134 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the other with deep focusing, which bear the same relation to one another 

 as the positive and negative of a photograph. They show that we have 

 hero a case of an optical effect. There are, however, frequently to bo 

 found figures in which the dark appearance of the striatums is to bo 

 regarded as a true darkness of the anatomical structure ; not the ex- 

 pression of diffraction, but an absorption of the light-rays. Sachs * says, 

 • The dark colour of the contractile substance rather depends principally 

 upon tho opposition offered by the very dense gelatinous mass to the 

 passage of light ; the greater part of tho incident light between <ti and o- 3 

 is absorbed.' 



Sachs is hero speaking of the fresh living musclc-fibrc,f in which 

 the doubly refracting substance, at least under ordinary conditions and 

 with the ordinary adjustment, does in fact appear dark. I must, however, 

 deny this and similar statements to the effect that there is anywhere in 

 the living muscle-fibre a substance which ' absorbs tho greater part of tho 

 incident light.' All parts of the fibre which arc not granular arc rather 

 to be regarded as absolutely transparent in layers of the thickness 

 with which the Microscope is concerned, i. e. if there is an absorption it 

 is not appreciable. The ' dark layers ' which are not granular, and also, 

 of course, tho ' bright layers,' are always optical effects. If there were 

 an apjireciable absorption it would also be observed when the light 

 travels parallel to the axis of the fibres. Since a reflection of the rays 

 must take place where there are granules in the fibre, it is an open 

 question whether light is absorbed by the granules. 



The second source of error, which seems to me to be too much over- 

 looked, takes effect when the fibres are examined in polarized light ; not 

 every bright lino which is seen between crossed nicols is necessarily to 

 be regarded as the expression of a doubly refracting layer. 



The plane of polarization is also turned by diffraction, and it is 

 impossible to say whether in this case the rotation of the plane of 

 polarization does not also take place by refraction and reflection. In 

 some fibres examined for this purpose I have found the maximum bright- 

 ness from Q and Z between crossed nicols to be always in the same 

 azimuth, which contradicts such an explanation of the layer Z which is 

 generally regarded as doubly refracting. 



Finally, there is one remark which I cannot refrain from making. 

 It is fully established, in my judgment, as I have said, that there are 

 living muscle-fibres for which the old idea of composition by alternate 

 layers of singly and doubly refracting substance does not hold good ; 

 several layers can be distinguished. On the other hand, however, we 

 must not ignore the fact that living fibres are observed in which only 

 two old layers can be seen with certainty, and that this is the more 

 certain in proportion as the fibres (assumed to be living) are more fresh. 

 It may well be asked what then is essential and typical in muscle- 

 fibres. One may well hold the view that it is more natural to assume 

 that in certain cases we fail to distinguish a part of the layers than 

 to imagine an irregularity in the structure of the fibres. We must 

 remember, however, that Eollett's investigations did not in general 

 establish a type of numerous layers, but that the image varies from one 



* Reichert and Du Bois Reymonrl's Arch., 1872, p. 633. 



t This is not expressiy stated, but follows from the fact that in the passage quoted 

 he is opposing Heppner, who speaks expressly of the living fibre. Arch. f. Mikr. 

 Anat., v. (1869) p. 139. 



