PROPERTIES OF VOLUNTARY MUSCLE.
I. The great property of muscle is irritability or the power of responding when irritated. The response is in the form of contraction, that is when the muscle is irritated or stimulated it responds by shortening itself, so that its ends are brought nearer and it becomes thicker at the middle. The muscle does not shorten itself all at once, but the contraction passes quickly over it in the form of a wave. The usual way in which a muscle is stimulated is by nervous action. The nerve-tubes end, it has been seen, in the fibres, and when an impulse reaches the fibres by the nerve the fibres become irritated and shorten themselves. Muscles, however, will respond to other than this usual stimula tion. (1) Mechanical means, such as pricking or pinching, will irritate them and cause them to contract. (2) An electrical current has the same effect. (3) Heat also is capable of pm clueing contractions as well as (4) chemical irritations. The purpose of contraction is ob vious. If one end of the muscle be fixed, and the other attached to something which is free to move, when the muscle shortens itself it will bring whatever is attached to the one end nearer to the other.
A muscle will respond by a single contrac tion to each irritation if it be sufficient in amount, and will thereafter relax to be ready for the next contraction. Sometimes the irri tations may follow so quickly after one another that the muscle has barely relaxed after one contraction when it is again called on to con tract. If the irritations be still faster, then the muscle may not have time to relax at all between each one. The result is that the muscle remains contracted and rigid. This is the condition of a muscle in cramp, and is called tetanus. For instance, when a man seizes the handles of a galvanic machine while it is at work, if it be strong enough he finds he ' cannot let go, his fingers being firmly bent over them. This is due to tetanus of the muscles that bend his figures, tetanus produced by a series of galvanic shocks causing contractions, the shocks being so rapid that the muscles have no time to relax and remain strongly ton tracted.
It is to be noted that when a muscle con tracts owing to the stimulus received from a nerve, it is not the nerve that supplied the force for contraction. The power of contrac tion is inherent in the muscle substance, and the stimulus only alli•ds the opportunity for its display. The muscle has energy stored up within it as a barrel of gunpowder has energy stored up within it. The barrel of gunpowder, however, would stand harmless so long as it was left alone. As soon, however, as a lighted match is applied, the energy of the gunpowder is liberated and an explosion occurs. So the stored up energy of a muscle requires merely to be set free to perform work. The ner vous stimulus acts the part of the lighted match in liberating the imprisoned force of muscle.
IL Muscles by their contraction are able to do work. Thus if to the end of the muscle that is free to move a weight be attached, when the muscle contracts it will lift the weight. It is found, curiously enough, that a muscle con tracts better when it has some weight to lift than when it has none. Up to a certain limit, with increased weight there is increased work done. The increased resistance seems to call forth increased action of the muscle. When the limit has peen passed the muscle quickly fails. Similarly a muscle works best with a certain degree of rapidity, but if the irritations follow one another too quickly, if the contrac tions are too rapid, the muscle becomes ex hausted and fatigue arises. "It is the pace that kills." III. Muscles develop heat by contraction.
IV. Living muscle has been shown to possess a certain amount of electrical force, which is diminished by contraction, but increases again with rest.
V. Elasticity is a property of this tissue, a property in virtue of which the muscle, after stretching, is capable of returning to its pre length.
VI. By tonicity is meant the condition of tension in which a muscle normally is. It is this condition of tension or tone that causes a muscle, when cut, to gape by separation of the edges of the wound.