[Illustration: FIG. 5.--Voluntary muscle (Leroy). _A_, Three voluntary fibers in long sections: _a_, three voluntary muscle fibers; _b_, nuclei of same; _c_, fibrous tissue between the fibers (endomysium); _d_, fibers separated into sarcostyles. _B_, Fiber (diagrammatic): _a_, dark band; _b_, light band; _c_, median line of Hensen; _d_, membrane of Krause; _e_, sarcolemma; _f_, nucleus. _C_: _a_, Light band; _b_, dark band; _c_, contracting elements; _d_, row of dots composing the membrane of Krause; _e_, slight narrowing of contracting element aiding in production of median line of Hensen.] In life muscle appears more or less translucent and is contractile and alkaline, but in death it loses its translucency and becomes rigid, at the same time giving off in decomposition much carbon dioxide, so that its reaction is acid. This phenomenon of the muscles becoming rigid in death is called rigor mortis and occurs generally a few hours after death, though it may come at once or be considerably delayed. It may last anywhere from a few moments to several days but generally lasts from twenty-four to thirty-six hours. It is probably due to the formation in the muscle of myosin, a substance which probably comes from myosinogen in the living muscle and which is closely akin to the fibrin of blood. Probably the myosin or what precedes it causes clotting of the muscle just as fibrin or what precedes it causes clotting of the blood. [Illustration: FIG. 6.--Three voluntary muscle fibers from an injected muscle, showing network of blood capillaries. (Hill.)] The muscles vary in shape in different parts of the body, being long and slender in the limbs and broad and flat in the trunk. They are attached chiefly to bones but also to cartilages, ligaments, and skin, either by means of tendons, which are cords or bands of white inelastic fibrous tissue, or by means of aponeuroses, membranous expansions of the same nature. Most voluntary muscles consist of a belly and two ends or tendons. The origin is the fixed point from which it acts while the movable point upon which it acts is known as its insertion. =Action of the Muscles.=--When attached to bones, muscles are distributed in three ways: 1. When it is necessary to produce much motion rapidly, a short muscle is used. 2. When a part needs to be moved far and much contraction on the part of the muscle is, therefore, needed, the muscle is very long, as in the case of the sartorius muscle, which shortens half its length. 3. Finally, where less distance has to be covered but greater power is required, tendons are used, as in this case the contraction is powerful but does not carry the part far.