Muscle-fibres exhibit all degrees of specialization. The simplest, “plain muscle-fibres,” are found in the wall of the alimentary canal, of bloodvessels, of ducts, in the tissue of the spleen, in the skin, and elsewhere. Each fibre is a fusiform cell. Save for its central nucleus and a little granular protoplasm in which the nucleus is embedded, the cell may show no architectural features. But in most varieties of plain muscle, and especially in that of the alimentary canal, the substance of the fibres is striated longitudinally. This is visible evidence of the orientation of the molecules of protoplasm and metaplasm in the direction of the long axis of the fibre. It shows that the streaming of particles occurs along these lines. It is, as it were, a diagram of the lines of force.
Heart-muscle has been described already (p. 224). Its striation, which is both transverse and longitudinal, is so delicate as almost to defy microscopical analysis. The transverse striæ are the darker and more distinct. But close examination shows that the transverse striæ do not indicate the direction in which the particles of cell-substance are oriented. They are oriented longitudinally. The cell is a bundle of rods of substance A, embedded in substance B. The transverse markings are very thin lines which cross the bundles at right angles.
The third variety of muscle is the kind by which locomotion is effected. It is present in large masses—all the red tissue to which the term “meat” is commonly applied. It accounts for about 35 per cent. of the body-weight. This kind of muscle is not composed of single cells, but of compound cells, or cell-complexes, termed “fibres.” A fibre may attain a length of upwards of 2 inches, with a breadth of about ¹/₅₀₀ inch. In most cases the fibres are attached by one end to a bone, by the other to a tendon; and since they are shorter than the muscle as a whole, the tendon commences as a membrane which covers the surface of the muscle, sloping to it from the bone to which by their other ends the fibres are attached. A fibre is developed from a single cell. The cell elongates, its nucleus divides, and the daughter-nuclei divide until several hundred have been formed; but cell division does not follow. The result is a cylindrical mass enclosed within a delicate membranous sheath, the sarcolemma. In the early stages of its development its nuclei are in the axis of the fibre, but subsequently they are displaced outwards. In the most highly specialized muscle, known as the “white” variety, they lie just beneath the sarcolemma (_cf._ Fig. 16, B).