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 compared, 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

valve. But it would be as absurd to say that the will produces the muscular force, as to attribute to the driver the strength and swiftness of his machine.

The initial fact and the indispensable condition of movement in the human body, just as in all other heatengines, is the production of heat. The body produces heat by the combustion of its own materials.

The combustion of our tissues is an expenditure necessitated by movement. How is this expenditure employed and how does the muscle make use of the heat produced? This problem is far from being solved, but we know that in the body, as in the machine, there is an intimate connection and a constant relation between the quantity of heat expended and the amount of work done. Muscular work is, like the work of all thermodynamic apparatus, subject to the principle of the mechanical equivalent of heat.

In mechanics, work is measured by a unit called the kilogrammetre, and heat by a unit called the calory. The kilogrammetre is the quantity of work necessary to raise a weight of one kilogramme through a height of one metre, and the calory is the quantity of heat necessary to raise the temperature of a kilogramme of water one. degree centigrade.

Physicists tells us that the mechanical equivalent of heat is 425 kilogrammetres. That is to say, the quantity of heat necessary to raise the temperature of a kilogramme of water from o° C. to 1° C. would be capable, when transformed into work, of raising a weight of 425 kilogrammes through a height of one metre.

Such is the theory; but in practice there is always much work lost, and the most perfect apparatus hardly makes use of nine or ten per cent. of the heat produced : the rest is lost in the machine, which becomes heated. In the human body, the heat lost, from the point of view of work, is considerable, and as the body absorbs that which is not used, its temperature is raised during muscular exercise.

Considering the enormous quantity of heat lost during muscular work, it is at first sight astonishing that the

body in which so much heat is distributed, does not rise to a higher temperature. As a matter of fact, we find that the temperature of a man doing violent work is only 1° C. or 2° C. higher than that of a man at rest.

This constancy in the temperature of the human body is due to a regulating mechanism with which the system. is provided and which works in the following manner.

When the body is warmed, the superficial bloodvessels dilate and receive a greater quantity of blood drawn from the internal organs. Thus we always see the skin of a hot man redden and swell. The blood which passes through the skin cools very quickly by radiation, and, since the circulation is very active in the body of a person doing work, the liquid which has given up its heat soon has its place taken by another wave of blood which becomes cool in its turn. In a few minutes all the blood in the system has been thus exposed at the surface of the body.

The surface of the skin considered as a means of refrigeration, does not only radiate heat, but also makes use of the evaporation of cutaneous perspiration. Every one knows the cooling power of an evaporating liquid. Everyone can also confirm the comfort derived from sweating when working at a high temperature.

Where there is excessive cooling, the mechanism is the opposite to that which we have just described. The capillaries contract under the influence of cold, and their calibre is lessened, driving the mass of blood forcibly into the internal organs. In the depth of the tissues the blood, protected from the external cold, can preserve its heat.

It is commonly said that work produces heat in the body; but the opposite is the truth: the heat is the cause and not the effect of the work. It is necessary to supply heat to a steam-engine in order to make it do work, and a considerable time elapses from the moment when the fire is lighted to that when the locomotive starts. In our system the affair is not such a long one, but the work is not absolutely instantaneous, and there is always an appreciable interval between the willing and