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before the contraction begins, for the muscle requires a certain time, called the latent period, before it commences to contract.
The origin of the electric currents of muscle will be discussed with nerve currents, to which the reader is referred.
III. Temperature Change.-Long since it was observed in the human subject that the temperature of muscles rose during their activity. In frog's muscle a contraction lasting three minutes caused an elevation of .18° C. A single contraction is said to produce a rise varying from .001° to .005° C., according to circumstances.
The production of heat is in proportion to the tension of the muscle. When the muscles are prevented from shortening, a greater amount of heat is said to be produced.
The amount of heat has also a definite relation to the work performed. Up to a certain point the greater the load a muscle has to move, the greater the heat produced; when this maximum is reached any further increase of the weight causes a falling off in the heat production. Repeated single contractions are said to produce more heat than tetanus kept up for a corresponding time.
The fatigue which follows prolonged activity is accompanied by a diminution in the production of heat.
IV. Change in Form.—The most obvious change a muscle undergoes in passing into the active state is its alteration in shape. It becomes shorter and thicker. The actual amount of shortening varies according to circumstances. (a) A muscle on the stretch when stimulated will shorten more in proportion than one whose elasticity is not called into play before contraction, so that a slightly weighted muscle shortens more than an unweighted one with the same stimulus. (6) The fresher and more irritable a muscle is, the shorter it will become in response to a given stimulus ; and, conversely, a muscle which has been some time removed from the body, or is fatigued by prolonged activity, will contract proportionately less. (C) Within a certain limit, the stronger the stimulus applied the shorter a muscle will become. (d) A warm temperature augments the amount of shortening, the amount of contraction of frogs' muscles increasing up to 33° C. A perfectly active frog's muscle shortens to about half its normal length. If much stretched and stimulated with a strong current it may contract nearly to one-fourth of its length when extended. Muscles are seldom made up of perfectly parallel fibres, the direction and arrangement varying much in different muscles. The more parallel to the long axis of the muscle the fibres run, the more will the given muscle be able to shorten in proportion to its length.
The thickness of a muscle increases in proportion to its shortening during contraction, so that there is but little change in bulk. It is said, however, to diminish slightly in volume, becoming less than 10 oo smaller. This can be shown by making a muscle contract in a bottle filled with weak salt solution so as to exclude all air, and to communicate with the atmosphere only by a capillary tube into which the salt solution rises. The slightest decrease in bulk is shown by the fall of the thin column of fluid in the tube.
Since a muscle loses in elastic force and gains but little in density during contraction, the hardness which is communicated to the touch depends on the difference of tension of the semifluid contractile substance within the muscle sheath.
THE GRAPHIC METHOD OF RECORDING MUSCLE CON
TRACTION. In order to study the details of the contraction of muscle, the graphic method of recording the motion is applied. The curve may be drawn on an ordinary cylinder moving sufficiently rapidly. Where accurate time measurements are required, it is better to use one of the many special forms of instruments, called myographs, made for the purpose. The principle of all these instruments is the same; namely, an electric current, which passes through the nerve of a frog's muscle connected with the marking lever, is broken by some mechanism, while the surface is in motion ; the exact moment of breaking the contact can be accurately marked on the recording surface, by the lever which draws the muscle curve, before the instrument is set in motion. The rate of motion is registered by a tracing drawn by a tuning fork of known rate of vibration.
In order that the muscle-nerve preparation may not be injured by the tissues becoming too dry, it is placed in a small glass box, the air of which is kept moist by a damp sponge. This moist chamber is used when any living tissue is to be protected from drying.
The first myograph used was a complicated instrument devised by Helmholtz ; in which a small glass cylinder is made to rotate rapidly by a heavy weight, and when a certain velocity of rotation is attained, a tooth is thrown out by centrifugal force, which breaks the circuit of the current passing through the nerve of the muscle. The tendon is attached to a balanced lever, at one end of which hangs a rigid style pressed by its own weight against the glass cylinder. When the circuit is broken the muscle contracts, raises the lever, and makes the style draw on the smokedglass cylinder.
