ferent moments, we remark a very manifest acceleration of the current at the moment when the animal puts the muscle in action, as in chewing oats. There is a more considerable flow of blood to the organ in action, and the debit of its nutrient artery is increased.

But this is not all. The acceleration noted in the vascular region which supplies blood to the muscle is propagated gradually to the great vessels, then to the heart and to the entire circulatory system. After some minutes the blood is flowing in all the arteries, even in those most remote from the head, with the same speed as in those of the masseter muscle, and in the end the circumscribed movements of mastication will be found to have caused a more frequent pulse. We can understand that this effect will be more prompt and more intense when the movement, instead of being confined to a small group of contractile fibres, involves powerful muscular masses, as in violent exercises.

The increased frequency of the pulse during work results from a sort of aspiration of blood towards the muscles undergoing contraction. It is a vital law that every organ in activity draws towards it a greater quantity of nutrient fluid than it does when in a state of repose. This law has been verified even in regard to the secreting glands. We cannot enter into the intimate explanation of this phenomenon; suffice it to say that it is invariable, and that every stimulus of a vital function causes a flow of blood towards the organ thrown into activity. Ubi stimulus, ibi fluxus. This is a formula. which expresses the fact without explaining it.

Thus the excitement produced by the nervous flow to the contractile fibre draws to the muscle a greater quantity of blood, and, in order to supply this increase, the blood flows more quickly towards the excited organ. The increased frequency of the pulse is a physiological, not a mechanical, phenomenon, as at first sight we are inclined to think. It is not the pressure of the muscles swollen by contraction which quickens the circulation in the arteries: it is rather a kind of aspiration by the muscles, which have become more thirsty for blood.

This explanation is confirmed by the indications of a manometer connected with an artery. The instrument shows a fall of pressure in the vessel at the moment of work.* The pressure would rise, on the other hand, if the acceleration were due to a driving back of blood by the muscles.

Muscular contraction may, however, be called into play as a mechanical cause, capable of quickening the blood current in the veins and capillaries. The compression resulting from the swelling of muscles in action may thus become a factor in the acceleration of the blood current during work. In any case, however, this is merely an accessory factor.

Whatever the cause, the quickening of the blood current during exercise is a constant phenomenon. Constantly, therefore, at this time, the organs are traversed by a greater quantity of blood. There is an active congestion of all the organs during violent exercise: hence more active performance of function.

It will not be devoid of interest, after considering the phenomena, to study the consequences of the active congestion which accompanies every energetic movement. This active congestion is the really useful period of bodily exercises, that to which they owe their fortifying power. A man during this period of hyperactivity of all his organs benefits from a considerable increase of his nutritive forces. All the organs and all the tissues of the body are the seat of a more active circulation, and we know that the nutrition of an organ is in proportion to the quantity of blood which passes through it.

Under the influence of the greater quantity of blood which traverses the lung, this organ, fellow-worker with the heart, becomes more active, and introduces into the system a greater quantity of air. The vital combustions, thanks to this freer supply of oxygen, are more energetic and more complete. If we may make a simple comparison, exercise increases the intensity of the vital

• Chauveau, Comptes rendus de l'Académie des Sciences, 1857.

combustions, just as lowering the blower of a fire-place makes the combustion of the wood more active by increasing the draught.

The duration of this salutary period of exercise varies with different individuals. With some persons a considerable muscular expenditure is necessary to bring about this expansion of the vital forces due to active congestion: these are the strong natures, and those accustomed to exercise. With feeble natures, in persons accustomed to complete rest, and to muscular inaction, the least movement, the shortest walk, produces a similar result. The student calls a game of billiards exercise; for the pugilist who is training for a prize-fight, the use of dumb-bells is an exercise which employs without fatiguing him.

Perhaps the most interesting effect of this active congestion of the organs under the influence of bodily exercise is, that experienced by the brain. All thinkers have noticed that physical exercise is favourable to brain-work. The Peripatetics disputed while walking, and found their arguments more easily when the body was warmed by exercise. "Walking and movement," said J. J. Rousseau, "favour the action of the brain and the work of thinking."

Stimulation of the brain may be very great under the influence of active congestion brought about by muscular action. It is possible to be made drunk by movement, and in certain brains predisposed either by their native organisation, or by exalted ideas or passions, muscular exercise is often the prelude to actions resembling those of intoxication and even of madness. The war-dances of savages, the contortions of dancing dervishes, produce without the assistance of any alcoholic drink, a state of cerebral hyperexcitement capable of bringing about the most violent nervous phenomena.

It is related that the Gauls, in the midst of the excitement of battle, were sometimes seized with a sort of intoxication which made them furious and insensible to wounds.

Without going as far as intoxication, exercise in the

end produces in every one a slight excitement, a kind of animation. The young girl dancing becomes animated, and would spend night and day forgetful of fatigue: a quarter of an hour's waltzing puts her into the same condition as does a glass of champagne. A vigorous horse becomes animated by a gentle gallop, and sometimes becomes so excited as to be attacked by a kind of madness, and runs away.

All these phenomena result from a moderate cerebral congestion. The apparent effects of exercise on the individual are, in short, similar to those produced by alcohol the same flush, the same bright eyes, the same active demeanour.

Exercise has a stimulating action on all the organic functions, because it renders the circulation in all the organs more active. The blood makes all parts of the system share in the stimulation which the will has sent to the muscles to put them in action, and this stimulation is the more marked because the blood is warmed by the friction against the walls of the vessel through which it flows more rapidly, and warm blood is more stimulating.

Thus, when the limbs move, the internal organs cannot remain inert, and the whole organism performs its functions with more energy under the influence of muscular contraction.

CHAPTER III

HEAT.

The Human Motor and Heat-Engines. The Mechanical Equivalent of Heat-Heat is a Cause of Movement, not its Effect -Heat Lost-How the Temperature of the Body is regulated -Effects of Heat on Muscle-Experiment of Marey on Caoutchouc-Observation of Daily Phenomena-Muscle Heated and Muscle Numbed by Cold--Gestures of Anger-Why we Make preliminary Passes in Fencing-The Hare which has just been put up-Effects of too high a Temperature-Death of Muscle at +45° C.

I.

THE human body has been co:npared, inasmuch as it is a source of motion, to a machine driven by heat. We know that no machine creates force. The most perfect motors do nothing more than transform one force into another. The human motor transforms heat into movement.

In the steam engine it is easy to trace the connection between cause and effect. The movement of the wheels is due to a rod set in action by the piston; the piston moves on account of the pressure of steam, and the latter owes its expansile force to the heat which it has absorbed. Finally the heat itself is due to the combustion of carbon. The combustion of carbon is found on analysis to be the cause of the movement of the locomotive.

In the body the motor power of the muscles comes neither from the nerves, the spinal cord, nor the brain. We know that these three organs only transmit to the muscles the stimulus of the will. The will itself is not the source of the motor force; it orders the movement and sets the machine in action, just as the driver gives the first impulse to the locomotive by opening the steam