340 RESPIRATION 



quently became ill, and sometimes died or became paralyzed. 

 The risk of these attacks increased with the pressure and the 

 duration of exposure to it, but they never occurred except on 

 return to atmospheric pressure. Divers are exposed to the highest 

 pressures, and in divers the attacks were of the most dangerous 

 character. In the worst cases the diver began to feel faint a few 

 minutes after return to surface ; soon he became unconscious 

 and his pulse disappeared; and in a few minutes he was dead. 

 In other cases his legs became paralyzed, and cases of "diver's 

 paralysis" used to be not uncommon in British hospitals. In the 

 slighter cases, very common among workers in caissons and tun- 

 nels under construction, there is severe pain, known to the work- 

 men as "bends," in one or other of the limbs, or in the body. 

 Another of the common slight symptoms is itching of the skin. 

 Various other nervous symptoms are also met with, the whole 

 complex being designated as "caisson disease" a somewhat mis- 

 leading name. 



Paul Bert investigated on animals the nature of compressed 

 air illness or "caisson disease," and found that it is due to libera- 

 tion in the blood and tissues of bubbles of gas consisting almost 

 entirely of nitrogen. In the rapidly fatal cases the heart becomes 

 filled with a mass of bubbles which stop the whole circulation. 

 In the cases of paralysis bubbles have obstructed the circulation 

 and so caused necrosis of parts of the spinal cord ; and it is evi- 

 dent that the bubbles may produce the most varied symptoms 

 according to the positions in which they are formed. 



The cause of the bubble formation was evident. At the high 

 pressure the blood in the lungs is exposed to greatly increased 

 partial pressures of nitrogen and oxygen, although, as shown in 

 Chapter II, there is no increased pressure of CO 2 . As, in ac- 

 cordance with Henry's law, liquids take up in simple solution a 

 mass of any gas proportional to its partial pressure, the blood in 

 the lungs takes up in the compressed air an extra amount of nitro- 

 gen and oxygen proportional to the increased pressure. The extra 

 oxygen disappears at once when the blood reaches the tissues, but 

 the extra nitrogen does not disappear, and gradually saturates 

 the whole of the tissues till they are charged with nitrogen at the 

 partial pressure existing in the air breathed. When the external 

 atmosphere is reduced to normal, the internal partial pressure of 

 nitrogen is of course far above the atmospheric pressure. The 

 blood and tissues are therefore supersaturated with nitrogen and 

 bubbles begin to form. These bubbles consist primarily of nitro- 



