*0008 mm. After 

 the action of very weak acetic acid it is the source (at least very fre- 

 quently) of the transversely striated marking of the muscle fibre of the 

 vertebrates. 



Krause holds a very peculiar view in respect to the structure of muscle 

 (fig. 280). He regards the dark cross-line just mentioned as the optical 

 expression of a delicate transverse parti- 

 tion springing from the sarcolemma. 

 which divides the interior of the muscle 

 fibre into a number of diskoid compart- 

 ments built up one over the other. The 

 contents of such a compartment, then, 

 would consist from below upwards of 

 (1) half of a transparent transverse 

 zone ; (2), of a dark zone occupying the 

 middle (i.e., of a transverse disk of sar- 

 cous elements); and (3), of another half 

 of a transparent cross zone (see fig. 280). 

 Krause believes also in the existence 

 of a delicate lateral membrane, invest- 

 ing closely the sides of the sarcous 

 elements and ends of its transparent 

 appendages, and uniting with the trans- 

 verse membrane. In this way he sup- 

 poses the elementary structures of the 

 striped fibres to be formed the so- 

 called " muscle caskets." In longitudinal rows they constitute the fibrillse. 

 This author also believes the clear longitudinal and transverse cementing 

 medium to be liquid, and that during contraction the layers of fluid flow 

 from the end surfaces to the sides. 



Almost at the same time, however, Hensen observed the dark transverse 

 zone to be divided in its middle by another transparent 

 cross-line of weaker refracting power (fig. 281, a). This 

 is now known by the name of the " middle-disk" of 

 Hensen. The views regarding its nature are very 

 various. By some (Krause^ Heppner) it is regarded as 

 an optical illusion, while others (MerJcel. Engelmamn) 

 maintain its presence in the living fibre. The last 

 view, of course, does away with the pre-existence of 

 the sarcous elements. They would have to consist 

 either of three portions, two dark terminal, and -a 

 central transparent, or could only be products of coagu- 

 lation, assuming a homogeneous constitution after death, 

 composed of the dark matter of the transverse zone and 

 middle disk. 



Finally, minute granules have been remarked, arranged 

 in rows, at each side of Krause 's transverse lines (Flonel, MerJcel). 

 These rows have been named " accessory disks" (Engelmann) (fig. 282). 



Fig. 280. "Muscle caskets," a; &, flbrillae 

 forming transverse disks at c; c, sarcc- 

 lemraa. 



Fig. 281 Muscle-fibre 

 of the lancelet (am- 

 fhioxus). a, the 

 " middle disk " of 

 Hensen; b, transpa- 

 rent transverse zone 

 (alcohol prepara- 

 tion). 



