240 MUSCULAR FIBRES IN POLARISED LIGHT, BY E. BRUCKE. 



become thicker, and the transverse strise to approximate. Each 

 sarcous element must consequently change its form, and be- 

 come shorter and thicker. If such a change of form result 

 from any force acting in an elementary solid body, the opera- 

 tion of that force must extend as far as the individual mole- 

 cules, the optic constants must be changed, and it is not 

 conceivable that they should be so changed that the ordinary 

 and extraordinary ray, after they have traversed equal thick- 

 nesses in the same direction, should present again the same 

 difference in velocity that they offered under similar circum- 

 stances before the change of form. 



But it is quite a different matter if the sarcous elements are 

 groups of solid doubly refracting bodies, of which each indi- 

 vidual remains unchanged in form in the act of contraction. 

 The form of the whole group that is, of the sarcous element 

 is here changed by an alteration in the arrangement of the 

 several corpuscles, just as in a company of soldiers groups of 

 various breadths and depths are produced by changes in the 

 position of the several individuals. In the latter case the 

 optic constants are not altered in the act of contraction, and 

 the rays on this account, if they have traversed equal thick- 

 nesses in the same direction, must constantly exhibit the same 

 differences in velocity, whether the muscle be in the relaxed or 

 in the contracted condition. 



Since we have a measure of the difference of velocity in the 

 colours which appear under the polarising microscope, we are 

 enabled to answer the question experimentally, whether the 

 optic constants of the contractile substance change during 

 contraction to any considerable extent or not. All the investi- 

 gations I have directed to this point have had a negative 

 result; i.e., I have never seen any alteration of colour that 

 could not be entirely referred either to changes in the thickness 

 of the layer traversed, or in the angle which the rays under- 

 going interference make with the optic axis. As, therefore, I 

 have in vain sought after a change of the optic constants, I 

 must maintain that the sarcous elements are not elementary 

 and simple solid bodies, but groups of smaller doubly refractile 

 bodies. These doubly refracting bodies I have called Disdia- 

 clasts, after the phrase employed by Erasmus Bartholin, the 



