THE REGULATION OF BREATHING 293 



mously under various conditions 25,000 times as great as the limit 

 within which that of the arterial blood varies during rest. Since an 

 increase of 2 millimetres of CO 2 pressure in the blood increases the 

 resting ventilation of the lungs by over 100 per cent., and yet causes 

 a scarcely measurable alteration in the hydrogen ion concentration of 

 the blood, it is obvious that the sensitivity of the respiratory centre is 

 extremely great. 



Any increase in the percentage of oxygen breathed, and of the 

 pressure of oxygen in the blood, does not diminish the excitability 

 of the centre to C0 2 when the breathing is normal. The breath, 

 however, can be held longer, and more work can be done while it is 

 held, when the lungs are previously filled with oxygen than with air. 

 Athletes may run a quarter or half mile more easily and quickly if they 

 breathe oxygen before, and start with the lungs full of oxygen. Breath- 

 ing oxygen before and after the race prevents stiffness. The explana- 

 tion is that the greater oxygen supply lessens the formation of lactic 



FIG. 158. ^ANESTHETIZED DOG. 

 Upper tracing, respiration; lower tracing, blood-pressure; white space =20 seconds. 



Between the arrows ^ nri was breathed. 

 (_;(_) 9 



acid in the muscles. This is proved by the fact that lactic acid appears 

 in the urine which an untrained man passes in the next hour after 

 a hard run ; but if he wear a breathing apparatus (such as is used for 

 rescue work in mines, etc.), and breathe oxygen during the run, little 

 or no lactic acid appears in the urine. The increase of acid concentra- 

 tion tells particularly against the efficiency of the heart and skeletal 

 muscles. Forced breathing can be carried on much longer and to a 

 greater extent when oxygen is used instead of air, probably because 

 the forced breathing interferes with the circulation in the brain. 



When a very low percentage of oxygen is breathed, there results 

 marked hyperpnoea (Fig. 158), but of less sudden onset than that 

 produced by C0 2 . The same is true in animals when blood de- 

 ficient in oxygen is injected into the peripheral carotid (Fig. 159). 

 The effect of oxygen-want is not noticed until the O 2 per- 

 centage falls below 14. On greater reductions e.gr., to 5 per cent 



