240 SPECIAL HISTOLOGY. 



§ 78. 



The muscular fibres in the trunk and extremities are, in 

 general, so associated, without the existence of any divisions, 



what may be the case in such sections made with the double knife, we do not know, 

 — but in those accidental longitudinal fractures of the muscular bundles of Man, 

 Mammals, aud Insects, which constantly occur, the edges of the discs are most dis- 

 tinct. Again, without making any section at all, the discs may, especially in Insects, 

 be traced, by altering the focus of the microscope through the entire thickness of the 

 bundles. The argument in the text, in fact, proves too much ; for if the fibrils are 

 visible over the whole transverse section, their dark parts (discs) alternating with 

 the light ones, must be as visible in a section, made in any longitudinal plane, as they 

 are on the surface. However, that the appearance of discs should be absent in any 

 longitudinal section of striped muscle is, to us, simply incomprehensible. 



With regard to the thoracic muscles of Insects, it is to be observed, in the first 

 place, that they do not represent ultimate fibrils, but non-fibr Mated primitive bundles. 

 Dr. Auber, in a valuable paper in ' Siebold and Kblliker's Zeitschrift' (H. 3 and 4, 

 1853), has already shown that there is no defined line of demarcation to be drawn 

 between these and the ordinary muscles of Insects, the two forms passing into one 

 another by the peculiar flat bundles of the Libellulidae, though he still considers the 

 thoracic muscles to represent ultimate fibrils. His sole argument, however, is their 

 resemblance to the ultimate fibrils of the higher animals, which we think loses all force, 

 when we consider a fact that he has overlooked, namely, that the muscles of the 

 legs, &c. present a very beautiful, though very delicate, fibrillation — the fibrils being 

 not more than j^j — 30005th °f an i nc ^ m diameter ; that is, not more than from one 

 third to one sixth the diameter of the thoracic muscles (fig. 93 A, 3,4, 5). Examined 

 carefully with a high power (600), with a good definition, the edges of the discs, 

 which under a lower power appeared very sharp and even, are seen to be dis- 

 tinctly granular, and to be composed of minute, somewhat fusiform or rounded par- 

 ticles, not more than ^oath of an inch in diameter, distinct from one another, and 

 imbedded in the general transparent matrix, which is marked by fine longitudinal 

 lines running between the rows of particles. Occasionally, the broad, dark discs appear 

 to be separated by a delicate line ; and this line, if carefully examined, is found to be 

 composed of similar, but far more minute and paler particles (4, 5, 7). However, this 

 appearance, though very common, is not to be met with in all the bundles. Acetic 

 acid swells the muscle up, and renders the sarcous elements still more distinct, 

 though the whole becomes very pale. If dilute ammonia be added to such a bundle, 

 so as to neutralise the acid, it resumes its original dimensions, and almost its original 

 appearance, except that the sarcous elements have often a wonderful sharpness of 

 outline (93 A, 7). 



The thoracic fibres, treated with acetic acid, become exceedingly pale, and the dis- 

 tance between the discs is much increased (93 A, 2). The latter often assume a 

 granular appearance, but not so distinctly as in the former case; nor have we been 

 able to detect any fibrillation of the intermediate substance, nor any minute sarcous 

 elements, in them. They share the former character, however, no less than the 

 latter, with multitudes of unquestionable muscular bundles — so that taking into con- 



