cases of skeletal and of cardiac muscle-the passage of a single wave of change has been followed from origin to end along the

FIG. 149.-WAVE OF CONTRACTION IN FROG'S VENTRICLE, TAKEN BY TWO LEVERS
RESTING UPON IT NEAR THE APEX AND NEAR THE BASE.

x marks the moment of excitation, which is applied near the base, i.e. in imitation of the normal origin of the contraction. The contraction of the base begins sooner and lasts longer than that of the apex.

excitable tissue-electrically, as we shall see on p. 390, and also mechanically, as is illustrated above.

6

Strength; work.-The absolute strength of a muscle is measured by the weight that a muscle just fails to lift on being stimulated, the weight being applied as an after-load.' This weight varies with the sectional area of muscle, not with its length; i.e., the absolute strength of a thick muscle is greater than that of a thin muscle; and the absolute strength of a long muscle is the same as that of a short muscle, if both are of the same thickness. The absolute strength of tetanised frog's muscle has been found to be about 3 kilogrammes per 1 square centimeter sectional area; of human muscle it is greater, viz. 5 to 10 kilogrammes per 1 square centimeter. This is for voluntary contraction; the strongest possible artificial tetanus only reaches to of the voluntary maximum. The absolute strength in tetanus is about double that of a single contraction in the frog, and no less than ten times its value in human muscle (Fick).

The work done by a muscle is measured by the product of weight raised x height. A muscle that shortens 10 millimeters raising a weight of 100 grammes, does work equal to 1,000 gramme-millimeters, or 1 grammeter. Leaving out of account the weight of the muscle itself, no work is done when muscle contracts without raising a weight or overcoming some resistance. A tetanised muscle does work only at the beginning of tetanus, when it raises a weight; during the tetanus, while the weight is kept up, the contraction is static, and no work is done. If we measure the work done by a muscle made to raise a succession of increasing weights, we shall find that it increases to a maximum, from which it declines with further increase of weight.

Ꮓ

Work done thus depends upon three factors-(1) amount of weight, whether as 'load' or as

after-load'; (2) length of muscle; (3) sectional area of muscle. Length x sectional area is equivalent to bulk or weight; the maximum work that can be done is therefore dependent upon the bulk or weight of the muscle in action. It has been found that for frog's muscle this maximum under favourable circumstances is in a simple contraction nearly 1 grammeter per 1 gramme, and in a short tetanus about 4 grammeters per 1 gramme.

An ordinary labourer at work does about grammeter per gramme muscle

per second; i.e., taking his muscle to be 20 to 25 kilogrammes, his work during 8 hours =115,000 to 144,000 kilogrammeters. A man during an ordinary walk does work at about the same rate. A long-distance bicycle-racer was calculated to do work for 8 hours at the rate of 1 grammeter per gramme per second, the weight of his muscle being estimated at 25 kilogrammes; a rowing-man was calculated to work at the same rate, but only for a short time; the work of a short-distance runner has been estimated at between 2 and 2.5 grammeters per gramme per second. According to Marey, the work done in walking and running on level ground amounts to between 10 and 20 kilogrammeters per step.

The muscular strength is tested on man by means of the dynamometer, which in its ordinary clinical form consists of a strong oval spring to be grasped in the hand, the value of the flattening being shown by an index and scale graduated in pounds or kilogrammes. The value of the instrument is much diminished by the fact that habit and knack' are very important factors in the maximum squeeze its indications are of

1 This is a low estimate; 1 horse-power = 77 KgM. per sec.; 1 man-power = " horse-power, or nearly 10 KgM. per sec., i... 288,000 KgM .per 8 hours.

greater value if the instrument is converted into a dynamograph; the manner of maintenance of a prolonged maximum effort for regular periods, or the character of a series of maximum efforts for regular periods, may then be recorded on a slowly-travelling surface, e.g. a cylinder on the hour-axis of an ordinary clock; and from such records an estimate may be formed of the muscular strength, of its rate of decline in a succession of efforts, and of its rate of recovery from such decline.

FIG. 151.-DYNAMOGRAPH.

By means of an instrument devised by Mosso, and termed by him the ergograph, similar estimates may be made, with the advantage that they may be represented here much shorter than it numerically expressed in terms

Short straight spring and long lever is in reality).

of work as kilogrammeters, although in other respects the isometric method is preferable. The half-supinated arm is attached to a horizontal support, a cord from a ring round the

Each group of lines is the effect of 30 maximal efforts, each lasting two seconds. Intervals of rest for one minute between successive groups.

middle finger passes over a pulley and carries a weight, the hand is kept in position by two tubes into which the index and ring fingers are inserted: the successive elevations of the weight by flexion of the middle finger are recorded in the usual manner.