Fick introduced a flat recording surface moving by the swing of a pendulum, by which the abscissa is made a segment of a circle, and not a straight line, and the rate varies, so that the different parts of the curve have varying time values. The curves given in the following woodcuts are drawn with the Pendulum Myograph.
Du Bois-Reymond draws muscle curves on the smoked surface of a small glass plate contained in a frame, which is shot by the force of a spiral spring along tense wires, and on its way breaks the contact. The trigger used for releasing the spring sets a tuning fork at the same time vibrating (Spring Myograph).
SINGLE CONTRACTION. In response to an instantaneous stimulus, such as occurs in the secondary coil on breaking the primary current, a muscle gives a momentary twitch or spasm, commonly spoken of as a single contraction, which is of so short duration, that without the graphic method of recording the motion we could not appreciate the phases which are seen in the curve.
The curve drawn on the recording surface of a pendulum myograph, by such a single contraction, is represented in Fig. 185. The short vertical stroke on the abscissa, or base line, is drawn by touching the lever when the muscle is in the uncontracted state, and indicates the time of stimulation. The upper curved line is drawn by the lever during the contraction of the muscle.
In such a curve the following stages are to be distinguished :
1. A short period between the moment of stimulation and that at which the lever begins to rise, during which the muscle does not move. This is known as the latent period. In the skeletal muscles of the frog this period lasts nearly .o1 sec.
2. A period during which the lever rises, at first slowly, then more quickly, then again slowly, until it ceases to rise. This stage has been called the period of rising energy. It lasts about .04 sec.
3. When the highest point is attained the lever commences to fall, at first slowly, then more quickly, and at last slowly.
Curve drawn by a frog's gastrocnemius on the Pendulum Myograph ; below is seen the
tuning-fork record of the time occupied by the contraction. Parallel to the latter is the abscissa. The little vertical mark at the left shows the moment of stimulation, and the distance from this to the beginning of the rise of the curve gives the latent period, which is followed by the ascent and descent of the lever.
There is then no pause at the height of contraction. The stage of relaxing has been called the period of falling energy. It occupies, when quite fresh, about the same time as the second period, viz., about .04 sec.
Thus, a stimulus occupying an almost immeasurably short time sets up a change in the molecular condition, which, taking nearly lo sec. to run its course, and requiring ito sec. before it exhibits any change of form, then in 1to sec. attains the maximum height of contraction, and, without waiting in the contracted condition, spends to sec. in relaxing.
The latent period which appears in a single contraction curve
drawn by a muscle stimulated in the usual way, through the medium of a nerve, is not entirely occupied by preparatory changes going on in the substance of the muscle, but a certain part of the time recorded as latent period corresponds to the time required for the transmission of the impulse along the
This may be shown by stimulating first the far end of the nerve, and then the muscle itself. In this case two curves will be drawn, having different latent periods, that obtained by direct stimulation of the muscle being shorter, and representing the real latent period, while the longer one includes the time taken by the impulse to travel along the piece of nerve between the electrodes and the muscle.
Wave of Contraction.-If one extremity of the muscle be stimulated without the aid of the nerve (it is best to employ a muscle from a curarized animal), the contraction passes along the muscle from the point of stimulation in a wave which travels at a definite rate of 3-4 metres per sec. in a frog, and 4-5 metres per sec. in a mammal. Reduction of temperature and fading of vital activity cause the velocity of the wave to be lessened, until finally the tissue ceases to conduct ; then only a local contraction occurs, severe stimulus causing simply an elevation at the point of contact. This seems analogous to the idiomuscular contraction, which marks the seat of severe mechanical stimulation after the general contraction has ended.
VARIATIONS IN THE PHASES OF A SINGLE CONTRACTION.
The latent period varies much in different kinds of muscle, in the same kind of muscle of different animals, and in the same individual muscle under different conditions. As a rule, the slow-contracting muscles have a longer latent period. Thus the non-striated slow-contracting muscles found in the hollow viscera have a latent period of some seconds. The striated muscles of cold-blooded animals have a longer latency than the same kind of muscle in birds and mammalia. The same gastrocnemius of a frog has a shorter latent period when strongly stimulated, or when its temperature is raised, and vice versa.
The latent period is considerably lengthened by fatigue. If