a given time shall not be greater than that which the respiratory organs can dispose of in the same time.

The habit of practising an exercise or of performing a work brings a man or an animal instinctively to regulate the intensity of his muscular effort by his respiratory power, in such a manner that there may be an equilibrium between the amount of carbonic acid produced by the muscles and the amount eliminated by the lungs. Thus it is that every man, every animal, comes to adopt in the exercise of running, a pace-or rather a ratefrom which he cannot depart under pain of breathless

ness.

In horse racing, certain animals are charged with making the running. They force themselves to the front at the start, endeavouring to lead their adversaries into an extremely rapid gallop. The aim of this manœuvre is to force the other horses to exceed their paces, while a confederate holds himself in check in order to take the lead when the others begin to be exhausted. A horse which exceeds its paces is, from the physiological point of view, an animal which produces more carbonic acid than it is able to eliminate. This causes a rapid intoxication, which paralyses its action. To win the race, a horse is almost always obliged to provide at a given moment all the swiftness of which its legs are capable, and consequently, to exceed its paces. But the art of the jockey is not to let it exceed its paces till the last possible moment, so that it may not be exposed to this inevitable intoxication till close to the goal.

Nevertheless, if a violent exercise is performed continuously for a certain time, breathlessness is always produced in the end, although the individual does not exceed his paces. Let us suppose a case in which the muscular work produces a quantity of carbonic acid just equal to that which can be eliminated by the lungs. At first there will be no breathlessness, as there is an equilibrium between production and elimination. But if the work goes on, respiration in the end becomes embarrassed. Running at a moderate pace, for instance,

which we can do for five minutes without losing breath, will produce breathlessness in a quarter of an hour, although the pace remains the same.

It is because, the quantity of work remaining the same, the respiratory power is diminished by the very continuance of the exercise. In the very act of working disturbances are produced in the functions of the respiratory apparatus. The circulation of the blood through the lungs is increased, and there results at first an active congestion. Later however, we have passive congestion as a result of fatigue, of the forcing of the right side of the heart, the contraction of which is not sufficiently powerful to drive the blood through the pulmonary vessels. Further, the nerve-centres, strongly stimulated by the carbonic acid brought to them by the blood, react on the movements of the lung in a reflex manner, and respiration becomes short, precipitate, and irregular.

The congestion of the lungs, the derangement of the respiratory movements, the excitement, and then the enfeeblement of the heart-beat, are so many secondary factors of breathlessness, which we shall study in the next chapter. The part they play in the production of dyspnoea in the course of exercise is important, for they create obstacles to the free working of the lungs at the moment when there is need that these organs should work with their fullest power.

CHAPTER III.

BREATHLESSNESS—(continued).

Mechanism of Breathlessness-Reflex Disturbances of the Respiratory Movements-Physical Sensations and Moral Impressions Stammering Respiration -- Why we become less Breathless at a Fencing-School than in a Duel-Reflexes due to Carbonic Acid-Reflexes are at first Useful; they become Dangerous when Exaggerated-Dangers of Instinctive Movements-Part played by the Heart in Breathlessness—Active Congestions-Fatigue of the Heart-Muscle and Passive Congestion of the Lungs-The Influence of the Heart is Secondary -Cessation of Breathlessness notwithstanding the Persistence of Circulatory Disturbances after Exercise-Personal Observation; the Ascent of Canigou-Effort; its part in Breathlessness-Prompt Advent of Breathlessness in Wrestling-Sprint Running and Long-Distance Running-Our observations on the Rhythm of Respiration during Breathlessness-Inequality of Expiration and Inspiration during Breathlessness; Causes of this Inequality-Serious Phenomena of BreathlessnessAction of Carbonic Acid on the Muscular Fibres of the Heart.

THE essential condition of respiration is the presence in the lungs of atmospheric air and of venous blood, in order that the inspired air may give up its oxygen to the blood, and that the blood may rid itself, in exchange, of its carbonic acid. It is evident that any obstacle to the circulation of blood in the pulmonary capillaries or to the free entrance of air into the pulmonary air-cells, will render the respiratory act incomplete.

Now violent exercise causes a disturbance in the respiratory movements which renders them less efficient in drawing air into the chest, at the same time as it produces in the vascular system disturbances capable of hindering the pulmonary circulation.

Each of these two effects deserves attentive study.

I.

Exercise may have a direct action on the respiratory movements, for many muscular actions are performed with the aid of the muscles of the thorax or the back. These muscles, being used in the work, are momentarily distracted from their function as respiratory muscles. They can even stop respiration when they take their fixed point at the ribs for the purpose of moving the upper limbs. Effort, which we discussed at length in the chapter on Movements, is the type of the actions which stop respiration by fixing the thorax.

This action has important consequences on the circulation of the blood, and it is in this way that it especially affects the respiratory function. But it also momentarily hinders the interchange of gases, and this usually at a time when it is most needed. Stoppage of respiration during repose has no grave consequences, because it is always followed by a compensatory effect, by a series of longer and deeper respirations, which promptly eliminate the carbonic acid, of which the quantity retained in the body cannot be excessive while the muscles are at rest. But if the effort has taken place during work, it happens that the working of the lungs is hindered just at the moment when their action ought to be increased; the suspension of respiration shuts the passage by which carbonic acid ought to be eliminated, at the very time when the muscles are producing three or four times as much of this gas as usual.

Respiration, which hardly answered the needs of the system while the lungs were freely performing their work, becomes suddenly insufficient when the thoracic movements are so hindered. Thus the repeated stoppage of respiration during work may become a very efficient cause of dyspnoea, while in a state of repose it produced but a passing disturbance.

But effort, and the other muscular actions capable of suspending or hindering the play of the ribs, are not the

most frequent causes of the respiratory troubles which we observe during exercise. Respiration is often profoundly modified in its rhythm, its amplitude, and its frequency, without it being possible for us to consider these disturbances as a result of the direct action of the exercise performed. Very often we see that exercises, whose performance does not involve the use of the thoracic muscles, nevertheless profoundly affect the thoracic movements.

It is then by a reflex action that we must account for this indirect effect of exercise.

Reflex actions, capable of modifying the rhythm of respiration, have very various origins, and the lungs are very frequently exposed to their effects, being the most impressionable of all organs. In order fully to understand the reflex effects to which the lungs are subject, we must recall the fact that powerful impressions in general, whether physical or moral, tend to produce involuntary movements, and that these movements may take place just as much in the muscles of organic as in those of animal life.

When we pass near a room in which a person is taking a cold shower-bath for the first time, we hear sighs and suppressed groans. These inarticulate sounds, which resemble cries of distress, are simply reflex actions. The sensation of cold which the water causes when applied to the chest-walls is transmitted to the nerve-centres as a stimulus which produces abrupt expiratory and inspiratory efforts. The air is violently drawn into the chest, or driven out from it in an irregular manner, and causes in its passage vibrations of the vocal cords without the influence of the will. the impression produced by the cold water is too strong, the reflex action may culminate in a complete stoppage of respiration: it becomes impossible for the air to enter the chest, or, having entered, to get out again. Hence there is a kind of distress, a momentary suffocation which makes hydrotherapeutics very painful at first to impressionable patients.

If

Every violent physical sensation, wherever situated,