There are two kinds of muscle, distinguished as striated or voluntary and non-striated or involuntary.

Striated muscle is red in colour, and forms nearly one-half of the entire weight of the body. It clothes the bones of the skeleton and moves them in obedience to the will, hence the term "voluntary "muscle.

A voluntary muscle consists of an aggregation of bundles of fibres united by connective tissue in which blood-vessels and nerves ramify to nourish and innervate them.

A muscle fibre, as seen under the microscope, is a minute, pale, faintly yellow filament. It is composed of an outer sheath or sarcolemma, within which is contained a contractile substance.

The sheath is a very thin, transparent, structureless membrane. It possesses no power to contract, but, being elastic, is capable of accommodating itself to the necessary changes which its contents undergo.

The contractile substance enclosed in the sarcolemma consists of a number of delicate filaments placed side by side, termed fibrilloe. Each fibrilla is composed of a chain of minute bodies called sarcous elements.

These are united in such a way as to give the fibre a succession of transverse markings, hence the term striated muscle; other but less distinct striations occur along its length, as a result of the contact of the several fibrilloe.

Non-Striated muscle is of a pale grayish hue, and outers into the structure of hollow organs, such as the stomach and bowels, the uterus and bladder, the blood-vessels, the bronchial tubes, etc. etc. It consists of a number of minute spindle-shaped fibre-cells, about 1/4500 to 1/500 to 1/3500 inch in breadth and 1/600 to1/300 of an inch in length. Non-striated muscle is not under the control of the will, its movements are therefore involuntary, and carried on by reflex action.

Voluntary muscles are distinguished from one another by various names. Of these some refer to their action. Those which bend a joint, for instance, are termed flexors, while others which straighten it again are known as extensors. There are also levators, depressors, abductors, adductors, constrictors, dilators, etc. etc.

Others are distinguished by their length, as the long muscle of the back, longissimus dorsi, the short muscle of the tongue, hyo-glossus brevis. Size, form, position, direction, and other qualities are also invoked as a means of recognition.

Voluntary muscles, with few exceptions, exist in pairs - one on either side of the body or organ in whose function they are engaged. They are attached by their extremities to two or more bones, which they cause to move at the instigation of the will.

When in action one extremity of the muscle is fixed, the other is movable. The former is termed its origin, or the part from which it acts; the latter is its insertion, or the part upon which it acts and moves. In some instances the extremities are alternately fixed and movable; what is at one time the origin is at another the insertion. This is the case with the mastoido-humeralis, a long muscle running from the arm to the back of the head. If when the arm is fixed the muscle contracts, the head is drawn downwards and to one side; and conversely when the head is fixed, the arm is raised.