1208 PHYSIOLOGY 



sea-level. In air compressed to a pressure of 1261 mm. the percentage 



1261 



of oxygen was 16'79, corresponding to a tension of 16'79 X -= - : 



26'8 per cent, of an atmosphere of 760 mm. 



Similar results are obtained by altering the percentage of oxygen 

 in the air breathed. The oxygen tension or percentage in the inspired 

 air can be lowered from its normal of 20' 93 to 12 or 13 per cent, without 

 altering in any way the depth or rhythm of respiration, and in fact 



PIG. 505. Effects of oxygen lack. (ScoTT.) 



Upper tracing, diaphragm slip; lower tracing, carotid blood pressure. 

 During time indicated by signal, 5 per cent, oxygen in nitrogen was inhaled, 

 c = convulsion 



without any change being noticed by the individual who is the subject 

 of the experiment. A percentage of 13 per cent, of oxygen corresponds 

 to an alveolar content in oxygen of 8 per cent., and with a further 

 reduction of the oxygen content there is increased pulmonary ventila- 

 tion (Fig. 505), but the diminution in oxygen may be pushed to such an 

 extent that the patient becomes blue from the deficient aeration of his 

 haemoglobin, without any considerable distress being caused. In fact in 

 many cases the subject of such an experiment may lose consciousness 

 suddenly before he has been aware of any serious deficiency in his 

 aeration. 



The difference in the sensitiveness of the centre to increase of 

 carbon dioxide and lack of oxygen respectively is well shown by an 



