EVENING DISCOURSES. 643 



volume (1) of the tissue fluid, (2) of the body fat which dissolves the nitrogen — 

 remember the fat dissolves 5 or 6 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 beat six hundred or seven hun- 

 dred 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 saturate and desaturate. The rate of the circulation 

 and percentage of fat vary in different organs. There are parts quickly and 

 parts slowly saturated or desaturated. The joints, tendons, subcutaneous fat, 

 abdominal fat depots, 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 com- 

 monly form and lead to a stoppage of the circulation there, death of the tissue, 

 and paralysis. Bubbles in the subcutaneous fat, or fat depots of the belly may 

 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 ves- 

 sels are dilated and 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 may 

 suffer from asphyxia, symptoms of paralysis — arising from bubbles in the heart 

 and pulmonary vessels, or in the spinal cord. The caisson-worker when decom- 

 pressed stands quiet, and is subjected to the cooling effect of the expanding air, 

 and this constricts his cutaneous vessels and prevents desaturation of the 

 peripheral parts. The caisson-worker ought to be decompressed in an air-lock 

 which is comfortably warmed, and he ought to exercise himself hard in order to 

 keep "up the circulation and pulmonary ventilation, and so hasten desaturation. 



Haldane thinks that the body of man is about half-saturated in one hour, and 

 about saturated in 4 hours. Bornstein eays 6 or 7 hours are required for satura- 

 tion of the fat. Greenwood and I found that the urine, secreted by the kidney, 

 is about saturated after 10 minutes exposure to 4 atmosphere. About 20 minutes 

 were occupied in reaching this pressure. On decompression of a saturated animal 

 the viscosity of the colloidal blood prevents the formation of bubbles under 

 a certain difference of gas pressure. It is found by experience that it is safe 

 to decompress men in a minute or two from 2 atmospheres to 1. Since the volume 

 of a gas is halved at 2 atmospheres made \ at 4 atmospheres, £ at 8 atmospheres, 

 and the volume of a bubble is doubled on lowering the pressure from 8 to 4, 

 6 to 3, 4 to 2, or 2 to 1, Haldane concluded it was safe to come rapidly from 

 4 to 2, 6 to 3, or 8 to 4 atmospheres. The supersaturated tissues then give nitro- 

 gen to the blood, and the blood to the lungs, and the nitrogen escapes without 

 bubbling at the half-pressure stage, where a long pause is given. Successive 

 stages may be given when required to secure the desaturation of the body, each 

 stage by producing a safe degree of supersaturation accelerating the outgiving of 

 the dissolved nitrogen. The stage method of decompression initiated by Haldane, 

 and adopted by the Admiralty, has an advantage over the uniform in that it pre- 

 vents the further and perhaps dangerous saturation of the slow parts. Supposing 

 a diver had been for half an hour at 6 atmospheres pressure, if he were decom- 

 pressed on the old plan, slowly and uniformly, his fat would become further 

 saturated up to 5 atmospheres, while he was being decompressed from 6 to 5 

 atmospheres. On the other hand, if he is decompressed rapidly from 6 to 3, the 

 further saturation of the fat at pressures above 3 atmospheres is altogether pre- 

 vented. The stage method is of value to divers, who go down for short periods, 

 and do not work very hard, as it prevents the saturation of slow parts and 

 hastens the period of decompression. 



Caisson-workers who do 4 to 8 hours' shifts are practically saturated, but 

 they, too, are best decompressed by the stage method because it accelerates the 

 outgiving of the nitrogen by producing a safe degree of supersaturation of the 

 blood. The safety is greatly enhanced if hard muscular work is done during the 



