218 APPLIED PHYSIOLOGY 



oxygen.* This may possibly be the reason why the 

 East-End Jew who is accustomed to overcrowding 

 from infancy appears to suffer so little in health 

 from it. 



The limit of height at which respiration can still be 

 carried on is another practical question to which no 

 definite reply is forthcoming. We know that Whymper 

 on Chimborazo reached a height of 20,517 feet, and Sir 

 Martin Conway in the Himalayas got as high as 

 22,600 feet, but it is probable that even higher altitudes 

 than these could be reached if time were given for the 

 subject to become accustomed to them. It is an interest- 

 ing fact that the higher one goes the more easily oxygen 

 appears to enter the blood, perhaps because of the 

 greater mobility of its molecules at reduced pressures. 

 No doubt some of the benefits derived from residence 

 in the high altitude health resorts are to be thus 

 explained. 



An impetus has recently been given to the study of 

 the effects of increased atmospheric pressure on the body 

 by the use of caissons in engineering operations, and by 

 the observance of cases of ' caisson disease.' The result 

 of breathing compressed air is naturally to cause the 

 blood to take up a much larger proportion of dissolved 

 gas, especially of nitrogen. When the pressure is 

 relieved nitrogen is liberated in the blood- stream in the 

 form of bubbles which may block some of the smaller 

 capillaries. It is to such blockage that the symptoms 

 of caisson disease are now believed to be due. As two 



* See Haldane and Lorrain Smith, Journ. of PhysioL, 1897-98, 

 xxii. 231. 



