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But such is not the case, as there is no special " flying sickness " brought about solely by the pursuit of aeronautics. Although certain authorities have inclined to recognize some mechanical effects owing directly to the reduction of atmospheric pressure upon the body, this is only of importance in connexion with the air enclosed within the cavity of the middle ear and to a lesser .extent as regards gas inside the intestines. Changes of absolute pressure of the atmosphere produce no mechanical effects since the altered pressure is transmitted equally in all directions through the semi-fluid body tissues. The suggestion has also been made that, owing to the diminution of atmospheric pressure, the airman may be liable to a special disease, somewhat akin to that experienced by the diver or the worker in compressed air. The cause of " diver's palsy," " caisson disease," or " com- pressed-air illness " is now thoroughly well established. When man is subjected to an increased air pressure he dissolves in the fluid portion of his blood a considerable amount of nitrogen from the surrounding air. When the air pressure is diminished, this nitrogen is again given off. If the diminution in pressure be rapid, then bubbles of gas are liberated inside the blood vessels, in the same way as bubbles of gas are liberated when fluid is removed from a siphon of aerated water. These bubbles then circulate in the blood and produce symptoms, according as they become lodged in the various parts of the body.

At first sight, therefore, it might be supposed that an airman making an ascent, in other words subjecting himself fairly rapidly to a diminution of the surrounding air pressure, might be liable to symptoms arising from the same cause as does " diver's palsy." This, however, is not the case, since the diminution in pressure is not sufficiently great or rapid to bring about any liberation of gases held in the blood plasma. In " diver's palsy " and " caisson disease " one is dealing with a reduction of pressure of from two to five atmospheres, whereas in flying one is generally dealing at most with a diminution of pressure of a little more than half an atmosphere, which is reached relatively slowly, and is easily within the margin of safety for the rate of decompression in compressed-air work. The idea, therefore, that airmen are subject to any special " flying sickness " of this nature may be dismissed.

Because it is stated that there is no " flying sickness " it does not mean, however, that flying may not cause bodily breakdown. Flying imposes a very definite stress upon the body, especially when flights are carried out for long periods at high altitudes. When to this is added the stress of offensive and defensive war- fare in the air it is obvious that bodily breakdown as the result of " strain " is likely to ensue. But the signs and symptoms of " flying strain " are varied and might occur in an individual quite apart altogether from flying. In the World War it was found that " flying strain " was most generally characterized by a gradual loss of power to fly high, associated in varying degrees with symptoms of respiratory, cardiac and nervous derangement, such as breathlessness on exertion, quickened heart-beat, exaggerated reflexes, marked tremor of fingers and eyelids, and loss of neuromuscular control as exemplified by power to balance on one leg. Mental symptoms, generally in the form of anxiety neurosis, might or might not be present. In many cases it was difficult to say whether breakdown was to be attrib- uted primarily to the effects of flying or to the nervous strain of aerial warfare, but such symptoms were frequently found to occur in those who had taken no part in active service in the air.

In order to appreciate the correct medical measures which must be taken in respect of the care of flying personnel, it is necessary in the first place to consider the human machine in relation to flying. The aviator provides the controlling and coordinating mechanism on which the satisfactory performance of the aeroplane depends. The pilot adds the aeroplane to him- self the " joy-stick," engine controls and so forth are append- ages to his hands, the rudder bar an extension to his feet. By appropriate movements of his upper and lower limbs man is now able to fly, just as previously by appropriate arm and leg move- ments he was able to indulge in games or to control other forms of mechanism, as, for example, a motor-car.

To acquire the art of flight, therefore, a number of controlled and coordinated movements are necessary. It is common ex- perience that certain people are found heavy-handed or heavy- footed and not likely to acquire the art of flying. In the apt pupil these coordinated movements are at first all made as the result of conscious effort, but later they pass into the realm of the automatic, so that eventually the expert pilot does not have to think how he flies he just wishes his machine to perform a certain evolution and it occurs.

No elements come into the mechanical problem of flying that are not required for driving a motor-car or taking part in various sports; some men have more aptitude for flying than others; just as some have more aptitude for games.

To initiate the coordinated movements necessary for flying, the pilot relies upon certain sensory impressions. Vision is the most important. Without facilities for using his eyes a man is not able to fly. It has been found that experienced pilots cannot satisfactorily perform even a simple evolution with the eyes- blindfolded. It is also well known that pilots cannot fly level in fog and may even get upside down. This is due to the temporary eclipse of the sense of vision ; unaided by instruments, man will never be able to fly in a fog successfully.

Besides good visual acuity it has been found that harmonious working of the muscles moving the eyeballs is necessary, particu- larly for successful landing, and is lacking in a great percentage of bad landers. By careful training it has been found possible to bring about good visual judgment of distance and to turn bad landers into good ones.

For successful flying, next to vision and perhaps almost equally important, come the sensations from the skin and muscles. A pilot flies very largely by the " feel " of his machine. In addition to the " feel " of the controls, he derives much information from the " feel " of his seat, from the direction and change of direction of the wind on his face. He is also aided by hearing the singing of the wind in the wires. Hearing is of importance also in flying in so far as it enables a pilot to detect a failing engine, to operate wireless and to hear a telephone above the roar of the engine.

According to some people it has been thought very necessary that a man should have a good sense of balance, but experience has shown, as already mentioned, that " balance sense " is not sufficiently developed in any man to enable him to fly level in a fog.

But for flying it is not sufficient to be endowed with a mechani- cal and mental aptitude; a consideration of prime importance is physical endurance to resist the stress of high flights or flights of long duration. For endurance it is particularly important that a man be fit as regards his respiratory and circulatory mechanisms. This has been shown by the examination of fit pilots as well as of subjects who have been deemed in need of a rest or who have broken down as the result of flying strain.

The examination of successful flying officers showed that they were possessed of an efficient respiratory capacity. The examina- tion of officers taken off flying through " flying strain " showed that their capacity was very much diminished. It was found by careful observation that this fall was due chiefly to ineffective working of the " exhaust " or expiratory side of the respiratory " bellows." The individual had lost his power to expire fully to the greatest extent. He, therefore, could not empty his lungs satisfactorily. Such a condition makes for deficient ventilation and the subject becomes very like a motor-engine in which the exhaust valves are defective and incomplete scavenging of the cylinders results. Hence we find that the airman in this condition easily gets breathless on the ground and certainly can- not fly to heights at which formerly he did not notice anything abnormal in his breathing.

For endurance and high flying, therefore, it is especially im- portant that a flier have an adequate " bellows capacity " and that the " bellows " be particularly effective on the exhaust side. An efficient expiratory force is, therefore, very necessary to the pilot.

Examination of successful flying officers also showed that the effective pilot is possessed of an efficient circulatory system.