296 



APPEARANCES OF MUSCLE UNDER POLARISED LIGHT. 



which may be drawn regarding its structure and the changes which occur in contraction, from 

 the appearance of stained preparations, and tends to show that the chromatic substance of the 

 earcostyles the substance which forms the sarcous elements is anisotropous, while the sub- 

 stance or fluid of the clear intervals as well as the sarcoplasm is isotropous. In muscles which 

 have been treated with acid and in which the sarcous elements are destroyed all appearance 

 of double refraction is found to have disappeared. 



It has been shown by Ranvier that the appearance of a tissue under polarised light 

 affords, when taken by itself, no guide to its structure. For the same tissue or part of a 

 tissue may appear either light or dark between crossed Nichols, according to the direction 

 and character of the " stress " to which it may have been exposed, in the same way that a 

 film of indiarubber, which is normally isotropous, becomes anisotropous when stretched. 

 Looked at however in conjunction with other facts, and especially with the results of methods 

 of staining, the appearance under polarised light may afford important confirmation, or the 

 reverse, of the deductions which may be drawn regarding structure by the employment 

 of these methods : this is exhibited by the observations upon muscle which have been above 

 detailed. 



Briicke has applied the theory of Bartholin (invented to explain the phenomena of double 

 refraction in crystals of Iceland spar, and which supposes that those crystals are compounded 



Fig. 341. MUSCULAR FIBRE OP AN INSECT, EXHIBITING PART OP A 

 SO-CALLED "FIXED WAVE OF CONTRACTION." (Engelmann.) 



The right half of the figure shows the effect produced by the 

 same fibre when examined under polarised light, the rows of sarcons 

 elements then appearing light on the dark field caused by the crossed 

 Nichols' prisms. 



R, part at rest and extended ; H, part passing into contraction ; 

 C, contracted part, a, intermediate disk ; b, accessory disk ; c, prin- 

 cipal disk. 



of minute doubly refracting particles (disdiaclasts)), to the 

 doubly refracting substance of muscle, and has applied the 

 same name (dlxdiaclasts) to the particles of which he supposes 

 that substance to be composed, and which would appear to act 

 upon the light like positive, uniaxial, doubly refracting crystals. 

 Under certain circumstances, as after the action of water or 

 salt solution, the muscular substance is apt to break down into 

 a cloud of fine doubly refracting particles which are either 

 themselves the disdiaclasts or represent groups of them. 



Historical. Until Bowman published, in the Philosophical 

 Transactions for 1840, his important work on the structure of 

 muscle, the whole subject was exceedingly obscure. The view 

 which Bowman took of the constitution of muscular substance, 

 namely, that it is composed of a series of particles joined 

 together closely side by side into disks, and less intimately iinited 

 end to end into " fibrils," long occupied a dominant position 

 in this branch of histology. Kolliker however (1851), laying 

 stress upon the fact that the muscular substance is much more 

 apt to break up into " fibrils " than into disks, looked upon the 

 appearance of the latter as altogether secondary, and regarded 

 the " fibrils " as the actual elements of the muscle, the alternate 

 dark and light portions in the course of each fibril being of 

 essentially the same nature, although differing somewhat in 

 their optical properties. Afterwards (1867), recognising that 

 the so-called fibrils might be composed of finer elements, or 

 ultimate fibrils, Kolliker was led to term the structures formerly 

 known as fibrils " muscle-columns," the areas of Cohnheim repre- 

 senting the transverse sections of those columns. The fibrillar con- 

 stitution of muscle has also been consistently urged by G.Wagener. 

 W. Krause (1868) introduced an entirely new idea into the conception of the subject, by 

 looking upon the intermediate line in the light stripe as a continuous disk or membrane, 

 united laterally to the sarcolemma, and thus dividing the whole fibre into a series of flat com- 

 partments, these being again subdivided longitudinally by partitions (seen on transverse section 

 as the clear lines bounding Cohnheim's areas), so that little cases (Muskel-Wxtohen) are thus 

 formed (fig. 342, A). Each such case contains, according to Krause, a portion of the dark disk 

 (muscle-prism) in its middle part, and portions of the light disks (fluid) at either end, and 

 Krause supposed that in contraction this fluid changes its situation, becoming shifted to the 

 periphery of the dark substance, and that in this way the muscle is diminished in length, and 



