HISTOETCAL. 297 



proportionately increased in breadth (fig. 342, B). Subsequently, however, recognising the 

 existence of longitudinal elements within the muscle-prism, Krause described the fluid as 

 passing between these and separating them more from one another during contraction (C). 

 About the same time (1868), Hensen described the stripe which bears his name. 



Fig. 342. DIAGRAMMATIC REPRESENTATION OF 



A MUSCLE-CASK UNDER A VERY HIGH MAG- ABC 



NIFYING POWER, (W. Krause.) 



A, at rest ; B, condition in contraction, 

 former view; C, condition in contraction, pre- 

 sent view, an, muscle-prism, consisting of a 

 bundle of muscle-rods ; is, fluid isotropous 

 substance. 



The next prominent writer upon the subject was Merkel (1872), who described the trans- 

 verse membranes of Krause as being double, and who corroborated Hensen's description of 

 the existence of a thin line or disk in the middle of the dark stria. But the most important 

 difference in Merkel's account occurs in his description of the process of contraction. Accord- 

 ing to Merkel, the anisotropous substance of the dark stria first of all becomes diffused over 

 the whole muscle compartment, so that the fibre acquires a homogeneous appearance, and 

 then at a later stage becomes accumulated against the transverse membranes, while the 

 isotropous substance on the other hand is accumulated on either side of Hensen's disk, so 

 that the position of the two substances is thus reversed. In a subsequent communication (1881) 

 a somewhat modified view of the changes in contraction was taken by Merkel. 



Merkel was followed by Engelmann (1873), according to whose description, a muscular 

 fibre consists of a succession of superimposed parts or compartments, which are partitioned off 

 from one another by thin disks or membranes Krause's membranes or intermediate disks (fig. 

 341, a, a). Within each compartment thus marked off is a series of disks, varying in their 

 refractive power and in their action upon polarised light, as follows : Next to an inter- 

 mediate disk comes a layer of isotropous clear substance, within which may be distin- 

 guished a thin disk of dark substance (ft), having in ordinary muscles the appearance of a 

 line of dots, the accessory disk (granule-layer of Flogel) ; then comes a broad disk of 

 anisotropous substance (principal disk, <) occupying the greater portion of the muscle- 

 compartment, and sometimes bisected by a narrow pale band, which lies exactly in the middle of 

 the compartment, and is distinguished as the middle disk or disk of Hensen (not seen in the 

 figure). Beyond the broad anisotropous disk come in inverse succession isotropous substance 

 with accessory disk, and intermediate disk, and so on in the next compartment. 



When contraction is about to supervene in any part of a muscular fibre, the changes, 

 which according to Engelmann may be observed, are the following : While the intermediate 

 disks approach one another, the successive disks within each muscle-compartment become less 

 distinct, and the fibre loses in great measure at the part in question (that namely in which 

 the contraction is beginning), its striated appearance. The stage in question was accordingly 

 termed by Engelmann the homogeneous stage (fig. 341, H.). As the contraction progresses, 

 transverse striae again make their appearance, in consequence of the gradual darkening of 

 the accessory disks and concomitant clearing up of the principal disk, so that now each inter- 

 mediate disk with its juxtaposed accessory disk forms a distinct dark isotropous band, these 

 alternating with the narrowed and now bright-looking principal disks of anisotropous sub- 

 stance (fig. 341. C.) The reversal of the striae in contracting muscle is ascribed by Engelmann 

 to changes in refrangibility in the several substances which compose the disks of the muscle- 

 compartment, accompanied by an increase in the volume of the principal disk at the expense 

 of the isotropous substance. 



Both Merkel and Englemann attach considerable importance to the occurrence of the 

 intermediate stage, in which the striaa become indistinct ; but it is probable that the homo- 

 geneous appearance of that part of the fibre which is passing into or out of full contraction 

 is due to a shifting of the longitudinal elements of the muscle, owing to their being unequally 

 pulled upon by the more completely contracted part. A similar mechanical shifting of the 

 muscle-columns, accompanied by disappearance of distinct transverse striation, is often produced 

 in teasing the tissue. Moreover the so-called homogeneous stage is often not observed in con- 

 tracting muscle. It must therefore be regarded as an adventitious appearance. 



Heitzmann (in 1873) seems to have been the first to notice the reticular appearances of muscle 

 which had been treated with gold and acid, but these appearances were first fully described 

 by G-. Retzius in 1881, who showed that the dots or enlargements upon the longitudinal lines 

 of the muscular substance are actually only the optical sections of the fibres of transverse 

 networks, which Retzius regarded with great probability, as extending from and continuous 

 with the protoplasm surrounding the nuclei of the muscle fibre. 



Camoy's theory of the constitution of cell protoplasm of a contractile reticulum and enchy- 

 lema, which was about this time beginning to rise into importance, had a marked influence 



