the weight be so applied that it does not extend the muscle before contraction, but only bears on it the instant it commences to shorten, the duration of the latent period increases in proportion to the weight the muscle has to lift.

The duration of the single contraction of striated muscle varies in different cases and under varying circumstances. With submaximal stimulation the length of the curve increases with the strength of the stimulation. When the maximal strength of stimulus (i. e., that exciting a maximal contraction) is reached, no further lengthening of the curve takes place.

The greatest difference is observed in the muscles found in different kinds of animals. The contraction of some kinds of muscle tissue (non-striated muscle of mollusca, for example) occupies several minutes, and reminds one of the slow movement of protoplasm; while the rapid action of the muscle of the wing of

Curves drawn by the same muscle in different stages of fatigue-A, when fresh: B, C, D, E, each immediately after the muscle had contracted 200 times. Showing that fatigue causes a low, long contraction.

a horsefly occurs 330 times a second. Various gradations between these extremes in the rapidity of muscle contraction may be found in the contractile tissues of different animals. The following table gives the rate of contraction of some insects' muscles, which may help to show the extent of these variations:

Among the vertebrata the duration of the contraction of the skeletal muscles varies considerably, according to the habits of the animal. The limb muscles of the tortoise and the toad take

a very long time to finish their contraction; other muscles of the same animals act more quickly, but do not attain the rapidity of contraction of the skeletal muscles of warm-blooded animals. The duration of a single contraction of the same muscle is also

Six curves drawn by the same muscle when stretched by different weights. Showing that as the weight is increased the latency becomes longer and the contraction less in height and duration.

capable of considerable variation. It seems to be lengthened by anything that leads to an accumulation of the chemical products which arise from muscle activity. Hence fatigue or overstimulation causes a slow contraction (Fig. 186).

Curves drawn by the same muscle at different temperatures. Showing that with elevation of temperature the latency and the contraction become shorter. (The muscle had been previously cooled.)

Moderate increase of temperature greatly shortens the time occupied by the single contraction of any given muscle. Excessive heat causes a state of continued contraction.

The reduction of temperature causes a muscle to contract more slowly, and when extreme, the muscle remains contracted long after the stimulus is removed.

The altitude of the curve which represents the extent of the

Curves drawn by the same muscle while being cooled. Showing that the latency and the contraction become longer as the temperature is reduced.

contraction varies in the same way as the latent period and the duration.

MAXIMUM CONTRACTION.

The extent to which a muscle will contract depends upon the conditions in which it is placed, and varies with the load, its

Pendulum Myograph tracings showing summation.

1. Curve of maximum contraction drawn by first stimulus, the exact time of application o which is shown by the small upstroke of the left hand of the base line.

2. Maximum contraction resulting from second simple stimulation given at the moment indicated by the other small upstroke.

3. Curve drawn as the result of double stimulation sent in at an interval indicated by the distance between the upstrokes, showing summation of stimulus and consequent increase in contraction over the maximum contraction."

irritability, the temperature, and the force of the stimulus. fresh muscle at the ordinary temperature, with a medium load, will contract more and more as the intensity of the current

employed increases. There is a limit to this increase, and with comparatively weak stimulation an effect is produced which cannot be surpassed by the same muscle with further increment of stimulus. The height of the contraction is the same for all medium stimuli while the muscle is fresh. This is called the maximum contraction, being the greatest shortening which can be produced by a single stimulus.

Summation.-Each time a muscle receives an induction shock of medium strength, it responds with a "maximal contraction," but this is not the maximum amount the muscle can contract with repeated stimulation. If a second stimulus be given while the muscle is in the contracted state, a new maximum contraction is added to the contraction already arrived at by the muscle at the moment of the second stimulation. If stimulated when the lever is at the apex of the curve, the sum of the effect produced will be equal to two maximum contractions.

If applied in the middle of the period of the ascent or descent of the lever, a second stimulation gives rise to 11⁄2 maximum contractions, and so on, in various parts of the curve, a new maximum curve is produced, arising from the point at which the lever is when the second stimulus is applied (Fig. 190).

During the latent period a second stimulation produces the same effect, but the summation only begins at the end of the latent period of the second contraction, when the effect of the first stimulus is as yet small. It is difficult to demonstrate the summation when the stimuli are very close, but if the second stimulus comes after an interval of more than 6 sec., summation can easily be appreciated.

This summation of effect also takes place when the stimulus is insufficient to produce a maximum contraction. The first few weak stimuli give rise to the same extent of contraction as if the muscle were at its normal length at the time of each successive stimulation. The following tracings (Figs. 191-193) show the effects of repeated stimulations applied at the various periods indicated by the numbers on the abscissa line.

TETANUS.

If a series of stimuli be applied in succession, at intervals less than the duration of a single contraction, a summation of con tractions occurs, which results in the accumulation of effect until the muscle has shortened to about one-half of the length it

Curve of tetanus resulting from 30 stimulations per second, drawn by a frog's muscle on a drum, the surface of which moves 1.5 centimetres per second. The stimulation commences at " 30," and ceases just before the lever begins to fall. No trace of the individual contractions of which the tetanus is composed can be recognized.

attains during a single contraction, or about one-fourth the normal length of the relaxed muscle; it then remains contracted to the same extent for some time, and does not shorten further, though the stimulus be increased in rate or strength. As long as the stimuli are continued, the various single contractions

caused by the individual shocks are fused together (Fig. 191); but if the intervals between the stimuli be nearly as long as the time occupied by a single contraction, the line drawn by the lever will show notches indicating the apices of the fused single contractions (Figs. 192 and 193).