REGULATION OF RESPIRATORY MOVEMENTS 1083 



pressure of oxygen (Fig. 506). Here, within wide limits, the partial pressure 

 of oxygen in the alveolar air is determined by its pressure in the inspired air. 

 Thus, if we take the same series of observations with a pressure of 646 mm., 

 the percentage of oxygen in the alveolar air was 13-19, corresponding to a 



tension of 13-19 x - = 10-4 per cent. At an atmospheric pressure of 

 i oU 



755 mm. the percentage of oxygen in the alveolar air was 13-97, corresponding 

 to a tension of 13-06 per cent., which we may take as the normal figure at the 



FIG. 507. Effects of oxygen lack. (SCOTT.) 



During tracing, diaphragm slip ; lower tracing, carotid blood-pressure. During 

 time indicated by signal, 5 per cent, oxygen in nitrogen was inhaled, c = con- 

 vulsion. 



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 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 

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

 there is increased pulmonary ventilation (Fig. 507), 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 



