THE REGULATION OF BREATHING 301 



until oxygen want again stimulates the centre. Group breathing may 

 be abolished by the giving of oxygen, or breathing 2 to 3 per cent. C0 2 . 

 It frequently occurs at high altitudes, owing to the diminished 

 partial pressure of oxygen in the rarefied atmosphere. It can be 

 produced experimentally in most people by forced breathing for two 

 to three minutes. After the subsequent apnoea, breathing returns 

 for the first few minutes in a periodic fashion. If, however, the lungs 

 are filled with oxygen instead of air at the end of the forced breathing, 

 the apnoea is of much longer duration, and breathing returns in a 

 perfectly regular manner. 



Under these circumstances, in addition to the ordinary respiratory 

 oscillations, rhythmic variations of pressure frequently appear in the 

 tracings of arterial pressure. These variations are known as Traube- 

 Hering curves. They can be evoked by the injection of a little 

 magnesium sulphate solution into the circulation of the dog (Fig. 168). 

 During the asphyxial rise of arterial pressure in the curarized dog, 



Fi3. 163. TKAUBE-HERING CURVES AFTER INJECTION OF MAGNESIUM SULPHATE. 



these curves occur, and also after injection of a large dose of morphia. 

 In conditions of anaemia of the bulbar centres, produced either by 

 tying the cerebral arteries, or compression of the brain, and after 

 injection of chloroform into the cerebral arteries, Traube curves 

 frequently become apparent. 



In periodic respiration of cerebral origin, the waxing and waning 

 of the blood-pressure seems to be due to the effect of the venous blood 

 on the vaso-motor centre, rather than on the heart. Oxygen-want 

 stimulates the respiratory and vaso-motor centres at the same time. 



In the case of morphia, the respiratory centre injured by morphine 

 does not react to the acid ions in the blood until these reach a con- 

 centration which injures the heart. The periodic fall of pressure in 

 this case is due to an asphyxia of the heart. When the breathing 

 starts, the heart recovers, and the blood-pressure rises. The breathing 

 ceases once more so soon as some of the carbon dioxide in the bloo:l 

 has been exhaled. 



