The mechanical energy developed by muscle during its contraction is derived from chemical energy liberated by changes in the constituents of the muscle itself. These are of the nature of oxidation, and during them oxygen is used up, and carbonic acid is liberated. But the oxygen is not necessarily present either around the muscle, or in the blood circulating through the muscle; it is stored up in some loose form of combination within the muscle.1

1It would appear that this force-yielding substance, or muscle-dynamite, as we may call it, is not present, at least in large quantity, in the muscles in a form in which it can be at once fired off. There appears rather to exist a substance yielding it, or dynamogen, which may be looked upon as corresponding to the zymogen of the glands, while the muscle-dynamite may be regarded as corresponding to the ferments of glands. Irritation of a nerve appears both to liberate muscle-dynamite

The form in which it is stored up has been compared by Lud-wig to gunpowder, a small quantity of which is fired off at each contraction.

One of the final products is carbonic acid; but there are intermediate products, one of them being sarcolactic acid; and these products tend to cause muscular fatigue.

Fig. 38.   Revolving cylinder for recording movements. The screws at the top are for fixing the cylinder in position. The brass pin is for making or breaking a current at a given time in the revolution.

Fig. 38. - Revolving cylinder for recording movements. The screws at the top are for fixing the cylinder in position. The brass pin is for making or breaking a current at a given time in the revolution. It does this by striking against a small key. The curve is described by the lever, Fig. 37. The abscissa, or zero line, is drawn by a fixed point, and serves to show the height of the contraction.

When they are washed out of the muscle by a current of blood, or of simple saline solution, the fatigue of the muscle is removed; and this removal is effected even more perfectly when the internal oxidation is rendered more complete by adding permanganate of potassium to the solution, or by the addition of minute quantities of potash. A mere trace of veratrine has also a similar effect in restoring the muscle after fatigue.

and to explode it, if we may so term it. The passage of a constant current through the muscle appears to liberate the muscle-dynamite from the dynamogen, but causes no expulsion except at the moment when the current is made or broken, or its strength altered. It must be carefully borne in mind that the idea of a muscle-dynamogen is at present simply theoretical, and must be looked upon not as a fact but rather as a means of remembering facts. According to A. Schmidt, however, the contraction and relaxation of muscle is closely connected with the formation and destruction of a ferment.

We find that the muscle does not immediately respond to a stimulus, but that a period elapses between the stimulus and the commencement of the contraction, which is on the average about the 100th of a second. This is termed the latent period.

During this period a chemical change is probably going on in the muscle, and it is evidenced by an electrical change known as the negative variation, or diminution in the natural current which passes from the longitudinal to the transverse section of the muscle.

The latent period is altered by fatigue. Loading the muscle shortens the latent period, until the load is just sufficient to extend the muscle. An increase of load above this, lengthens the latent period. Cold lengthens it; heat shortens it. Small doses of strychnine or veratrine shorten the latent period. Large doses of strychnine or veratrine, and also curare, lengthen it.

Summation Of Stimuli

During the latent period, the stimulus applied to a muscle excites chemical changes which result in contraction; but if the stimulus be very small, the chemical changes may be so slight that contraction does not occur. If the stimulus, however, be repeated several times, the changes which it induces in the muscle become sufficient to produce at first a slight contraction, and then one greater and greater, until the maximum effect is produced - this is called summation. It occurs not only in voluntary muscles, but in other contractile tissues, such as those of the medusa (vide Fig. 30, p. 110). A similar phenomenon occurs also in the heart, and has there received the name of 'the staircase.'

Summation Of Stimuli 67

Fig. 39. - Electro-magnet (after Marey) for recording time on a cylinder. When used to record time, the current is made and broken alternately by clockwork or by a tuning-fork. It may be used also to record the time of irritating or dividing a nerve, or of injecting a poison, etc.