92 MINUTE STRUCTURE OF MUSCLE. [BOOK i. 



lines giving rise to a longitudinal striation, sometimes conspicuous 

 and occasionally obscuring the transverse striation. In the muscles 

 of some insects each dim band has a distinct palisade appearance 

 as if made up of a number of ' fibrillse ' or ' rods ' placed side by 

 side and imbedded in some material of a different nature ; more- 

 over these fibrillae or rods may, with greater difficulty, be traced 

 through the bright bands, and that at times along the whole 

 length of the fibre. And there is a great deal of evidence, into 

 which we cannot enter here, which goes to prove that in all 

 striated muscle, mammalian muscle included, the muscle-substance 

 is really composed of longitudinally placed natural fibrillce of a 

 certain nature, imbedded in an interfibrillar substance of a different 

 nature. In mammalian muscle and vertebrate muscle generally 

 these fibrillae are exceedingly thin and in most cases are not 

 sharply defined by optical characters from their interfibrillar bed ; 

 in insect muscles and some other muscles, they are relatively large, 

 well defined and conspicuous. The artificial fibrillse obtained by 

 teasing may perhaps in some cases where they are exceedingly 

 thin correspond to these natural fibrillse. but in the majority of 

 cases they certainly do not. 



In certain insect muscles each bright band has in it two (or 

 sometimes more) dark lines which are granular in appearance and 

 may be resolved by adequate magnifying power into rows of 

 granules. Since they may by focussing be traced through the 

 whole thickness of the fibre the lines are the expression of discs. 

 Frequently the lines in the bright bands are so conspicuous as to 

 contribute a greater share to the transverse striation of the fibre 

 than do the dim bands. Similar granular lines (rows or rather 

 discs of granules), may also be seen though less distinctly, in 

 vertebrate, including mammalian, muscle. 



Besides these granular lines whose position in the bright band 

 is near to the dim bands, often appearing to form, as it were, the 

 upper edge of the dim band below and the lower edge of the dim 

 band above, there may be also sometimes traced another transverse 

 thin line in the very middle of the bright band. This line, like the 

 other lines (or bands), is the expression of a disc and has been held 

 by some observers to represent a membrane stretched across the 

 whole thickness of the fibre and adherent at the circumference 

 with the sarcolemma; in this sense it is spoken of as Krauses 

 membrane. The reasons for believing that the line really repre- 

 sents a definite membrane do not however appear to be adequate. 

 It may be spoken of as the " intermediate line." 



When a thin transverse section of frozen muscle is examined 

 quite fresh under a high power, the muscle-substance within the 

 sarcolemma is seen to be marked out into a number of small more 

 or less polygonal areas, and a similar arrangement into areas may 

 also be seen in transverse sections of prepared muscle, though the 

 features of the areas are somewhat different from those seen in the 



