374 



Mr. J. B. Haycraft. Upon the Cause of [Feb. 3, 



vestigation of mammalian muscle, while in the case of the insect one 

 of 300 diameters is quite enough. 



In the living and dead muscular fibre the whole of its substance is 

 doubly refracting. The observations of some modern observers 

 entirely agree with my own, in that, with crossed Nicols, the crests 

 (dark bands) and the centres of the valleys (bright stripes) appear 

 bright and therefore refract light doubly, and that there are two 

 dark bands on the slopes between them. (See fig. 2.) It does not 

 follow, however, that these two dark bands represent tracts of 

 isotropous substance. This is the point at issue. The dark lines 

 between the valleys and ridges which appear when the Nicols are 

 crossed have been interpreted as marking the positions of cross bands 

 of singly refracting substance, but this is a fault of reasoning. If the 

 fibre were smooth and cylindrical it would then follow, but the fibre is 

 not, as we have already insisted. These bands lie just on the sloped 

 parts of the fibre, those sections in fact which are oblique to the pass- 

 ing rays ; and the explanation is now quite easy, for the extraordinary 

 ray passing through the fibre is naturally deflected at these parts, and 

 does not reach the eye of the observer. Hence the body appears not 

 to transmit them at all at these parts. 



It is not difficult to explain the discrepancies between Brucke's 

 description of the bright stripe and my own. 



It is essential to be very scrupulous in the selection of a fibre for 

 examination. It must not be at all twisted, or sheared in the 

 slightest degree, for then the cross stripes are not at right angles to 

 the long axis, and as their width is several times their thickness (in 

 the length) overlapping will to some extent occur. Thi-s will certainly 

 lead to very confusing results, and the bright centre of the bright 

 stripe (valley) may well be overlooked. Moreover, the fibre should 

 be slightly stretched and as small as possible. 



It has previously been mentioned that in many preparations the 

 fibres split up transversely in a most regular manner, and unless the 

 cover-glass be pressed upon, the little disks remain in position with 

 narrow chinks between them. These chinks will be filled with the 

 isotropous fluid used for mounting, which will lead to very anomalous 

 appearances, and which may perhaps help to account for some of 

 Brucke's statements. These fallacies may be avoided by a study of 

 the fresh fibres of insects' muscle. Dytiscus and Hydrophilus muscle 

 has received a laro-e share of the attention of histoloo-ists, but that 

 from the wasp or blue-bottle fly is quite as good. A leg should be 

 pulled from the trunk of a blue-bottle fly and this again forcibly 

 separated at the middle joint. A piece of muscle will project from 

 one of the segments, which may be cut off and examined in a drop of 

 fluid expressed from the thorax of the fly. The polariscopic effects 

 may then be made clearly out in the still contracting fibres. I have 



