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). The feature of this type of muscle is its transverse striation, almost mathematically regular. Commonly striated muscle is spoken of as “voluntary,” because, for the most part, it is under the control of the Will; but the term, in so far as it implies a connection between structure and mode of actuation, is misleading. Transverse striation is evidence of capacity for rapid action. The muscles which the Will directs exhibit promptitude; but striated muscle, which is not under the direction of the Will, is found in certain situations—_e.g._, the upper part of the œsophagus. Conversely, many animals can voluntarily call into action muscle which is not striped. A turkey erects its feathers by setting in motion little groups of “plain” fibres, which pull on elastic tendons attached to the tips of the buried ends of their shafts. Plain muscle contracts less promptly and relaxes more slowly than the striped variety. Cardiac muscle is quicker in acting than plain, but does not hold the contraction so long.