1146 PHYSIOLOGY 



ordinary apnoea is entirely due to deficiency of carbon dioxide tension in the 

 respirator}" centre, and that although the vagus nerve is inhibitory of 

 respiration, it is impossible to summate a series of vagus inhibitions by 

 artificial respiration so as to produce a lasting cessation of respiratory 

 movements. The chief use of the vagi in respiration seems to be for main- 

 taining, by frequent inhibitions, the excitability of the respiratory centre 

 at a maximum. 



Miescher distinguished three types of apnoea, viz. : 



Apnoea vera, due to the washing out of CO 2 from the lungs, and the consequent 

 reduction of the tension of this gas in the blood. 



Apnoea vagi, a stoppage of respiration caused by stimulation of the inhibitory fibres 

 of the vagi. This stoppage is limited, as we have seen, to the immediate duration of 

 the stimulus (whether electric or produced by distension of the lungs). 



Apnoea spuria. Stoppage of respiration by stimulation of other nervous or sensory 

 surfaces. Thus when & duck plunges there is immediate stoppage of respiration, 

 which may last four or five minutes if the animal remains so long under water. The 

 same stoppage may be produced by pouring water on the beak. 



ITiG. 528. Forced breathing of air for two minutes, followed by apnoea for two 



minutes, and periodic (' Cheyne-Stokes ') breathing for about five minutes. 

 At A, sample of alveolar air contained 2 , .11*44 per cent.; C0 2 , 5'58 per cent. 

 Second sample at B, 2 , 13'55 per cent.; CO 2 , 5*57 per cent. (DOUGLAS and 

 HALDANB.) 



' CHEYNE-STOKES ' BREATHING 



If a man desires to hold his breath for some time he takes first a series 

 of deep breaths. The result is to diminish the carbon dioxide tension in the 

 alveoli and therefore to take away the need and the desire to breathe until 

 the carbon dioxide tension rises to normal as the result of the continued 

 formation of carbon dioxide. By continuing forced fespiratory movements 

 for a minute or two, the carbon dioxide tension both in the alveoli and in 

 the blood may be brought down to a very considerable extent. As a result 

 there is a prolonged period of apncea. During this period of cessation of 

 respirations however, the oxygen is being used up, and the tension of this 

 gas in the alveoli may fall to such an extent that the respiratory centre is 

 excited by lack of oxygen before the carbon dioxide tension in the alveoli 

 has risen to its normal value. As a result of the excitation by oxygen lack, a 

 few breaths are taken, the carbon dioxide tension is once more lowered 

 and the stimulation due to the oxygen lack disappears. There is thus 

 again a cessation of respiration. These periods of cessation alternate with 

 periods of respiration, so that we get a condition of periodic breathing which 

 is spoken of as Cheyne-Stokes respiration. During the period of apncea 



