640 EVENING DISCOURSES. 



Out of thirty autopsies done on fatal cases of caisson illness, in nineteen 

 gas-bubbles were visible in the blood-vessels ; of the other cases most' were old- 

 standing lesions of the spinal cord. 



The paralysis so often produced is due to a local death and degeneration of 

 the spinal cord, produced by bubbles blocking the circulation there (Von 

 Schrotter, Heller, and Mager). 



Proofs that nitrogen gas dissolved in the body fluids and fat is the cause of the 

 illness are the following. The blood collected from the artery of an animal while 

 under pressure, and analysed with the gas-pump, shows that the amount of dis- 

 solved nitrogen varies with the pressure. Roughly, 1 per cent, per atmosphere 

 is dissolved (Bert, Hill, and Macleod). 



Exposed to 1 atmosphere at body temperature, blood dissolves just about 



1 per cent. N, to 2 atmospheres 2 per cent., to 3 atmospheres 3 per cent., and so 

 on. The tissue fluids take up the dissolved gas from the blood, and with time 

 the whole body becomes saturated, according to Dalton's law. The saturation 

 of the body fluids takes time, since the blood forms but 5 per cent, of the 

 whole body weight, and it is the blood alone that comes in direct contact in the 

 lungs with the increased atmospheric pressure. Probably about 5 kilograms of 

 blood circulate through the lungs per minute, and this blood conveys the 

 absorbed nitrogen to the 60 kilograms of tissues. The arterial blood saturated 

 in the lungs yields the nitrogen to the tissues, and returns to be saturated again 

 in the lungs. Those tissues which are plentifully supplied with blood will 

 become saturated rapidly, while less vascular areas, and parts in a state of vaso- 

 constriction, will saturate very slowly. 



C. Ham and I exposed rats to 10 to 20 atmospheres, killed them by instant 

 decompression, and then, opening their bodies under water, collected and analysed 

 the gas set free therein. We obtained in this gas CO, 6'7 to 16 per cent., 



2 2-1 to 8'7 per cent., N 80 to 87 per cent., and a volume of N greater than 

 that calculated according to solubility of N in tissue fluid. Some of the excess 

 we found was due to air swallowed while under pressure, the rest to solution of 

 N in fat. 



M. Greenwood and I have tested upon ourselves the rate of saturation, using 

 the urine as a test fluid. We were compressed in a large boiler, placed at our 

 disposal by Messrs. Siebe, Gorman & Co. The chamber was fitted with electric 

 light and telephone, and taps for slow decompression. The pressure was raised 

 by means of a diving-pump driven by a gas engine. We drank a quart of water 

 before entering, and collected samples of urine at varying pressures and times. 

 The urine, collected in sealed bulbs, was evacuated by the blood gas pump. We 

 found the urine secreted in the next ten minutes after reaching any given pres- 

 sure is saturated with N at that pressure. 



To demonstrate the bubbling off of nitrogen on rapid decompression, I have 

 spread the web of a frog's foot or wing of a bat over the glass window of a pres- 

 sure chamber. The circulation of the blood is projected on a screen by aid 

 of microscope and arc light. We can thus observe the circulation under 20 

 atmospheres of air, and watch the bubbles forming in the capillaries on rapid 

 decompression. Recompression diminishes the size and finally drives the bubbles 

 again into solution. 



When the larger mammals are exposed to high pressure, such as 8 atmo- 

 spheres, for an hour or so, and are then rapidly decompressed, they usuaily die 

 in a few minutes. Small mammals, such as mice and rats, may escape, owing to 

 the small bulk of body and rapid respiration and circulation. The young of 

 rabbits, cats, &c, also escape more frequently than old animals. This is due 

 rather to their smaller weight and more rapid circulation than to the youth of 

 the body tissues. Paralysis in the limbs follows too rapid decompression, or the 

 animals fall over and become unconscious. Noise of gas bubbles gurgling in the 

 heart may be heard. Respiration becomes embarrassed, and the animals die. On 

 dissection, the peritoneal cavity may be found distended with gas, or the stomach, 

 and gas may be seen in the intestine. A part of this gas arises from the fer- 

 mentative processes of digestion, and from air swallowed during compression. 

 The veins of the portal system, the venas cavae, are seen to contain chains of 

 bubbles; the heart is full of froth. Small haemorrhages may be present in the 

 lungs. The edges of the lobes of the lung are emphysematous, blown out by the 

 ranid decompression. The fat often is full of small bubbles, so too are the con- 



