October 26, 191 1] 



NATURE 



57i 



once by pain and anaesthesia, spasm, and paralysis. In 

 the medulla oblongata arrest of the circulation will stop 

 respiration, and bubbles lodging there may produce 

 immediate death. Lodging in the arteries of the great 

 brain, bubbles may produce hemiplegia, aphasia, blindness, 

 or mental disturbance. 



Among men some are affected and others not. We can 

 look for an explanation in the varying state of the blood, 

 in fatness, in the varying vigour of the circulation and 

 respiration and the effect of fatigue, in vaso-motor 

 changes which alter the relative volume of circulating 

 blood in viscera and muscles, and in the fermentative 

 processes going on in the alimentary tract. The young 

 man who is in perfect health, with powerful heart and 

 deep respiration, can expel the dissolved nitrogen from his 

 Jungs far more rapidly than the old, the fat, the in- 

 temperate, or one who is over-fatigued by excessive labour. 

 The records of caisson works seem to show that most men 

 under twenty escape, while the percentage of cases 

 increases with age, and is highest for men above forty ; 

 that long shifts increase the number of cases ; that men 

 who work the air-locks, passing material through, and 

 undergoing frequent and short-lasting compression and de- 

 compression, are not affected. The longer the shift the 

 more complete the saturation of the body ; the higher the 

 pressure the greater the risks and the graver the symptoms. 

 The records show that practically no cases occur with a 

 pressure below 2 to 2J atmospheres absolute, even though 

 the decompression period be made only a minute or two. 



At the Rotherhithe Tunnel the decompression period was 

 three minutes, and the maximal pressure +22 lb. No 

 cases of any gravity occurred. Nevertheless, we proved 

 that the workers had excess of nitrogen in their bodies 

 after decompression. We gave them a quart of beer to 

 drink in the tunnel thirty minutes before decompression 

 to provoke diuresis, and made them empty their bladders 

 just before, and again ten minutes after, decompression. 

 Their urine yielded more than the normal volume of N. 

 The urine, passed immediately after their decompression, 

 obviously effervesced. 



Influence of Fatness. 



As the fat holds five or six times as much nitrogen in 

 solution as the blood, it saturates and desaturates slowly. 



J. F. Twort and I have found 35-55 per cent, of nitrogen 

 dissolved in olive oil which had been exposed to 73 atmo- 

 spheres. The risk of exposure to compressed air varies 

 with the fatness of the animal (Boycott and Damant). 

 Greenwood and I have found fat pigs weighing 100 to 

 120 lb. are more susceptible than smaller pigs 50 to 60 lb. 

 The bubbles once set free in the subcutaneous fat of pigs 

 may stay there for days after decompression, as we have 

 found to our cost, for it has seriously damaged the sale 

 of the animals to the butcher, since the fat does not bleed 

 white, but remains pink and mottled. All the results prove 

 that fat men should be excluded from compressed-air work 

 at pressures above two atmospheres absolute. 



The varying percentage of fat in the blood, chyle, and 

 liver must be an important factor in the evolution of 

 bubbles in the blood. The less fat in the food eaten by 

 caisson workers the better. 



Ventilation and Illness. 



Much has been made of the impurity of the air as a 

 contributory cause of caisson sickness, in particular, of the 

 percentage of CO.. The ventilation of the tunnels built 

 by the London County Council under the Thames have 

 been carried out at enormous and needless expense in 

 order to keep the C0 2 percentage down to a very low 

 level. The work of the English physiologists is against 

 this view. Divers generally work with 1, 2, or even 3 

 per cent, of an atmosphere CO. in their helmets. We have 

 exposed ourselves to 3 to 4 per cent, of an atmosphere 

 CO, without untoward results, beyond increased frequency 

 of respiration, which prevents any increased concentration 

 of CO. in the body. 



Recently I have carried out many experiments on students 

 sealed up in a small air-tight chamber, and found, as 

 Haldane has, that it is the heat, moisture, and stillness of 

 the air which cause discomfort and fatigue, and not the 



excess of CO. or deficiency of oxygen in the air breathed. 

