374 Mr. -Leonard Hill [May 25, 



Our object has been to prove that such high pressures can be 

 endured safely if the decompression period be made long enough, to 

 study on ourselves the rate at which the body absorbs nitrogen on 

 compression and gives it up on decompression, the effect of compressed 

 air on the respiratory exchange and general metabohsm of the body, 

 and the effect on the sensations and mental state. We find the voice 

 becomes high pitched and nasal in tone, and loses its individual 

 character. The pitch rises with the pressure. The fine vibrations 

 of the lips which cause whistling and whispering cannot be produced 

 in the dense air at 4 atm. The mechanism of respiration and the 

 output of COo as far as we have tried them are unaltered,* and no 

 noteworthy change occurs in the circulation. We had no sense of 

 the pressure and could not estimate its height. Mr. Greenwood after 

 decompression from 92 lb. suffered from pains in the forearms, which 

 were of some severity and lasted a few minutes. On one occasion I 

 had some small patches of ecchymoses in the subcutaneous fat of the 

 chest. Otherwise, except nervousness, we have endured no symptoms 

 of any note. 



The Relief of Compressed Air Illness. 



Recompression was a method which naturally suggested itself to 

 the earliest workers in caissons (Pol and Watelle, 1854). Bert showed 

 the value of it experimentally, and v. Schrotter, and Macleod and 

 myself have demonstrated the same. In the frog's web, as I have 

 shown you, the bubbles go into solution on recompression, and the 

 circulation may even recommence. If bubbles have formed in the 

 nervous system, the recompression must be carried out quickly, for 

 anaemia lasting some minutes will produce death of the nerve cell. 

 In the case of bubbles embarrassing the heart, recompression may 

 immediately and entirely relieve the symptoms. 



Smith, at Brooklyn, and E. W. Moir, at the Hudson tunnel, in- 

 troduced a recompression chamber, or " medical lock," for the work- 

 men, and Moir has seen many men entirely restored from coma, etc.. 

 by recompression followed by slow decompression. A medical lock is 

 now the rule at all works. The men should live in barracks at the 

 works, so that the medical lock may be always at hand, for they often 

 are not affected till some half hour or so after leaving the caisson. 

 If the recompression is delayed it can do little good. 



The safety of divers and caisson workers can only be assured by 

 increasing the period of decompression. For pressures above +1*5 

 atm. the locking out period should be extended. For pressures of 



* To study the respiratory exchange we have used the ingenious method of 

 Geppert and Zuntry, and Haldane's method for measuring the tension of CO., 

 in the lungs. The COj tension regulates the ventilation of the lungs, and we 

 find this regulation continues to act up to G atm, There is the same partial 

 pressure of COg in the lungs at 6 atm. as at 1 atm. 



