nutrition. The muscles, the tendons, the aponeuroses, even the bones undergo tractions and pressures which are so violent that ruptures of all kinds would be produced had not a progressive state of habituation gradually consolidated them. Accidents of all kinds, rupture of muscles, lacerations, dislocations, are frequently seen during exercises of strength. Other more serious lesions, such as hernia, laceration of the lung, rupture of great vessels, and even of the heart, are liable to occur unless the internal organs are perfectly sound. Organs which have undergone some degree of degeneration are soon incapable of resisting the violent strain of effort.

Finally it is necessary, to avoid overwork, that the work should be gradually increased and not done in the. largest quantities till after complete training. If a man who attempts an exercise of strength is too freely provided with reserve materials, these undergo dissimilation in mass, and an excessive quantity of wasteproducts is produced. This causes auto-intoxication by poisonous substances, alkaloids or others.

Thus are to be explained the fevers of overwork, often taken for typhoid fever, in young recruits. Facts show that these fevers have a special predilection for the services which need forced labour, the Artillery for instance.

To perform exercises of strength with impunity, the food must be sufficiently abundant to supply the losses undergone. If the food is not sufficiently restorative, the work is done at the expense of the materials of the body; the worker becomes thin and is quickly exhausted.

Exhaustion will also result from excessive work which exceeds a man's strength even if he is well fed. If a man wishes to get out of his muscles a quantity of force, out of proportion to their contractile power, he is obliged to make an energetic effort of will, and he needs a great expenditure of nervous energy to excite more powerfully the weak muscular fibre. He can thus perform a work beyond his strength, but it is by taking "from his

nerves that which his muscles are unable to give. In this case the exercise of strength will not have its usual result. of economising nervous energy. A work of the nerve-centres is necessary to increase the irritability of the muscles. We do not know exactly in what this work consists, but we can determine its effects. The excessive exertion of the will in the work leads quickly to nervous exhaustion. The man becomes thin, eats and sleeps ill; he suffers from overwork by exhaustion. It is thus that we see horses rapidly waste and get ill, although they are well fed, if they are forced to draw too heavy a load, and their ardent and generous nature impels them to go on working up to the last limit of their strength.

CHAPTER IV.

EXERCISES OF SPEED.

Accumulation of Work by the rapid Succession of Movements. Common results of works of Strength and Exercises of Speed; the "Thirst for Air"; Breathlessness - Strength and Speed combined-" Forced" Exercise; its Dangers--Part played by the Nervous System in Exercises of Speed-Law of Helmholtz; the Loss of Time in Muscle; "Latent Period"-Irritability of Muscle; its part in Exercises of Speed; its Variability in different Kinds of Animals; the Snail and the Bird-Its Variability in different Individuals and different Nations: Dutch Rowers-Effects of Exercises of Speed-Effects due to the Accumulation of Work; they resemble the Effects of Exercises of Strength-Effects due to Expenditure of Nervous Energy; resulting Influence on Nutrition-Why these Exercises cause Loss of Weight.

I.

WE call those exercises which need the very frequent repetition of muscular movement, exercises of speed.

There are great differences among the various exercises of speed as regards the intensity of work. Many of them are typical violent exercises: running, for instance. Many on the contrary need so insignificant an expenditure of force that they hardly deserve the name of exercise. A pianist playing scales, notwithstanding the extreme speed of the movements of his fingers, performs but a trifling muscular work.

The essential character of exercises of speed is the rapid multiplication of muscular movements. A series of inconsiderable efforts, often repeated, thus allow the performance in a short time of a considerable quantity of work, without bringing into play very important muscular masses. In fact ten movements each needing

an expenditure of force of ten kilogrammetres must represent the same work as one movement in which the expenditure of force is a hundred kilogrammetres, and we can easily understand that ten rapid movements may be performed in the same time as one very slow one. As regards the quantity of work done, an exercise of speed may thus be absolutely equivalent to an exercise of strength.

Exercises of speed, as well as exercises of strength, may then produce a great quantity of work in a short space of time. From this common condition are derived certain identical effects, breathlessness for instance. But each of these kinds of exercise has its own character, from which are derived very different results. The one kind needs that the muscles should contract with all the energy they possess; the other does not need this, but the muscles must pass at very short intervals, and a great number of times in succession, from repose into action.

The essential character of exercises of speed, that to which their very remarkable physiological effects are due, is this frequently repeated change of the muscles from the condition of relaxation to that of contraction.

We have thus to study exercises of speed from two very different points of view: (1) the rapidity with which the work accumulates, (2) the speed with which the movements succeed each other.

The rapid accumulation of work depends upon two factors which are, the quantity of work represented by each muscular effort, and the number of these efforts in a given time. Whether the work accumulates owing to the intensity of the efforts or to their number, the results are the same. Thus the breathlessness will be the same after 100 movements each representing 10 kilogrammetres, as after 10 movements each representing 100 kilogrammetres, if, in both cases, the same quantity of work has been done in the same time.—A man who slowly goes up a staircase with a heavy burthen on his shoulders is doing a work of strength. A man running as fast as he can along a level road is performing an

exercise of speed. Both of them do a great quantity of work in a very short time, one by slow movements each representing a great expenditure of force, the other by rapid movements, each of which taken alone represents a very much smaller quantity of work, but which, when multiplied, make in the end a considerable expenditure of force.

Thus the exercises of speed can, as well as the exercises of strength, lead to an accumulation of work. The man who runs is taking, quite as much as the man who wrestles, exercise in "large doses."

In this manner speed can supplement force, and enable certain persons, whose muscular development is feeble, to benefit from the general effects of violent exercise, without needing intense efforts which they would be unable to perform. The intensity of the combustions of work is in proportion to the total quantity of force expended, whether this expenditure has been made in mass at one effort, or has been made in successive fractions, by little efforts at very short intervals. Now the formation of the waste-products of combustion, such as carbonic acid, is in proportion to the intensity of the combustion, and it is from the quantity of carbonic acid accumulated in the system that the intensity of the respiratory need results, and from this the amplitude and frequency of the respiratory movements. The need of absorbing oxygen is intimately connected with the need for the elimination of carbonic acid, and thirst for air is the inevitable result of very intense muscular work, be its mode of performance strength or speed.

Exercises of speed produce, quite as inuch as exercises of strength, this thirst for air which is to respiration what appetite is to digestion. Skipping, running, and the numerous children's games whose essential character consists in rivalry of speed amongst the players, are as valuable, and more, as exercises of strength, from the point of view of respiratory hygiene. A child which has just been playing at running has absorbed without making any painful muscular effort, in simple "play," a greater quantity of oxygen than one which has been