 The putting on of powerful electric fans, by whirling the 

 air and cooling the body, gives very great relief, even 

 when there is 4 to 5 per cent, of CO. in the chamber. 



In open-air treatment the coolness and movement of the 

 air are the essential qualities which promote health by 

 stimulating the activity, the metabolism, and nervous well- 

 being of the body. 



Hot, moist, still air causes fatigue by taxing the cool- 

 ing mechanism of the body ; blood is sent to the skin to 

 be cooled which ought to be going to muscle and brain. 

 Fatigue increases the danger of decompression by making 

 the circulation and respiration less efficient. The heat 

 causes more blood to come to the skin and a more com- 

 plete saturation with nitrogen there. The cold in the 

 decompression chamber — due to expansion of the air — 

 causes vaso-constriction and repels the blood from the skin, 

 and so stops its desaturation. We have lost pigs by taking 

 them from the warm caisson into the cold air. 



Over-hot and moist — that is, under-ventilated — caissons 

 have, therefore, a higher morbidity. To secure efficient 

 work, the wet-bulb temperature must be kept below 75 F. 

 (Haldane). The men should not pass from a warm caisson 

 to a cold air-lock and a cold outside world. They should 

 go through a warm lock to a warm room. 



Hot, moist atmospheres are very disadvantageous to 

 health and work. If the wet-bulb temperature is high in 

 the caisson, the current of air should be increased or 

 electric fans used to cool the workers. Electric fans have 

 enormously increased the efficiency and health of Europeans 

 in the tropics. An excess of CO.' in the air-lock, or diver's 

 helmet, during decompression is favourable, as it increases 

 the pulmonary ventilation and the outbreathing of nitrogen. 

 Haldane advises the air-pump to be slackened purposely. 

 There is no harm in breathing 1 or even 2 per cent, of CO.. 



Methods of Decompression. 



The safety of compressed-air workers depends on the 

 relation of the period of decompression to that of com- 

 pression. 



The period of the saturation or desaturation of the body 

 with nitrogen depends on the relation between the circu- 

 lating volume of the blood and the volume (1) of the tissue 

 fluid (2) of the body fat which dissolves the nitrogen — 

 remember the fat dissolves five or six times as much as 

 the tissue fluid. The more often the whole volume of the 

 blood circulates round the body, the quicker will be the 

 saturation or desaturation. The smaller the body, the 

 more often does the volume of blood course round _ it. A 

 mouse's heart beats six hundred or seven hundred times a 

 minute against a man's seventy (F. Buchanan). The 

 circulation and rate of respiratory exchange are twenty 

 times faster in the mouse. In the case of a man, the 

 smaller man, the leaner and harder the man (less fat and 

 tissue fluid), the quicker will his body saturate and de- 

 saturate. The rate of the circulation and percentage of fat 

 vary in different organs. There are parts quickly and parts 

 slowlv saturated or desaturated. The joints, tendons, sub- 

 cutaneous fat, abdominal fat dep6ts, are relatively slow 

 parts. The white matter of the brain and spinal cord has 

 much fat in it, while the grey matter has little fat and a 

 more active circulation. In the white matter of the spinal 

 cord bubbles commonly form and lead to a stoppage of the 

 circulation there, death of the tissue, and paralysis. 

 Bubbles in the subcutaneous fat, or fat depflts of the 

 belly, mav be compared to stones scattered in the fields, 

 and bubbles in the spinal cord to rocks thrown down on 

 the main railway lines of London. 



Muscular work increases the circulation and pulmonary 

 ventilation five or six, even ten, times if the work is very 

 hard. In warm, moist caissons the cutaneous vessels are 

 id the circulation accelerated, and this makes the 

 saturation of the peripheral parts quicker than in the case 

 of the diver, who is surrounded with cool water. The 

 diver also does not work so hard and so long as the 

 caisson worker. Therefore the caisson worker suffers far 

 more from " bends." The diver goes to much greater 

 pressures for short times, and after a quick decompression 

 mav suffer from asphyxia, symptoms of paralysis — arising 

 from bubbles in the heart and pulmonarv vessels, or in the 

 spinal cord. The caisson worker when decompressed stands 



NO. 2 19 1, VOL. 8/] 



