EVENING DISCOURSES. G41 



nective tissues. Bubbles are seen in the joints, and may appear in the aqueous 

 humour of the eye. On opening the skull, bubbles are seen in the veins of the 

 brain. The bubbles are not restricted to the veins, but may also be seen in the 

 arteries. The ooronary vessels of the heart often show chains of bubbles. On 

 microscopic examination, the bubbles are seen in the capillaries; here and there 

 they run together and form larger bubbles, sometimes rupturing the walls of 

 the vessel, and compressing the surrounding tissues. In the larger animals 

 decompressed from 100 lb. in 4 to 7 seconds, we have found the cells of the 

 liver, kidney, &c, vacuolated or even burst by bubbles. The gas set free in 

 the heart can be collected and analysed ; about 80 per cent, of it is found to be 

 nitrogen (Bert, Von Schrotter, Hill, and Macleod). Catsaras lowered dogs in a 

 diving dress to depths of 43"7 m., and after about an hour rapidly drew them 

 to the surface. He found bubbles set free in these dogs just as in those exposed 

 in a pressure chamber. 



In animals which escape without any severe symptoms, some gas bubbles may 

 be found in the veins even six hours later. This shows how long it may take for 

 nitrogen gas once set free as bubbles to escape from the lungs, and explains why 

 caisson workers may suddenly be seized some half-hour or more after leaving the 

 works. In such cases the bubbles may be swept from the abdominal veins— 

 where they do no harm — into the heart, and impede the action of this organ, 

 or they may penetrate the pulmonary circulation and enter the arterial system, 

 and block up, perchance, the coronary arteries, or others in the brain or spinal 

 cord. 



The blood is a colloidal solution, and it takes time for the nitrogen to come 

 out of solution and for the small bubbles to run together to form visible bub- 

 bles. The gas bubbles tend to collect in the veins, as the blood travels quickly 

 through the arteries and slowly in the veins. It is only when the gas in the 

 veins becomes sufficient in amount to produce foam in the heart, or when gas 

 bubbles block up arteries of vital import, that grave symptoms arise. The place 

 where bubbles in the arteries must always produce serious results is the central 

 nervous system. In the liver, kidneys, muscles, fat, &c, bubbles may embolise 

 small arteries and produce no grave effect, but in the spinal cord the interrup- 

 tion of the blood supply to any group of cells or tract of fibres is evidenced at 

 once by pain and amesthesia, spasm, and paralysis. In the medulla oblongata 

 arrest of the circulation will stop respiration, and bubbles lodging there may pro- 

 duce 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 explana- 

 tion 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 lungs far more rapidly than the old, the fat, 

 the intemperate, 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 over forty; that 

 long shifts increase the number of cases; that men who work the air-locks, pass- 

 ing material through, and undergoing frequent and short-lasting compression and 

 decompression, 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 2^ atmospheres absolute, even though the decompression 

 period be made only a minute or two. 



At the Eotherhithe 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 decom- 

 pression. We gave them a quart of beer to drink in the tunnel 30 minutes before 

 decompression to provoke diuresis, and made them empty their bladders just 

 before, and again 10 minutes after, decompression. Their urine yielded more 

 than the normal volume of N. The urine passed immediately after their decom- 

 pression obviously effervesced. 



1911. T T 



