as the' intoxications due to waste products, and the exhaustion due to diminution of the mass of the organic tissues. We shall call them the dynamic phenomena of fatigue, because they seem to manifest themselves simply by a loss of force, without any lesion, any chemical change, any loss of substance, being discoverable in the organ. When we hold the arm outstretched, at the end of five minutes we are forced to lower it by fatigue.

What has happened in the muscles during the short time contraction has lasted? The fatigue is not due to the formation of waste-products; five minutes of contraction would not suffice profoundly to alter the nutrition of the muscles; it is not due to material frictions of the fibres; it would last longer. We can only suppose," in the case we are considering, that there is a dynamic effect, a loss of the energy contained in the muscle, without any anatomical change being appreciable in its tissue.

This dynamic fatigue of muscle is quite similar to that which may be observed in nerve-elements after over-use. If a nerve be stimulated very frequently, it ends by losing for a time the power of transmitting stimuli: similarly the nerve-centres lose their auto-motor power when they have been in action too long.

We call an organ which has thus momentarily lost its specific energy exhausted; but we must not confuse this dynamic exhaustion with the organic exhaustion which we have described, and which is characterised by the diminution of certain anatomical elements. In exhausted nervous tissue, we do not see any diminution in the mass of material molecules, but simply a lessened manifestation of the energy peculiar to these molecules.

Can we discover in fatigued nerves nutritive disturbances at present ill-understood? Everything leads to this belief, for we know that nervous tissue becomes heated and congested when in action. Its work is subject to the same physiological conditions as that of muscle, and its fatigue should be subject to the same laws. But in muscle we have been able to establish that there is an exhaustion of muscular contractility, which

seems due, in certain cases, to a similar expenditure of the energy of the fibres, independently of any intoxication by products of dissimilation, and of any material loss in the organ. We cannot then refuse to admit, among the phenomena of fatigue, a series of phenomena due to a simple loss of vital energy in consequence of the very activity of the element which has been at work. We must make a provisional category of these facts under the title of dynamic fatigue, and admit that this form of fatigue is due in the nerves and nerve-centres, to a too great expenditure of the force which we will call, for want of a better name, nervous energy.

Nervous energy, like heat and electricity, results from 'the liberation of a force which existed as latent energy in the molecules of the nervous substance, from which certain circumstances have caused its discharge. A bar of red hot iron plunged into water cools by loss of heat which the cold liquid abstracts from it. A nerve which is stimulated in order to cause a muscular contraction seems to be deprived by its work of transmission of a certain quantity of energy, and just as the red hot iron had only a definite quantity of heat to give up, so the nerve when at rest had only a limited quantity of nervous energy at its disposal, which has been expended in the work.

The analogy up to this point seems satisfactory; it ceases to be so when we consider that the heat lost by the cooled iron is not spontaneously reproduced, while the nerve, left to itself, recovers its energy after a little time. The provision of force which has been exhausted is renewed without any other condition being necessary than a temporary cessation of expenditure,

Let us suppose there is a large reservoir in which the water gradually accumulates from a very feeble source of supply. Open the reservoir and use the water contained in it to move a water-wheel: after a time the supply of water is exhausted and the wheel no longer turns. But the conduit does not cease to feed the reservoir, and if the outflow is stopped the mass which gradually accumulates will soon become sufficient to

move the wheel again. Such is, in default of a satisfactory explanation, the simile we propose, that we may make the succession of the phenomena understood.

We have been obliged to give the reader as complete and clear a theory of fatigue as was possible; the gaps and imperfections in this chapter will be excused, in consideration of its novelty. No author has hitherto arranged after a methodical plan all the phenomena which may be a consequence of work, or has endeavoured to determine their laws.

The phenomena of fatigue are local or general, immediate or consecutive. If we endeavour to sum up the physiological laws according to which these phenomena are evolved, we shall see that they relate to four orders of causes:

1. Material lesions of the motor organs

2. Auto-intoxication by the waste-products of work.

3. Exaggerated use of the living tissues.

4 Dynamic exhaustion of the motor elements.

CHAPTER XI.

REPOSE.

Repair of the Animal Machine-Cleansing of the Organs; Elimination of the Waste Products of Combustion-Diminution of Combustions during Repose--Fall of Temperature and Depression of Vital Functions during Sleep-The Duration of Repose must vary according to the Form of Fatigue-Short Period of Repose necessary to dissipate Breathlessness-The Runners of Tunis-Difference in the Rapidity of Elimination of the various Products of Dissimilation-Dynamic Effects of Repose; they are still unexplained-Influence of Periods of Repose in relation to the Conservation of Energy-The English Boxers.

I.

IN a steam engine, barring accidents, the work continues as long as the fire is kept up.

In the human body, in spite of the richest diet, muscular movement becomes impossible after a certain period of exercise, and the work is necessarily stopped: the organism has need of repose. The human machine can only work intermittently. But this apparent imperfection is in reality the result of a great superiority to the steam engine. Repose is wanted because of the power of repair possessed by the living organism.

The machine at work is slowly but fatally used up: the more work it does, the less it becomes fitted for work. We can calculate in advance the amount of work in kilogrammetres which an apparatus or instrument will be able to perform before it is used up. A cannon is useless after a certain number of shots has been fired. The more a machine performs, the more it

deteriorates, and loses its fitness for performance. Contrariwise, the more the living body works, the more resistant, and the fitter for work it becomes. It is a law of vital movement that function strengthens the organ, whereas the working of a machine wears out its wheels.

The organs of the human body repair the losses which they have suffered during work, and make in compensation new acquisitions; now it is a law of life that the losses of work are not repaired during the work, but only after it is over. A period of repose is then necessary that the organs may repair the losses they have suffered during the period of activity.

What is the nature of the actions which join in repair of the organs after a period of activity? These actions are numerous and complicated; some of them are known to us, but we are still ignorant of many.

Repair of the organs is, strictly speaking, a complete renovation of the organs. A muscle which works makes waste, that is to say is the poorer for certain portions of its tissue which are detached from the organ and rejected. In their place the blood, drawn to the muscle in abundance by the very act of contraction, carries to it new materials which are installed in place of those which have been eliminated. Every moment a fresh particle is being detached as waste, and its place being taken by a molecule of new formation. In this manner the muscle is in the end entirely renovated, and it is thus that the process of nutrition makes the new instruments of work.

Thus the body is a machine the wheels of which are constantly renewing themselves and undergoing continual repair. It is owing to this repair that the body is not worn out by work.

The blood current passing through a muscle exercises on it a true process of cleansing by disembarrassing it of the waste products of combustion resulting from work. This cleansing takes rather a long time, for according to our observations, twelve, and even twentyfour hours are sometimes necessary for the elimination