CAISSON DISEASE  . In order to exclude the water, the air pressure within a caisson used for subaqueous works must be kept in excess of the pressure due to the superincumbent water; that is, it must be increased by one atmosphere, or 15 lb per sq. in. for every 331 ft. that the caisson is submerged below the surface . Hence at a depth of too ft. a worker in a caisson, or a diver in a diving-dress, must be subjected to a pressure of four atmospheres or 6o lb per sq. in . Exposure to such pressures is apt to be followed by disagreeable and even dangerous physiological effects, which are commonly referred to as caisson disease or compressed air illness . The symptoms are of a very varied character, including pains in the muscles and joints (the " bends "), deafness, embarrassed breathing, vomiting, paralysis (" divers' palsy "), fainting and sometimes even sudden death . At the St Louis bridge, where a pressure was employed equal to 44 atmospheres, out of 600 workmen, ttq were affected and 14 died . At one time the symptoms were attributed to congestion produced by the mechanical effects of the pressure on the internal organs of the body, but this explanation is seen to be untenable when it is remembered that the pressure is immediately transmitted by the fluids of the body equally to all parts . They do not appear during the time that the pressure is being raised nor so long as it is continued, but only after it has been removed; and the view now generally accepted is that they are due to the rapid effervescence of the gases which are absorbed in the body-fluids during exposure to pressure . Experiment has proved that in animals exposed to compressed air nitrogen is dissolved in the fluids in accordance with Dalton's law, to the extent of roughly r % for each atmosphere of pressure, and also that when the pressure is suddenly relieved the gas is liberated in bubbles within the body . It is these bubbles that do the mischief . Set free in the spinal cord, for instance, they may give rise to partial paralysis, in the labyrinth of the ear to auditory vertigo, or in the heart to stoppage of the circulation; on the other hand, they may be liberated in positions where they do no harm . But if the pressure is relieved gradually they are not formed, because the gas comes out of solution slowly and is got rid of by the heart and lungs .

Paul Bert exposed 24 dogs to pressure of 7-91 atmospheres and " decompressed " them rapidly in 1-4 minutes . The result was that 21 died, while only one showed no symptoms . In one of his cases, in which the apparatus burst while at a pressure of 91 atmospheres, death was instantaneous and the body was enormously distended, with the right heart full of gas . But he also found that dogs exposed, for moderate periods, to similar pressures suffered no ill effects provided that the pressure was relieved gradually, in x-11 hours; and his results have been confirmed by subsequent investigators . To prevent caisson disease, therefore, the decompression should be slow; Leonard Hill suggests it should be at a rate of not less than 20 minutes for each atmosphere of pressure . Good ventilation of the caisson is also of great importance (though experiment does not entirely confirm the view that the presence of carbonic acid to an amount exceeding 1 or 14 parts per thousand exercises a specific influence on the production of compressed air illness), and long shifts should be avoided, because by fatigue the circulatory and respiratory organs are rendered less able to eliminate the absorbed gas . Another reason against long shifts, especially at high pressures, is that a high partial pressure of oxygen acts as a general protoplasmic poison . This circumstance also sets a limit to the pressures that can possibly be used in caissons and therefore to the depths at which they can be worked, though there is reason to think that the maximum pressure (44 atmospheres) so far used in caisson work might be considerably exceeded with safety, provided that proper precautions were observed in regard to slow decompression, the physique of the workmen, and the hours of labour . As to the remedy for the symptoms after they have appeared, satisfactory results have been obtained by replacing the sufferers in a compressed air chamber (" recompression "), when the gas is again dissolved by the body fluids, and then slowly " decompressing " them . See Paul Bert, La Pression barometrique (1878) ; and Leonard Hill, Recent Advances in Physiology and Biochemistry (1906), (both these works contain bibliographies); also a lecture by Leonard Hill delivered at the Royal Institution of Great Britain on the 25th of May 1906; " Diving and Caisson Disease," a summary of recent investigations, by Surgeon Howard Mummery, British Medical Journal, June 27th, 1908; Diseases of Occupation, by T . Oliver (1908); Diseases of Workmen, by T . Luson and R .

Hyde (1908) .