BREATHING IN RAREFIED AND COMPRESSED AIR 421 



cyanosed, and the surface of the body cold. Many of the cases are fatal ; 

 indeed, death may be almost instantaneous. Such cases are common in 

 careless diving when the divers, to return the more quickly, screw up the 

 outlet valve in their helmets so as to fill their suits with air, which car- 

 ries them to the surface, where they decompress themselves by opening 

 the valve. 



Autopsies of persons dead of caisson disease have shown, as a rule, 

 intense congestion of the viscera, hemorrhages in the spinal cord and 

 brain, and ecchymoses on the pleura and pericardium. In some cases 

 interlobar emphysema of the lungs and laceration of the spinal cord and 

 brain have been noted. 



The Cause of the Symptoms 



The cause for the symptoms is not, as was at one time supposed, that 

 the pressure drives the blood from the peripheral into the deep regions 

 of the body, including the nerve centers. Such a process is impossible, 

 because the fluids of the body and all tissues, even the bones, are full 

 of fluid are incompressible. Pressure applied to any part of the body 

 will be immediately distributed equally to every other part. If this were 

 not so, life would be impossible during any variation of atmospheric pres- 

 sure. It is now clearly established that all the symptoms of caisson disease 

 are due to decompression, and not, in the slightest degree, to the mechan- 

 ical effect of the pressure itself (Paul Bert, Leonard Hill and Macleod 34 ). 



When an animal is under pressure, its tissue fluids dissolve a large 

 amount of gas. They absorb it in obedience to the law of solution of a 

 gas in a fluid, which states that the amount of gas dissolved in water is 

 directly proportional to the partial pressure of that gas in the atmos- 

 phere; at two atmospheric pressures twice as much gas will pass into 

 solution as at zero pressure (Dalton's law). So long as the gas is in 

 simple solution, it does not in any way change the physical condition of 

 the blood and tissue fluids. If, however, the animal is suddenly decom- 

 pressed (i. e., the pressure of air surrounding it is reduced to zero), the 

 dissolved gas will be so quickly thrown out of solution that bubbles of 

 it are set free. These bubbles act as air emboli, sticking in the pulmonic 

 capillaries or blocking up a terminal artery in the brain; or they may be 

 large and tear the capillary wall and so lead to hemorrhage. If these 

 bubbles are produced in the posterior spinal roots, intense pain results; 

 if in the anterior, motor paralysis. Frothing of the blood in the heart im- 

 pedes the action of the organ and death soon follows. 



The following experiments furnish proof of this explanation: A frog 

 was placed in a small steel chamber connected with, a cylinder of com- 

 pressed air and provided with two windows by which a strong arc light 



