CHAP, ii.] RESPIRATION. 489 



centre is interesting since it shews by how many safeguards the 

 working of the respiratory centre is carefully adapted to the 

 needs of the economy. 



304. Apnoea. When we attempt to hold our breath, we 

 find that we can do this for a limited time only ; sooner or 

 later a breath must come ; but, as is well known, the time dur- 

 ing which we can remain without breathing may on occasion 

 be much prolonged, if we first of all take a series of deep 

 breaths. The breath sooner or later inevitably follows because 

 at last the natural impulses proceeding from the respiratory 

 centre become too imperious to be any longer held in check by 

 the impulses of volition passing down to the centre from the 

 brain. The fact that a series of deep breaths, a thorough ven- 

 tilation of the lungs, postpones the victory of the unconscious 

 centre, shews that such a ventilation in some way delays the 

 development of the natural respiratory impulses. A similar 

 but still more marked delay may often be seen in an animal 

 under artificial respiration. If in a rabbit artificial respiration 

 is carried on very vigorously for a while, and then suddenly 

 stopped, the animal does not immediately begin to breathe. 

 For a variable period no respiratory movements at all take 

 place, and breathing when it does begin occurs gently and nor- 

 mally, only passing into dyspnoea if the animal is unable to 

 breathe of itself ; and even then the transition is quite gradual. 

 Evidently during this period the respiratory centre is in a state 

 of complete rest, no explosions are taking place, no respiratory 

 impulses are being generated, and the quiet transition from this 

 condition to that of normal respiration shews that the subse- 

 quent generation of impulses is attended by no great disturb- 

 ance. Not only is the centre at rest, but it is less irritable 

 than the normal ; impulses along the vagus or other nerves 

 which otherwise would produce respiratory explosions are now 

 ineffectual. This state of things is known as that of apncea, 

 the converse of dyspnoea ; and the longer pause in breathing 

 mentioned above as possible after unusual ventilation of the 

 lungs may be regarded as a brief apnoea. 



Now it seemed natural to suppose that such a state of rest 

 of the respiratory centre was brought about by the more than 

 necessarily ample supply of oxygen afforded by the previous 

 increased inspiratory movements ; and indeed it was main- 

 tained that apnoea was the result of too great, just as dyspnoea 

 is the result of too little arterialization of the blood reaching 

 the respiratory centre. It was argued that owing to the in- 

 creased vigour of the artificial respiratory movements the hemo- 

 globin of the arterial blood, which in normal breathing is not 

 quite saturated with oxygen, became almost completely so, and 

 that at the same time the quantity of oxygen simply dissolved 

 in the blood became largely increased and its tension largely 



