PROGRESS OF MICROSCOPICAL SCIENCE. 127 



as volume is concerned, the contractile substance is in excess over the 

 fluid in the state of rest, whereas in the state of contraction the con- 

 trary is the case. 



In polarized light, and under high magnifying powers (600-800), 

 it is to be noticed that the contractile substance as well as the terminal 

 disk, is anisotropous ; whereas all other parts are isotropous. In the 

 state of contraction, ^the median membrane remains dark between 

 crossed Nicol's prisms, it is therefore to be regarded as isotropous. In 

 the intermediate state the whole fibre appears anisotropous, by which 

 it is proved that during this state the contractile substance fills out 

 the whole division. 



In a second memoir in the same journal (vol. ix., part 2), Merkel 

 occupies himself chiefly with the appearances presented by the striped 

 muscular fibres in polarized light during the different stages of rest 

 and contraction. From these observations, he finds that the appear- 

 ances of polarization cannot be studied successfully on intact muscular 

 fibres, but only on very thin parts of them ; and, further, that the 

 straining of muscular fibres with logwood produces effects which are 

 similar to those observed in polarized light. These observations have 

 perfectly affirmed the assertions made by Merkel about the structure 

 of the muscular fibre and the displacement of the contractile substance 

 during contraction. 



T. W. Engelmann describes his microscopical observations on 

 the striped muscular tissue in Pfliiger's ' Archiv,' vol. vii., part 1. 

 He studied the striped muscular fibres of arthropoda. They were 

 observed in a moist chamber without the addition of any reagent, 

 for reagents produced very marked changes. The fibres were 

 in a perfectly fresh and living condition, showing still veiy lively 

 contractions. 



Each muscular fibre is divided into a number of divisions of equal 

 sizes by transverse dark membranes — intermediate disks, which are 

 closely imited with the sarcolemma. Each division contains in the 

 centre a bright, slightly refractive transverse median stripe — median 

 disk of Hensen, on each side of which lies a dim, highly refractive 

 band — the transverse disk ; then comes on each side a bright, slightly 

 refractive band — isotropous substance ; then a dark, highly refractive 

 stripe — the lateral disk ; and finally, again, a thin, bright, slightly 

 refractive band — isotropous substance ; so that each division contains 

 between the two intermediate disks, one median disk, two transverse 

 disks, then two isotropous bands, two lateral disks, and finally, again, 

 two isotropous bands. 



(a) The intermediate disk, or the membrane of Krause, is dis- 

 tinctly to be recognized as a separate structure in the perfectly fresh 

 fibre in the state of rest, when examined without a reagent, and if the 

 height of a muscular division exceeds • 008th part of a millimetre. 

 In those cases where the lateral disk is very dark, and is in close con- 

 tact with the intermediate disk, this latter may easily escape observa- 

 tion ; it can, however, be brought into view by slightly stretching the 

 muscular fibre. The intermediate disk appears under the microscope 

 as a single dark line, being a homogeneous, highly refractive mem- 



