REGULATION OF THE RESPIRATORY MOVEMENTS 1133 



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

 oxygen was 16-79, corresponding to a tension of 16-79 X - = 26-8 per 



i OVJ 



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 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 



FIG. 620. 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 = con- 

 vulsion. 



oxygen of 8 per cent., and with a further reduction of the oxygen content 

 there is increased pulmonary ventilation (Fig. 520), 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 experiment of Haldane's, in which the 

 same person breathed in and out of a bag, in the first place allowing the carbon dioxide 

 produced in respiration to accumulate, and in the second removing the carbon dioxide 

 by means of soda lime, so that the sole effect of respiration was to produce a continual 

 diminution in the percentage of oxygen. In the first case, when the carbon dioxide was 



