CHAP. IL] RESPIRATION. 635 



reaching the respiratory centre, or by generating or increasing in 

 that blood some substance or substances tending to affect the 

 nutrition of the respiratory centre, affect the working of the all 

 important breathing mechanism. And the affection so wrought 

 has generally an adaptative character, it generally tends to protect 

 the organism against the evil effects of the change. 



374. 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 during which 

 we can remain without breathing may on occasion be much 

 prolonged, if we first of all take a series of deep breaths. It is 

 probable, though perhaps not distinctly proved, that when we 

 breathe voluntarily, or when by an act of the will we hold the 

 respiratory apparatus in any one respiratory phase, the nervous 

 impulses, generated by the will, do not pass down by a direct and 

 independent course to the respiratory muscles, but that the will 

 makes use or modifies the activity of the bulbar and spinal nervous 

 respiratory mechanisms. 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 

 ventilation 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 (the effect is not so well seen 

 in a dog) 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 normally, 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 subsequent 

 generation of impulses is attended by no great disturbance. 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 apnosa, 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 neces- 

 sarily ample supply of oxygen afforded by the previous increased 

 inspiratory movements ; and indeed it was maintained that apnoea 



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