^{page 343 note 1} La Pression Barometrique, 1878.

^{page 343 note 2} Luftdruckerkrankungen, 1900; also v. Schrötter, , Der Sauerstoff in der Prophylaxie und Therapie der Luftdruckerkrankungen, 2nd edition, 1906Google Scholar. The former work contains a very full abstract of all previous investigations on the subject.

^{page 343 note 3} This Journal, vol. III. (1903), p. 401Google Scholar (and references there are given): See also Recent Advances in Physiology, 1906, pp. 233–255Google Scholar.

^{page 343 note 4} Proceedings of the Royal Society, vol. LXXVII. p. 442, 1906; vol. LXXIX. p. 21, 1907;Google ScholarBritish Medical Journal, 07 7th, 1906, 02. 16th, 1907, 06 22nd, 1907.Google Scholar

^{page 344 note 1} The Report of this Committee, which has recently appeared as a blue-book, contains a full account of the experimental investigations on Diving, carried out under its anspices at Portsmouth, off the West Coast of Scotland, and elsewhere, during the last two years: also a short summary of the experiments detailed in the present paper, and many data as to the occurrence of compressed-air illness in connection with diving and other work in compressed air. The conclusions and recommendations of the Committee are summarised at the beginning of the Report.

^{page 345 note 1} Journal of Physiology, vol. XXXII. (1905), p. 229.Google Scholar

^{page 345 note 2} Proc. Roy. Soc., B, vol. LXXVII. p. 442.Google Scholar

^{page 346 note 1} Nagel's Handbuch der Physiologie, vol. I., 1905, p. 63.Google Scholar

^{page 346 note 2} Proc. Roy. Soc., vol. LXXIX. B, 1907, p. 366.Google Scholar

^{page 346 note 3} Haldane, and Smith, Lorrain, Journal of Physiology, vol. XXV., 1900, p. 340.Google Scholar

^{page 347 note 1} This calculation is in principle similar to that made by Zuntz (Fortschritte der Medizin, 1897, No. 16), and worked out more fully by Heller, Mager and v. Schrötter (loc. cit). On account, however, of the discovery that fat has a very high coefficient of absorption for nitrogen, and that the blood volume in man is considerably less than was formerly supposed, our calculation gives a much slower rate of saturation per round of the circulation.

^{page 348 note 1} Haldane, and Priestley, , loc. cit., p. 245.Google Scholar

^{page 348 note 2} As a result of numerous experiments on man with the lung Loewy, catheter and v. Schrötter, (Untersuchungen über die Blutcirculation beim Menschen, 1905, p. 90Google Scholar) infer that the average rate of blood flow during rest is slightly faster. At present, however, there is some doubt as to the interpretation of results obtained by the lung catheter method.

^{page 348 note 3} Proc. Roy. Soc. B, vol. LXXIX., p. 21, 1907.Google Scholar

^{page 349 note 1} Boycott, G. W. M., Trans. Inst. of Civil Engineers, vol. CLXV., 1906.Google Scholar

^{page 350 note 1} The only method apparently available to determine the time of complete saturation in normal animals is to subject them to a series of experiments in which the pressure and decompression are kept constaut and the time of exposure varied, and to observe when the effects cease to become any worse. The method is open to obvious limitations.

^{page 351 note 1} In view of the enormous surface (probably more than 100 square metres) presented by the lung alveoli for diffusion it seems hardly possible to doubt that the blood during its passage through the lungs becomes saturated or desaturated to almost exactly the pressure of nitrogen in the alveolar air. According to the calculations of Loewy, and Zuntz, (Die physiologischen Grundlagen der Sauerstoff-Therapie in Michaelis' Die Sauerstofftherapie, Berlin, 1904Google Scholar), a difference in partial pressure of oxygen of less than 1 mm. of mercury would account for the diffusion of 250 c.c. of oxygen per minute through the alveolar walls. With a difference in partial pressure of nitrogen of two atmospheres, or 1520 mm. of mercury, between the blood and the alveolar air only about 70 c.c. of nitrogen would require to pass per minute in order to establish complete saturation, or desaturation, of the blood. The conditions are thus enormously more favourable for the taking up or giving off of this nitrogen than for the taking up of oxygen by diffusion during normal respiration.

