TEXTURES.] ANATOMY 857 far highly magnified. of muscle is found. The fasciculi are separated from each other by a delicate, areolar connective tissue, or perimysium. The size of the fasciculi varies in different localities ; in the hollow viscera they are so large that their arrangement can be observed with the naked eye; but in the skin, the walls of gland- ducts, &c., they can only be seen with the aid of the microscope. If a fasciculus be carefully torn up with needles it can be resolved into its constituent fibres, and the number of the fibres varies with the size of the fasciculus. The non-striped fibres are pale and almost colourless, with soft, ill-defined outlines, from ^Vffth- to ToVffth inch in diameter; they are rounded in form or laterally compressed, and are so easily flattened by artificial pressure, that they have erroneously been regarded as flat or ribbon-shaped fibres. When digested for a few hours in dilute nitric or hydro- chloric acid, and sometimes even without any reagent, the fibres may be resolved into elongated fusiform cells the contractile fibre-cells of Kolliker which vary in length from ^^th to *"6rt;h inch, and which taper off usually into attenuated ends. In the middle of each cell is a characteristically elongated, rod- shaped nucleus, and sometimes the substance of the cell is finely granular, or even faintly longitudinally striped. No cell wall or sarcolemma can be distinguished. In some localities, as was pointed out by Lister in the minute arteries in the web of the frog s foot, isolated contractile fibro- cells are wound spirally around the wall of the vessel. The Transversely Striped fibre is the characteristic tissue of the voluntary muscular system, and is found wherever energetic movements are to be performed. In these muscles the fibres are collected together in fasciculi, which bundles usually lie parallel to each other, and extend from the tendon of origin to the tendon of insertion. Each muscle is invested by a membranous sheath formed of connective tissue, the perimysium externum, which sheath gives off processes that dip into the substance of the muscle, so as to form delicate partitions between the fasciculi, and from these partitions still more slender prolongations of connec tive tissue, named perimysium internum, pass between the fibres. The number and size of the fasciculi vary with the size and texture of the muscle ; in some, as the deltoid and glutceus maximus, the fasciculi are large and coarse ; whilst in others, as the gracilis and omo-hyoid, they are much finer. The number of fibres in a fasciculus varies with its length and thickness, and the fibres which are adjacent to each other in a fasciculus lie parallel. The striped fibres are cylindrical or laterally com pressed; they usually taper off at their extremities, and apparently do not, even in muscles with long fasciculi, exceed l| inch in length. The transverse diameter of the striped fibres varies, in different localities in the human body, from yJ-$-th to ^iVffth inch, according to the measure ments of Kolliker. Much wider differences in diameter are found in the animal series, in insects the fibres being of extreme minuteness, whilst in cold-blooded animals they are much larger than in man and mammals. If a fibre be carefully separated from a fasciculus, and examined microscopically by transmitted light, transverse stripes may be readily seen to extend across it from side to side. These transverse striae are not mere surface marks, but, as Bowman pointed out, pass through its entire thickness, and lie parallel to each other. The stria- tion is due to the structure of the fibre, which consists of dark and light bands or discs, alternately dark and light. The discs differ in optical properties, for, as Briicke s observations show, the light discs refract light singly are isotropic; whilst the dark discs refract light doubly, and consist of an anisotropic substance. Busk and Huxley described in 1853 a dark line passing across the light disc, so as to sub divide it into two halves ; and this appear ance has also been figured by Sharpey, Krause, and others. It is believed to be due to the presence of a strongly refracting stripe in the middle of the feebly refracting disc. More recently Hensen has directed attention to a slender, feebly refracting stripe passing transversely across the strongly refracting disc, so as to subdivide it also into two halves. In addition to the transverse strice, the fibres not unfrcquently show markings which extend longitudinally, but these are irregular in position, do not cor respond to the whole length of the fibre, or necessarily pass through its entire thickness. _ mi, x 11 -x v i i Fifl. 55. A trans- The transverse and longitudinal mark- vu-soiy striped ings indicate that a muscular fibre has a "^uiar fibre. disposition to split up transversely or longitudinally into smaller particles. The transverse subdivision of the fibre is promoted by digesting a piece of muscle for some hours in dilute hydrochloric acid. If the fibres be then examined, gaps or fissures will be seen to extend transversely into the substance of the fibre ; and, if the digestion has been sufficiently prolonged, the fissures have extended completely across the fibre, and have subdivided it into a multitude of plate or disc-shaped bodies the muscular -fibre discs. These discs are the strongly and feebly refracting discs already described, and the transverse diameter of each disc corresponds to that of the fibre from which it has been derived. The longitudinal marks in the fibre are best seen by digesting a piece of muscle in strong spirit of wine, or in a solution of chromic acid. If a fibre so treated be teased out with needles, and the thin covering glass be smartly tapped, the fibre will split up longitudinally into multitudes of minute, elongated threads the muscular-fibre fibrillce. A fibrilla may be regarded as equalling in length the fibre of which it formed a part, and like the fibre is transversely striped ; but its breadth is not definite, and depends upon the minuteness with which the fibre has been split up in the longitudinal direction. If in the same fibre the processes of transverse and longitudinal splitting were to go on simultaneously, then the fibre would be resolved into an immense multi tude of rectangular particles the sarcous elements of Bowman. If these particles be regarded as the ultimate subdivisions of the fibre, then the discs may be conceived to be built up of a number of these particles, possessing similar optical properties, arranged side by side, so as to occupy the entire diameter of the fibre in any transverse plane: whilst the fibrillaa are built up of the particles arranged end to end, so as to correspond to the entire length of the fibre ; but in this longitudinal arrangement, particles with different optical properties, the one singly refracting, the other doubly refracting, alternate with each other with the utmost regularity. Another view of the structure of muscular fibre has just been advanced by E. A. Schafer. He describes the dark, or, as seen in a living fibre, the " dim discs," as traversed by multitudes of excessively fine, dark, rod-shaped particles parallel in their direction to the fibre itself, which extend into the contiguous bright discs, near the middle of which each muscle rod ends in a knob-like extremity, and the scries of knobs form a line of minute dark dots, passing transversely across each bright disc. The muscle rods are
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