Heat evolved. The chemical action taking place in muscle is of course accompanied with an evolution of heat, and its amount is increased during contraction. It has been found that, cæteris paribus, (1) more heat is evolved in the contraction of a stretched than in that of an unstretched muscle, (2) more heat is evolved

by a muscle that contracts without doing work than by a muscle that does work (Heidenhain). These facts have a far deeper significance than is superficially apparent. The first fact implies that pure muscle is not a mere machine discharging energy irrespectively of the work to be done, but that it evolves much or little energy according as it pulls against much or little resistance, and it is obvious that this relation between demand by tension and response by action is an important feature in the economy of natural contraction. The second fact is an illustration of the conservation of energy: if the total energy set free appears in the form of heat, there is more heat than if some of the energy goes off as work. The most favourable conditions for the demonstration of the heat of muscular contraction are therefore established by tetanising stretched muscle which cannot shorten and do work.

Left, +0.6°

Right, +0.5°
FIG. 153.

Left, +0.20

Two thermo-electric needles in the two gastrocnemii of a frog, and connected with a low-resistance galvanometer; the spot of light from the mirror records the deflection of the magnet upon a cylinder covered by sensitive paper and revolving round a horizontal axis. Excitation of the sciatic on one side for 1 min. causes increased heat in one muscle and deflection of the magnet; excitation on the opposite side gives deflection in the opposite direction. Periods of excitation indicated by breaks of the abscissa. In this instance the rise of temperature is considerably greater than that generally quoted, viz., according to Helmholtz, a rise of 0.1° with a tetanus lasting one minute; according to Fick, a rise of 0·002° with a single contraction.

Considered in a slightly different manner, these two important facts become still more intelligible. In the case of the first, which may be designated by the aphorism ut tensio sic calor,' we must be careful not to suppose that a muscle raising a heavier weight, i.e. under greater tension and doing more work, yields this mechanical energy at the expense of thermic energy; on the contrary, heat as well as work is increased with the heavier weight. The second fact becomes quite obvious if thus regarded:-like caoutchouc, muscle is warmed by passive elongation, and if, of two identical and equally weighted muscles, one pulls up and lets down its load (i.e. no work done), while the other winds up its load from a lower to a higher level (i.e. work done), the first muscle, alternately contracting and stretched, must be warmed more than the second muscle, that only contracts, and is not stretched.

As we have seen above, the muscles, even at rest, are constantly slightly warmer than arterial blood, and the difference is increased during contraction. Some idea of the amount of heat evolved by muscular contraction may be arrived at by comparing the temperature of the venous blood coming from tetanised muscles with that of arterial blood. If, for instance, through a given group of muscles 100 c.c. of blood have passed per minute, and have been raised to a temperature 0-2° above that of arterial blood, it follows that heat has been produced at the rate of 20 calories per minute in the muscles in question. Such an estimate does not, however, give the total amount of heat evolved; to it must be added the value of any rise of temperature of the muscle itself. If, for instance, the muscle in action weighs 50 grms. and its temperature is raised 0.1°, this value will be about 5 calories-to be added to the 20 calories as above estimated. And even then we have left out of account the heat dissipated by radiation from the muscle.

Direct estimations of the amount of heat evolved have been obtained on excised frog's muscles, and on mammalian muscle after the circulation has been arrested. Fick estimated that frog's muscle evolves 1 to 3 milli-calories per 1 gramme in a single contraction. Ludwig gives for dog's bloodless muscle a heat-production of 1 milli-calorie per gramme per contraction. It is probable that these values are below the value of normal heat-production of muscle in the body, more particularly in the case of mammalian muscle, which rapidly runs down after arrest of the circulation. Chauveau's observations performed under more normal conditions, upon the levator labii superioris of the horse, yielded a much higher value, viz., 300 milli-calories per gramme per minute during voluntary contraction.

The production of heat by muscular exercise is a matter of everyday experience, and the increased temperature of a single limb (e.g. the arm or the forearm) in consequence of muscular contraction is easily demonstrated on man by means of an ordinary thermometer with tenths of a degree legible; the temperature may rise as much as 1° or even 2° in consequence of strong muscular contractions repeated for two or three minutes. But this rise is not a pure muscular effect; it is mainly due to a more copious blood-supply, as may be proved by repeating the observation during arrested circulation (by a bandage); the rise is then very much smaller than that just obtained with unobstructed circulation. This result is in apparent contra