122 PRACTICAL HISTOLOGY. 



change in like manner the relative position of the optic 

 axis of this also, and thus to modify the color of the 

 field of view. 



Transversely striated muscle is not by any means the 

 only tissue which is doubly refracting, for the property 

 is possessed by the white fibrils of connective tissue, and 

 by bone, as well as by the plain muscular fibre cells. 

 But it is the only one which under certain conditions 

 exhibits alternate bands of singly and of doubly refract- 

 ing substance. It lias, however, been pointed out by 

 Ranvier that it is rather the conditions of growth and 

 formation of a tissue than differences of structure which 

 tend to determine differences in the optical properties of 

 the substance of which it may be composed And he 

 instances the case of cartilage, the matrix of which, 

 although undoubtedly composed of the same substance 

 throughout, is doubly refracting in those parts where the 

 cells, either from pressure or in progress of growth, have 

 come to assume either a flattened or elongated shape, 

 singly refracting where they remain rounded. 



Preparation 13. Transverse section of mus- 

 cle. In examining the fresh muscle of the water- 

 beetle, in either of the ways above described, it may 

 happen that a fibre is seen which is turned up 

 towards the observer, and in which, therefore, by 

 focussing the vertical part with the fine adjustment, 

 a view may be obtained just as if the fibre had been 

 cut transversely. It is then said to be seen in optical 

 section ; and for muscular tissue this is the only way 

 in which a section of the fibres, whilst they are quite 

 unaltered, can be observed. Nevertheless, when it 

 is not so much the structure of the fibres but their 

 shape and mode of arrangement into bundles that is 

 to be the subject of investigation, the tissue may be 

 hardened by weak chromic acid (} per cent.), or by 

 alcohol, and thin transverse sections may be cut 

 with a razor and mounted in glycerine, with or 

 without previously staining in logwood. There is 

 another method, that of freezing^ which has luvn 

 much recommended for hardening fresh muscular 



