EFFECTS OF ALTERED AIR PRESSURE 353 



ordinary state of affairs ; but as improvements were made in experimental 

 nietliods, the absorption was shown to follow physical lines ; it was then 

 held to apply to cases of muscular exercise, and now only to acclimatisation to 

 high altitudes. One might venture to say that the more accurate the methods 

 of investigation the better is it found that chemical and physical laws are 

 capable of explaining physiological jihenomena." — Bayliss, Principles of 

 General Physiology. 



Let us now consider what happens to the inland transport 

 service when the port becomes congested with incoming traflftc. 

 Compressed air is used in all the great sub-aqueous works of to-day, 

 in diving, in preparing foundations for bridges, in pier building, and 

 in the construction of tunnels or shafts through water-bearing 

 strata. It is well known that a large percentage of the men 

 working under those conditions suffer illness and many die. In 

 the construction of the Adour bridge 90 per cent, of the workers 

 suffered from " compressed air " disease, and in the boring of the 

 Hudson Tunnel 2 per cent, of the caisson workers died each month. 

 Compressed air sickness is characterised by its protean symptoms 

 ■ — loss of speech, blindness, deafness, transitory madness, vertigo, 

 loss of conciousness, emphysema, spinal paralysis, etc. None 

 of the symptoms, with the exception of some slight ear trouble, 

 ever occurs while the men are under pressure. " Mules lived about 

 a year in the Hudson tunnel and were healthy enough to kick and 

 bite at all comers," The illness seemed to come on during or 

 after decompression, and is now known to be due to the appearance 

 of bubbles of nitrogen in the tissues. Boyle, in the seventeenth 

 century, showed that bubbles of gas appeared in the humours of a 

 viper's eye when submitted to rapid decrease of air pressure under 

 an air pump. Paul Bert in a remarkable series of experiments 

 (1870-1880) proved that these bubbles were nitrogen and that 

 they might block up the capillaries in some part of the body and, 

 by cutting off that part from the blood supply, produce one or 

 other of the symptoms mentioned above. 



If merely the pressure of the surrounding air is increased, why 

 should nitrogen alone be set free on decompression ? When a 

 person is placed in compressed air, the blood passing through the 

 lungs dissolves the same volume of the atmospheric gases as it 

 does under normal conditions, but the weight of gas absorbed 

 will be increased above normal in proportion to the increase in 

 partial pressure of each gas in the alveolar air. Now we have seen 

 that the partial pressure of carbon-dioxide in alveolar air is a 

 constant, hence there can be no increase in the amount of carbon- 

 dioxide present in the blood during exposure to compressed air. 

 Oxygen is carried in two ways, (a) by haemoglobin, and (b) in 



B. 23 



