The fleshy covering of the skeleton is called muscle, and by means of it the bones are moved. There are two kinds of muscle—voluntary and involuntary ; voluntary muscle is called striped, or striated, and involuntary muscle unstriped or non-striated.

Voluntary muscle is composed of bundles of fibres, which are composed of smaller bundles called fasciculi, and these again are composed of still smaller fibres called fibrillae. These consist of little disc-shaped bodies united in such a way as to produce transverse markings, from which the muscle is called striped, or striated ; every fibre is enclosed in a sheath of connective tissue called sarcolemma. Numbers of these bundles of fibres, arranged side by side, and bound together by connective tissue, form a muscle.

Voluntary muscles are on the outside of the skeleton, and are under the control of the will.

Involuntary muscle is made up of bundles of fibres, which are composed of elongated cells, generally pointed at the ends. They have no sarcolemma.

Involuntary muscles produce movements over which we have no control, such as the motions of the stomach.

The muscle fibre of the heart differs from those of other involuntary muscles in being striated like those of voluntary muscles. Most voluntary muscles are attached to bones by tendons which are strong, white, shiny, inelastic bands, the continuation of the interstitial connective tissue in which the muscular fibres terminate. Some muscles have fleshy attachments to bones, and some are attached to ligaments, aponeurosis, cartilage, or skin. The attachments are called origin and insertion ; the origin is the end which is attached to the more fixed of two bones, the insertion is the end which is attached to the more movable bone ; the thick, fleshy part between is known as the body of the muscle.

Fig. 17.—Showing Action of Biceps.

1, Clavicle; 2, scapula; 3, humerus; 4, radius; 5, ulna; 6, short head of biceps ; 7, long head of biceps ; 8, body of biceps.

Muscles act by contraction ; when a muscle contracts it broadens and shortens, and consequently approximates the two ends, thus bringing the more movable bone to the more fixed one ; this bone is then brought back to its former position by the contraction of another muscle on its opposite side, which is antagonistic to the first, muscles being generally arranged so that each one has an antagonist. The arm gives a simple example of this. When the muscle on the front of the arm (biceps) contracts the forearm is drawn up, when the muscle on the back of the arm contracts the forearm is straightened. The muscles which bend the limbs are called flexors, those which straighten them extensors. This power of contraction in muscles must be borne in mind, as when from weakness or wasting it is lost, massage is one of the agents used for restoring it. Muscles are plentifully supplied with nerves, bloodvessels and lymphatics ; they form about half the weight of the body. There are over 300 voluntary muscles. Seventy-five per cent, of muscle consists of water.