^{page 354 note 1} It is evidently a mistake to assume that a given rate of uniform decompression, such as 20 minutes per atmosphere, is either necessary for safety in all cases, or would be actually safe except from some limit of pressure. From a pressure below this limit the rate will be unnecessarily slow, and from above it dangerously fast.

^{page 355 note 1} Proc. Roy. Soc. B, vol. LXXVII., p. 449, 1906.Google Scholar

^{page 355 note 2} One atmosphere or 760 mm. of mercury=14·7 lbs. per square inch, about 1 kilogram per square centimetre, 34 feet of fresh water, 33 feet of sea water. In this paper where pressures are defined in pounds or atmospheres without qualification, references is intended to the excess over atmospheric pressure as shown on gauges, not to the absolute total pressure.

^{page 355 note 3} Babington, and Cuthbert, , Dublin Quarterly Journal of Medical Sciences, vol. XXXVI., 1863, p. 312CrossRefGoogle Scholar. In the list of fatal cases given by Heller, Mager and v. Schrötter (Luftdruckerkrankungen, p. 1072), are entered two deaths at a pressure of 1·4 atmospheres. A perusal of Paul Bert's original account (La pression Barometrique, p. 401) shows that both the pressure and the cause of death are quite uncertain.

^{page 357 note 1} Whether the law holds good for pressures much exceeding six atmospheres is still doubtful, as no experimental data exist.

^{page 359 note 1} Heller, Mager and v. Schrötter recommend that at all depths decompression should be at a rate of at least 20 minites per atmosphere. This would imply a dealy of 25 minutes in coming up from 42 feet. Both common practical experience and our own experiments show that this excess of caution is quite unnecessary at small depths.

^{page 361 note 1} The regulations of the Dutch Government make the following method of decompression obligatory for work in caissons. The pressure is to be lowered at the rate of not more than 1/10th of an atmosphere in 3 minutes till 3 atmospheres of excess pressure is reached: then at not more than 1/10th of an atmosphere in 2 minutes till 11/2 atmospheres excess pressure is reached; and finally at not more than 5/10th of an atmosphere in 1½ minutes till normal pressure is reached. This method is still more unsuitable than uniform decompression, and would be very unsafe with high pressures.

^{page 362 note 1} This was fully realised by Catsaras who recommended a stay on the bottom of only 1 minutes at 30 fathoms.

^{page 370 note 1} Smith, Lorrain, Journ. of Physiology, vol. XXIV., p. 19, 1899CrossRefGoogle Scholar; Hill, and Macleod, , Journ. of Hygiene, vol. III., p. 401, 1903.CrossRefGoogle Scholar

^{page 371 note 1} One animal showed bends after decompression which was effected in 133 minutes by stages. After exposure at +75 lbs. for 3 hours in air this decompression gave 2 bends in 14 goats. There is therefore no evidence that the exposure to high pressure oxygen increased the susceptibility to caisson disease.

^{page 373 note 1} The delivery of the inlet tap should also be restricted, and the man in charge should have strict directions to take care that the rate of admission or discharge of air does not cause pain in the ears. of any of the men in the lock. To avoid pain a very slow rate of air admission may sometimes be needed, but with practice a rise of pressure of one atmosphere per minute is often not too much, so that any definite rule, limiting the rate to much less than this, seems scarcely desirable.

^{page 374 note 1} With the lock air-tight, and no ventilation, uniform decompression at any required rate could be easily secured by means of a reducing valve on an outlet, with a graduated tap beyond it, the arrangement being similar to the reducing valve and tap usually connected to a cylinder of compressed oxygen or gas used for limelight. If the delay in the lock is so long that ventiliation is required, or if ventiliation is needed in order to compensate for accidental leakage, it would be best to have an adjustable safety valve on the outlet, and adjust this by one pound at a time at the proper intervals.

^{page 374 note 2} We have some doubt as to whether the increased slowness of decompression after very long exposures would be altogether sufficient to meet the increased tendency to slight symptoms (“bends”). These are, however, of minor importance if all serious symptoms are prevented. We also think that with long shifts, exceeding a total of about 3 hours, still slower decompression would be needed for any men inclined to obesity. Such men should, therefore, be excluded in the medical examination which all men working in air at high pressures ought previously to undergo.

