184 B. T. LOWNR ON THE HISTOLOGY OF 



diffused and broad, whilst the dark bands become narrow, and 

 minute bright dots appear in a line in the centre of the dark band. 



Such appearances indicate, to my mind, difraction phenomena, 

 and are not inconsistent with the structure which I have indicated 

 above, the transverse stria} arising either from the moniliform con- 

 dition of the primitive fibrillse, or from an alternation of more and 

 less highly refractive material in the fibril itself. 



In the imago of the fly two totally distinct forms of muscle 

 exist, which possess well-marked histological characters differen- 

 tiating them from the muscles of the larval insect. These two forms 

 of muscle were recognized by Dr. Weismann,* by myself, f and more 

 recently by M. Viallaines.J They have been distinguished by the 

 last-named author as the muscles of the wings and the muscles of 

 the limbs. Neither term is very fortunate, however. All the 

 muscles of the imago, including those which move the wings, ex- 

 cept in the vibratory movement of flight, belong to the second 

 kind ; I shall, therefore, speak of them as the ordinary muscles of 

 the imago, and I shall designate the first kind as the muscles of 

 flight. 



The muscles of flight occupy the greater part of the thorax. 

 They consist, as M. Vialkines§ has pointed out, of enormously 

 enlarged colonnettes, and are entirely devoid of any investing 

 sheath. In transverse section each muscle consists of a few 

 gigantic fields of Cohnheim, each dotted over with a regular and 

 close punctation of minute highly-refracting granules. In optical 

 longitudinal section each fibre is seen to consist of very fine more 

 or less distinctly beaded fibrillar, embedded in a ground substance 

 of low refractive power, in which rows of very small oval nuclei are 

 distributed. 



The ultimate fibrillas exhibit alternate dark and light bands, 

 which may be regarded, perhaps, as the expression of the monili- 

 form character of the fibres, or possibly of an alternation of sub- 

 stance ; for although the beading of the fibres is often distinct, in 

 other specimens it is not recognizable. 



M. Ranvier|| believes the fibrillar of the wing muscles are capable 

 of still further subdivision. I am unable to determine that such a 



* Weisrnann, " Zeitscli fur ration Med.," Bel. xv., p. 60. 

 f " Anatomy of the Blowfly," 1870, London. 

 X L.c. 



| " Lecon (TAnutomie General sur le System Mnsoulaire,"' Paris, 1880. 



