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unconsciously " cleared " by swallowing, which under ordinary circumstances is sufficient to open the Eustachian tubes and equalize the pressure on both sides of the ear drum. Occasion- ally a very graduated self-inflation, just sufficient to open the tubes, may be required to dispel a sensation of fullness in the ears. If, however, owing to very marked obstruction of the Eustachian tubes, no equalization of pressure has taken place, then at 20,000 ft. the pressure in the external auditory meatus is approximately 380 mm., while in the middle ear it is still 760 mm. (ground level), a difference of 380 mm. tending to push the drum outwards. If, on the other hand, during the relatively slow ascent to this height equalization of pressure is made, but, owing to Eustachian obstruction, little or no equalization is made during a rapid descent, then on reaching ground level there is through the external ear a pressure of 760 mm. but only about 380 mm. in the middle ear, a pressure which forces the drum painfully inwards. Such an " invagination " of the drum is sometimes found immediately after landing in pilots who com- plain of deafness, discomfort or pain in the ears, headaches, dizzi- ness, nausea and, in certain cases, vomiting and fainting in the air. In less severe cases, inspection of the ear drums often shows marked distension of the blood vessels. On enquiry it is usually ascertained that the symptoms complained of have come on during descent or immediately after landing, and are in many cases attributable to difficulty in equalizing the pressure within and without the tympanic cavity. It has been found also that one-sided obstruction of the Eustachian tubes may cause vertigo and incoordination in the air. The importance to the aviator, therefore, of adequate ventilation and drainage of the middle ear through the Eustachian tubes under rapidly varying degrees of atmospheric pressure is manifest. Broadly speaking, any con- dition of the nose or throat which causes or is likely to cause post-nasal or pharyngeal catarrh is a potential factor in the causa- tion of Eustachian obstruction. Abnormal conditions of the nose, throat and ears which are apparently of trifling importance on the ground tend to become considerably aggravated in the air. Free nasal respiration and a healthy condition of the upper respiratory tract are necessary in the aviator.

From what has been written it will be seen that the medical measures to be taken as regards flying consist in (a) the careful selection of flying personnel; (b) the effective care of those selected.

In the main the case for careful selection has been presented. The great necessity of nervous stability, efficient respiration and circulation has been shown. Attention has also been directed to the important part played by vision, as well as to the necessity of a healthy state of the ears and upper air passages.

A word may be added here as to the importance of vestibular stability. As already mentioned, a man cannot fly level in a fog. In certain countries, particularly in the United States, great importance was at first attached to the supposed " motion- sensing functions " of the vestibular apparatus. On them the success or failure of candidates for flying was believed largely to depend. The sensitivity of the vestibular apparatus was tested by means of " rotation tests." As the result of special investigation, so great an importance is not assigned to these tests in England. Generally speaking, rotation tests therefore are only employed when a candidate gives a history of giddiness, train or swing sickness, suggestive of undue sensitivity of the vestibular apparatus.

At first no special medical examination was made for flying, but early in the World War medical officers with squadrons collected considerable evidence which proved that a special examination was necessary. They were constantly seeing pilots who were breaking down or had actually broken down from causes which should have precluded their admittance to the flying services.

In addition to visual defects, olitis media, and conditions resulting in Eustachian obstruction, numerous instances of gross nervous instability were observed amongst unfit flying officers, who could never have been accepted for the service had details of their past histories been elicited at a medical examina-

tion. In the selection of flying personnel the importance of the past history of the candidate cannot be overestimated.

Nowadays candidates in England, both for military and civil aviation, are submitted to:

I. A surgical examination, comprising, in addition to measure- ment of height and weight, observations as to any existing surgical abnormality, congenital or the result of injury or disease, which is likely to impair the efficiency of f he individual.

II. A medical examination, including enquiries as to previous occupation, family and personal medical history, an investigation of the various systems, including special tests for flying efficiency.

III. An examination of the eyes from the point of view of normal acuity of vision and also of good ocular muscle balance. Normal colour vision is also demanded.

IV. An examination of the ears, nose, throat and buccal cavity, including tests of hearing, the patency of Eustachian tubes, and, when deemed necessary, the sensitivity of the labyrinthine apparatus.

V. An assessment in which, after such further examination as appears necessary, a decision is formed as to the candidate's fit- ness for flying.

The special tests employed in the assessment of efficiency are as follow:

For respiratory efficiency :

1. Measurement of the respiratory capacity by means of a spirometer.

2. The length of time during which the breath can be held after full expiration and full inspiration.

3. Measurement of the expiratory force that is, the height to which the subject can force a column of mercury with the cheeks and lips held.

For circulatory efficiency:

4. The pulse rate sitting, standing and after regulated exercise (lifting the body weight on and off a chair five times in fifteen seconds).

5. Measurement of the systolic and diastolic arterial pressures.

For nervous stability and neuromuscular coordination:

6. Observation of knee jerks and other reflexes.

7. Observation of presence or absence of tremor of eyelids, tongue and fingers.

8. The ability of the subject to stand steadily on one leg for 15 seconds with the eyes closed and hands to side.

9. The ability of the subject to raise from table to shoulder level and replace again an unstable rod placed on a piece of board.

Tests for endurance and resolution (testing respiratory and circulatory 'efficiency and nervous stability):

ip. After full expiration and full inspiration, the length of time during which the subject can support with the breath held, a column of mercury at 40 mm., the rate of the pulse being counted meanwhile.

The standards for these tests, which are used as adjuncts to the clinical examination, have been set by the examination of efficient pilots who have rendered satisfactory aerial service. Results have also been obtained from larger numbers of pilots who have partially or wholly broken down.

The duty of forming a final decision as to the candidate's fitness for air work rests with the assessor, a medical officer of wide experience. His decision is based upon a review of all the facts and observations recorded by the examiners, checked and supplemented by an examination on his part of such points as appear doubtful.

Apart from the elimination of cases which fail to satisfy the requirements in respect of the special senses of sight and hearing or show signs of organic disease of a gross or potentially disabling nature, the assessor's main duty is to ensure that the accepted candidate is possessed of a mental aptitude and a degree of stamina and nervous stability adequate to withstand the stress of training and of subsequent service in the air.

In forming an opinion on these points, no attempt is made to determine the temperamental suitability of candidates by elaborate psychological methods. In most cases the assessor is able to gain an insight into the candidate's general " mental make-up " by interrogation as to his motives for wishing to fly, by ascertaining his keenness for sports and games and by ob- taining details as to his service, if any, in the war. The evidence as to the soundness of the stock from which the candidate