neither eat nor sleep. Many horses which are too nervous refuse their oats after a hard day's hunting. We do not see this capricious appetite in coarsely built animals which do collar-work all day.

In man it is remarkable to note the different effects on nutrition of exercises of speed and of exercises of strength. Navvies, porters, strong men at a fair, have usually a massive build which becomes more and more marked by the exercise of their profession. Runners, dancers, fencing-masters, are generally slender and thin.

If we wish to sum up in a few words the effects of exercises of speed, we see that we must distinguish between the effects due to the accumulation of work, and those due to the frequency of movements.

Exercise of speed has one point in common with exercise of strength; this is the great quantity of mechanical work which it can produce. The rapid succession of a great number of efforts, leads, in ultimate analysis, to the same results as the great intensity of a small number of efforts at longer intervals. We may say, to borrow an idea from therapeutics, that these two modes of exercise have for result the habituation of the system to "large doses" of work.

But exercises of speed have special results very different from those of exercises of strength. These results are not due to the great quantity of mechanical work done, but to the rapid succession of the movements. The speed of movements has on the system a peculiar influence, independent of their greater or less energy. It is by the nervous system that this influence is felt, and it is, in ultimate analysis, to an increased work of the nerve-centres that the very specialised effects of exercises of speed are due.

CHAPTER V.

EXERCISES OF ENDURANCE.

Conditions of Exercise of Endurance; Moderation of Efforts; Slow Repetition-Fractional Work-Conditions Inherent in the Worker. Men and Animals who have "Staying Power "-Need for perfect Equilibrium between the Intensity of the Work and the Power of Resistance of the System-Importance of Respiration in regulating Exercise of Endurance Exercise of Endurance makes the Functions more Active without Fatiguing the Organs-Association of the Great Functions with Moderate and Prolonged Muscular Work; Respiration more Active without Breathlessness; Circulation Quickened without Palpitation - Indications and Contra-indications for Exercises of Endurance-Parallel with Exercises of Speed-Why Children bear. Exercises of Endurance badly-Veterans and Conscripts.

I.

WE call exercises of endurance those in which the work must be continued for a long time.

In these exercises the expenditure of force is determined less by the intensity and rapid succession of efforts, than by their duration. It is necessary that the muscular effort shall not be too considerable, and the movements not too rapid, in order that fatigue under its various forms may not interrupt them too soon. So that an exercise of endurance is only moderate exercise if it lasts a short time, while it may become forced exercise if it be continued too long.

In these exercises the quantity of work done after a long time, at the end of a day, for instance, may be very considerable, but the expenditure of force is made in such small fractions that there is no painful muscular effort, nor any marked disturbance in the organic functions. So that a man performing an exercise of endurance may pass, almost without noticing it, to strong doses of muscular work.

The animal machine is made in such a manner as to be able to perform without fatigue movements of a determined intensity and speed. When these limits are not exceeded, no appreciable disturbance is produced in the system, and the work is done amidst complete tranquillity of the vital functions. Thanks to the perfect equilibrium between the muscular exertion and the power of resistance of the subject, he is able in exercises of endurance to go on working for a long time, and let the useful effects of work insensibly accumulate, without causing any disturbance to the various parts concerned in its performance.

We see at once the importance and usefulness of exercises of endurance when we have to do with a feeble system, with a person of low resisting power, to whom we wish to give the benefits of muscular work, while enabling him to avoid the dangers of fatigue. Similarly we are sometimes able to give a sick man a very energetic remedy by administering it to him in "fractional doses."

The division of work into fractional quantities sufficiently small to enable the system to support each one without disturbing its normal functions, such is the essential condition of exercises of endurance.

Another condition is necessary to constitute an exercise of endurance; the muscular efforts must be at intervals sufficiently long that the effect of a second may not be added to that of a first. Between two successive doses of work there must be a sufficient time for repose.

There are organs in the human body which perform a considerable work continuously throughout life. It is surprising, for instance, to think how the hollow muscle we call the heart, goes on contracting from birth till death, without ever suspending or slackening its work. This is because the cardiac muscle performs a work of endurance. The expenditure of force at each beat is well balanced with the power of resistance of the system of which it forms a part, and the interval between two beats is a time just long enough to rest the fibres.

But if some circumstance occurs which increases the work of the organ, as we see in constriction of the orifices for instance, or if the contractions become immoderately frequent, as is the case in palpitation, the conditions of work are changed. The heart, instead of having to do a simple work of endurance, has to do a work of speed or strength incompatible with continued work; the muscle becomes fatigued, its fibres lose their elasticity and their energy, there is overwork of the heart, and a condition of asystole comes on, of which death is the inevitable consequence.

Similarly in the muscles of animal life, increased energy, or more rapid succession of movements, tends to make the exercise of endurance pass into one of speed or of strength.

In an exercise of strength there is accumulation of work, because each muscular effort is very intense. In an exercise of speed there is multiplication of work, for the movements have little energy, but the rapid succession of efforts of small intensity leads in the end to an accumulation of work. In an exercise of endurance, on the contrary, the efforts being repeated at sufficient intervals, the work is fractional, for at any moment the quantity of work performed by the organism does not exceed its power of resistance.

Among exercises ordinarily practised, which are those we may call exercise of endurance? This question raises a first difficulty, for the same exercise may represent in turn a work of speed, a work of strength, or a work of endurance, according to the conditions in which it is performed.

Rowing, for example, is a work of speed in a rowingmatch, and a work of endurance in a long course. Walking, which is the type of exercises of endurance, may present the characters of an exercise of strength when it is performed on a very steep slope. Thus in certain ascents in which it is necessary to climb almost perpendicular slopes, each step represents a great expenditure of muscular force, and the tourist is obliged to interrupt his work as

frequently as if he were walking in the plain with a heavy burthen on his shoulders.

The conditions under which the man performing the exercise is placed have no less importance than the exercise itself in determining its character as one of endurance.

Exercise of endurance is characterised by the necessity for perfect equilibrium between the intensity of muscular effort and the power of resistance of the system. Now there is nothing so variable as the power of resistance of each individual. So that which is for one man an exercise of strength or of speed, becomes for another, stronger or better trained, a simple exercise of endurance. A canter is an exercise of speed for a cart horse, used only to walk; it is an exercise of endurance for a thoroughbred, which can sustain this pace for an entire day without stopping. Rowing seems an exercise of strength to a man who is learning; after a quarter of an hour he is out of breath. For a waterman it is an exercise which he can perhaps keep up a whole day without any fatigue.

There are then two conditions necessary to constitute an exercise of endurance: (1) a certain moderation in the violence of the exercise, (2) a certain power of resistance on the part of the system.

This is why the word "staying-power," which conveys the idea of length of time, applies rather to the qualities of the man or the animal, than to the nature of the work they perform. A work of endurance is one whose mode of performance enables it to be long continued; and a man or animal with "staying power are those whose system is fit to support prolonged work.

Certain persons are unable to perform the most moderate exercise without showing, after a very short time, the signs of extreme fatigue. There are others

who keep up with surprising power of resistance the most violent exercises, and for them exercises of strength and of speed become exercises of endurance.

Generally these differences in power of resistance, in the staying power of different people, are due to differences in their respiratory powers.