68 MUSCULAR TISSUE. 



slightly pressed, so as to flatten out the object, arborescent 

 branchings of the tracheae first attract attention. These air- 

 tubes consist, like the tracheae of mammalia, of parallel rings, 

 and entwine the muscular fibres with a network of fine, dark 

 capillaries, each of which follows a winding or spiral course. 

 The muscular fibres themselves, which either run parallel, or 

 cross each other in various directions, are in active movement 

 In some fibres this movement resembles that of a wave, which 

 rapidly progresses in the direction of its length ; in others, 

 when it is slower, it has a vermicular character. On more 

 careful examination it is seen that, during the progress of the 

 wave, the muscle swells, returning to its original thickness 

 immediately after. It is further observed that the dark paral- 

 lel striae come nearer together during the swelling, and that 

 the interval-s return to their original width after the wave has 

 passed. In the contents of a muscular fibre, when in a state 

 of rest, the following parts can be distinguished : (a) the dark 

 parallel cross stripes, which as we shall find, correspond to 

 thin parallel disks of less refractive isotropous substance 

 (called interstitial disks) ; (b) the portion intervening between 

 these. This, again, appears to consist*of two parts, viz., a 

 broader middle one of dull gray appearance, and on either side 

 of this a narrow, clear layer. The whole is made up of highly 

 refractive, anisotropous contractile substance, which is to be 

 regarded as the essential substance of the muscular fibre. The 

 dark cross-lines do not seem, under high powers, homogeneous, 

 but appear to consist of series of contiguous granules of equal 

 size. Many muscular fibres exhibit no other differences ; in 

 others, it is possible to distinguish lines running longitudinall}' 

 of greater or less extent, and which are so arranged that they 

 come between what appear to be dark granules of the inter- 

 stitial striae. With reference to these granules, it is not to be 

 supposed that the3' actually exist as such ; the appearance is 

 rather to be regarded as expressive of the fact that the dark, 

 interstitial transverse stripes are interrupted by clear, longi- 

 tudinal lines, the interval between the latter remaining dark 

 as, e. gr., in a check of which dark transverse lines are covered 

 b3' light longitudinal lines. In a fresh muscular fibre, as seen 

 under the microscope, the transverse interstitial disks are not 

 placed vertically, as we can satisfy ourselves by using the fine 

 adjustment, but are set at an oblique angle with the long axis 

 of the muscular fibre. In this respect a muscular fibre may be 

 compared to a roll formed of coins of different metals, so ar- 

 ranged that the thin dark disks alternate with thicker light 

 ones. If such a roll is laid on a plain surface, all the coins 

 lean in one direction, and present their edges to the eye, re- 

 garding them from above, just as the disks in a muscular fibre 

 do under the microscope. 



