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ESSENTIALS OF PHYSIOLOGY. 



Summarising the various factors which influence respiration, we see, 

 first, that the normal stimulus to the respiratory centre is the tension 

 of carbonic acid in the blood passing to the centre : an increase of this 

 tension stimulates the centre, and when the tension falls respiration 

 ceases. Secondly, when the blood is deficient in oxygen, lactic acid 

 passes into the blood and also stimulates the centre. Thirdly, impulses 

 passing along the vagus nerves from the lungs help to maintain the 

 normal rhythm of respiration, and make the centre more sensitive to 

 the chemical stimulus of carbonic acid. Finally, impulses from the 

 higher parts of the brain, or impulses reaching the centre by afferent 

 nerves may, and do, modify the respiratory movements. 



APNCEA. 



Reference has already been made to the inhibition of the respiratory 

 movements, which is known as apnoea. The most important condition 

 which gives rise to apnoea is a fall in the tension of carbonic acid in the 



FIG. 111. Forced breathing in man, followed by apnoea and subsequently by 

 Cheyne-Stokes breathing. (Haldane and Douglas. ) 



blood ; this occurs whenever the ventilation of the lungs is increased 

 without any corresponding rise in the production of carbonic or lactic 

 acid in the body, and may therefore be brought about either, in man, 

 by voluntary forced breathing (fig. Ill), or, in animals, by vigorous 

 artificial respiration. In these circumstances the apncea may last for 

 1 to 2 minutes, respiration beginning again as soon as the tension of 

 carbonic acid in the alveolar air returns to its normal level. That it 

 is due solely to a fall in the tension of carbonic acid is clearly shown 

 by the fact that, when the inspired air contains an excess of carbonic 

 acid, forced breathing is not followed by apnoea. 



A totally different form of apnoea is that produced by sudden dis- 

 tension of the lungs, and known as vagus apnoea (p. 273). This is 