^{page 376 note 1} A comparatively rapid fall in absolute pressure in the proportion of 2·2 to 1 is within practically safe limits, particularly if the previous period of continued exposure has not exceeded three of four hours.

^{page 379 note 1} The weights are only given in a few instances; from these it may be surmised that the dogs were small (5 to 12 kilos).

^{page 380 note 1} Of 22 animals whose blood was examined, 16 gave no reaction with M. melitensis at a dilution of 1 : 20, 3 gave some reaction at 1 : 20, while 3 animals gave complete agglutination up to 1 : 200 (XVIII A, XXVI A, XXIII A). Cultures from the blood, during life were negative, and when they eventually came to autopsy cultures of blood, spleen, liver, inguinal, axillary, mesenteric and mammary glands were negative as regards M. melitensis. The exact history of these animals could not be obtained, but there is practically no doubt that they had never been out of England.

^{page 382 note 1} Journal of Physiology, vol. XXXVI. (1907), p. 283.Google Scholar

^{page 383 note 1} Greenwood, (British Medical Journal, June 22nd, 1907, Supplement, p. 409Google Scholar) has recently found that high percentages of CO₂ do not increase the liability to decompression symptoms.

^{page 384 note 1} Nature, vol. LXXV., 1907, p. 330.Google Scholar

^{page 385 note 1} We are indebted to Dr H. W. Armit for this photograph.

^{page 395 note 1} i.e. pressure 100 lbs. positive; exposure for 1 hour; decompression in 1 minute.

^{page 395 note 2} Post-mortem experience shows that the stomach alone is distended, not the bowels.

^{page 399 note 1} The details of the experiments in this group are given in Appendix III.

^{page 399 note 2} The stage decompressions from 45 lbs. pressure are likewise all shorter than what we calculate to be safe. The stoppages are also imperfectly spaced. The proper spacing and duration of stoppages could not be calculated till the results of the experiments were known, and we realised the extreme slowness of saturation and desaturation.

^{page 402 note 1} The details were eaten by a goat. All the animals were about the same size.

^{page 402 note 2} British Medical Journal, 06 22nd, 1907, Supplement, p. 408.Google Scholar

^{page 406 note 1} The respiratory activity per unit of body weight, being proportional to the ratio of surface to mass, would of course vary but little in the goats, and would only be about a fourth greater in a goat of 15 kilos than in one of 30 kilos.

^{page 408 note 1} Journal of Physiology, vol. XXXIII. (1906), p. 499.Google Scholar

^{page 410 note 1} We have since examined this point by direct analysis of rats and guinea-pigs divided into susceptible and non-susceptible groups by decompression experiments. The results, which will be published in detail later, show that fatness is a very important factor in individual susceptibility to death.

^{page 410 note 2} Meeting of Physiological Society, Nov. 1906.

^{page 411 note 1} The vessels and bladder in the rabbit are so thin-walled that bubbles can be seen with certainty if they are present.

^{page 411 note 2} On the other hand dogs (showing no symptoms) killed after sudden decompression after 16 minutes exposure at 2·8 atmospheres, 5+ at 3·5, 12 at 4·5 and 5 at 4·7, showed no bubbles in the blood: these were however found in three other dogs after 10+ minutes at 4·0 atmospheres, 16 and 72 at 4·5.

^{page 413 note 1} Heller, Mager and v. Schrötter (p. 852) record a case in a dog fatal in 6 hours after decompression in which no free gas was found in the vessels.

^{page 413 note 2} This animal had however been recompressed and died under a state of recompression: see protocol e, Appendix III.

^{page 419 note 1} Two pregnant guinea-pigs were exposed for 1 hour at +100 lbs. and decompressed in 34 minutes by stages: they showed no systoms. On being killed 5 hours later, numerous bubbles were found in the amniotic fluid but nowhere else.

^{page 420 note 1} Zografidi, (Revue de Médecine, 1907, p. 159Google Scholar) records the finding of numerous bubbles in the peripheral vessels, but not in the heart, of a diver who was paralysed and died 33 days after decompression!

^{page 429 note 1} Reprinted from the Report of the Admiralty Committee on Deep Diving, 1907.