THE CONTROL OF THE RESPIRATION 375 



methods have been employed in the investigation of this subject. The 

 most important of these are as follows: (1) Breathing from a tank con- 

 taining varying mixtures of oxygen and nitrogen (Dreyer apparatus). 

 This apparatus was used extensively in the British Army in testing the 

 ability of candidates for the aviation service to withstand low oxygen. 

 Its greatest value is that the alteration in oxygen content of the inspired 

 air can be made either gradually or quickly. (2) Breathing from a 

 tank through tubes provided with valves which direct the expired air 

 so as to make it pass through an apparatus for absorption of the C0 2 , 

 before it re-enters the tank. The subject, therefore, re-breathes the air 

 of the tank from which he gradually absorbs the oxygen. In this method 

 the 2 -content of the inspired air falls gradually, and effects are pro- 

 duced which must be similar to those which would be caused by slow 

 ascent to higher altitudes. The rate at which the 2 falls can be varied 

 by altering the size of the tank. When a very rapid fall is desired rubber 

 bags can be used in place of tanks. An apparatus on this principle was 

 employed for testing aviators particularly in the United States Army. 

 (3) Breathing in an air-tight cabi.net containing properly arranged soda 

 lime absorbers to take up the C0 2 ; the oxygen decreases at a rate which is 

 inversely proportional to the size of the cabinet. (4) Breathing in a 

 strongly built steel chamber connected with a powerful pump by means 

 of which the chamber can be partially evacuated and the pressure main- 

 tained at any desired level. Such a chamber has been used by Haldane, 

 Kellas and Kennaway in important experiments, the results of which we 

 shall consider immediately. (5) Adding a sufficient percentage of carbon 

 monoxide gas to the inspired air to combine with a considerable propor- 

 tion of the hemoglobin of the blood and so render it incapable of carrying 

 the oxygen. 



The observations made by the use of these methods have been com- 

 pared with those made during life at high altitudes, particularly in con- 

 nection with mountaineering. The latter observations are of particular 

 importance in' the study of the adaptive processes which come into play 

 to render persons who have become accustomed to high altitudes immune 

 to the distressing symptoms from which others suffer. 



There is considerable variability in the reactions of different persons to 

 decreased oxygen and these cause symptoms which are partly subjective and 

 partly objective in nature. Slight differences are observed according 

 to whether the anoxemia is produced by a lowering of barometric pressure 

 (decompression), or by simply reducing the percentage of oxygen in 

 the inspired air. In the former case there is also a slight difference 

 between the symptoms following decompression in an experimental cham- 

 ber, and those observed on a high mountain. In a general way the symp- 



