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Observation has shown that there is a marked difference between the fit and unfit pilot in this respect. For example, the fit pilot is possessed of a regular, fairly slow pulse which gives the im- pression of a delightfully easy-working piece of mechanism. It is not greatly quickened by exercise and speedily returns to its normal rate. The pulse of the man unfit for flying, or unfit to learn to fly, is unduly quickened by exercise and takes con- siderable time to return to normal.

Circulatory efficiency also depends upon the pressure main- tained in the arteries both during and between the beats of the heart. With the beat of the heart the pressure in the arteries rises: during the rest period it falls. In some people it may fall greatly, in others but a little. The examination of successful fly- ing officers has shown that in them the fall is not great, whereas in the tired or inefficient individual the difference in the pressure during and between the beats is relatively large. The importance of a good pressure between the beats will be appreciated when it is realized that if the fall of pressure be great enough, fainting may result.

The efficiency of the circulatory mechanism of the body is intimately bound up with the efficiency of the respiratory mechanism. The abdominal cavity has sufficient vessel capacity to take the whole of the blood of the body and, in the upright or sitting posture, blood, by virtue of the effect of gravity, will tend to stagnate there unless its return to the heart is aided by the movements of respiration. In inspiration the downward thrust of the great muscle separating the chest from the abdomen, the diaphragm, acts like the piston of a pump and squeezes blood upwards into the heart, since it is prevented from escape in any other direction by means of valves placed in the vessels. During expiration the muscles of the abdominal wall and of the lower ribs squeeze inwards upon the abdominal contents and again force blood upwards to the heart.

The importance of these accessory pumps to the circulation is well exemplified in the crucifixion of a man. In the vertical posture the immobilization of the limbs and the restriction of the action of the respiratory and abdominal muscles cause blood to stagnate in the lower limbs and the abdomen, thereby con- tributing the principal cause of death.

Since in the machine the pilot is rendered relatively immobile in a sitting posture, it is of the greatest importance that he be possessed of efficient respiration and good abdominal tone, in order that an adequate circulation may be maintained. The importance of good abdominal tone is further emphasized by the following experiment. If a hutch rabbit, with its flabby, pendu- lous abdomen, be held in the vertical posture, it will soon become unconscious owing to the lack of tone of its abdominal wall; a wild rabbit, on the other hand, will not do so, owing to the fact that, on account of the exercise taken in its free open-air life, it has developed the tone of its abdominal musculature.

This emphasizes the value of sport in developing the respira- tory and circulatory mechanisms, and for this reason all airmen are advised to take up sports which, besides giving eye and limb coordination, also give physical endurance by toning up the respiratory and circulatory mechanisms. The importance of sports and games in the life of the flying man cannot be over- emphasized.

In addition to the power of endurance the pilot must also be possessed of quick perception and judgment, which, besides enabling him to learn to fly, will help him to meet any sudden emergency which may arise while he is in charge of his machine in the air. He must therefore possess good mental and nervous stability. Such stability is of even greater importance in the service pilot who may be called upon to undertake combatant service in the air.

Since 1878 it has been known that the chief cause of " mountain sickness " or " altitude sickness " is lack of proper oxygenation of the body owing to the rarefaction of the air breathed. Ex- periments conducted in rarefaction chambers as well as at high altitudes, such as Pike's Peak and Monte Rosa, have fully proved this point. In respect of life at high altitudes, however, a certain degree of bodily acclimatization takes place, which is

not the case in respect of flying. In an aeroplane the length of sojourn at high altitudes is insufficient to induce any ac- climatization, beyond possibly a transitory concentration of the blood plasma. In flying the effect of increasing altitude is in the first place a deepening of the respiration in order to secure the oxygen necessary to maintain the bodily functions. At the same time the heart quickens, and thus is established the beginning of a " vicious circle." For an increase in the rate of the heart-beat means an increase in the amount of work done by the heart, and this increased work entails an increased oxygen consumption, the supply of which is diminishing; thus each factor reacts unfavourably upon the other.

All the devices to render the respiration and circulation efficient will, therefore, be called into play to meet the changing conditions, so that with prolonged and repeated stress a break- down of the respiratory and circulatory mechanisms, involving also the nervous system, is to be anticipated, unless appropriate measures are taken to mitigate the ill effects. This has been found to be the case.

The effects of flying at great altitudes were observed as the result of the high flying which became necessary during the World War. In the earlier stages of the war such flying was the exception rather than the rule. Owing to the increasing altitudes reached by aeroplanes, however, it became eventually quite an ordinary event for high-flying aeroplanes to maintain an altitude of from 20,000 to 22,000 ft. for several hours. When this first took place it was found that after a time the pilots and observers began to suffer from the effects of prolonged exposure to such altitudes. In the air the chief among these effects were breath- lessness, muscular weakness and diminution of judgment followed by great bodily fatigue. This, when frequently repeated, led to the signs of breakdown already given.

Another effect of high altitudes was the onset of drowsiness or sleepiness. In some cases this was excessive and pilots have stated that they have fainted at great heights and cannot re- member landing, whereas they have actually been sufficiently awake to fly the machine and land it in their own aerodrome with verbal assistance from the observer.

At great altitudes there is, therefore, either a general slackening of moral and loss of offensive spirit or else a feebleness of judgment which may lead a pilot into unnecessary difficulties. The effects of high altitudes upon judgment are insidious and constitute for the aviator a subtle danger.

Some flying officers eventually complained of headaches which at times came on while in the air, but more usually after landing. Vomiting and bleeding from the nose were very rare indeed. Cases of syncope were infrequent.

As with " mountain sickness," the symptoms described above are chiefly due to oxygen want and it was found that with the provision of oxygen apparatus on high-flying machines these symptoms were greatly alleviated.

As is well known it has been shown that the administration of oxygen (i) tends to keep an efficient slow pulse; (2) tends to keep up a good arterial pressure; (3) keeps off the onset of dis- tressful breathing; (4) mitigates any ill effect due to excessive deep breathing; (5) increases the power for nervous concentration and muscular work.

In flying, particularly in high flying, it is important that the pilot be able to accommodate himself to the effects of diminished pressure upon the air enclosed within the middle ear and the air passages connected with the nose. Any hindrance, for example, to effective ventilation and drainage of the frontal sinuses in the brow may lead to headaches of varying duration. As regards the ear, the external orifice affords a wide passage by which alterations of air pressure are easily transmitted to the ear drum; on the other hand the Eustachian tubes, leading from the throat to the middle ear, are narrow passages which normally open only during the act of swallowing, and therefore do not so readily transmit changes of pressure. Any catarrhal condition or congestion of these tubes, therefore, tends to produce difficulty in the equalization of pressure within and without the tympanic cavity. Generally speaking, during an ascent the ears are