380 APNCEA. [BOOK n. 



The Effects of an increased supply of Air. Apncea. 



We have already (p. 861) seen that when artificial respiration 

 is carried on too vigorously, a condition of peculiar breathlessness 

 known as "apncea 1 " is brought about. We have further seen that 

 the essential feature of apncea consists in the blood containing 

 for the time being more oxygen than usual. In consequence of 

 this a longer time is needed before the deficiency of oxygen in the 

 blood of the capillaries of the medulla oblongata, or rather in the 

 nerve-cells constituting the respiratory centre, reaches the limit 

 which determines the discharge of a respiratory impulse. As we 

 have seen, the molecular processes of these cells are so arranged, 

 that whenever the oxygen which is available for their use sinks 

 below a certain level, respiratory explosions occur whereby a fresh 

 supply of oxygen is gained. We must suppose that the 

 changes going on in these cells, like those taking place in other 

 cells and tissues, are oxidative in character; but they possess this 

 peculiar feature, that the absence or diminution of oxygen acts as 

 it were as a stimulus, leading to an explosive decomposition. The 

 facts previously (p. 361) discussed lead us to adopt this view, though 

 we cannot explain why oxygen has this remarkable effect on these 

 particular cells. By increasing their available oxygen, the ex- 

 plosive action of the cells is deferred and diminished; that is, 

 apnoea is established. Similarly when the supply of oxygen is 

 diminished, the explosions are hastened and increased, that is, 

 dyspnoea is brought about. The different conditions of the respi- 

 ratory centre during apncea, normal breathing or eupncea, and 

 dyspnoea, are well shewn by the different effects produced by 

 stimulating the afferent fibres of the trunk of the vagus with the 

 same stimulus during the three stages. If the current chosen be 

 of such a strength as will gently increase the rhythm of normal 

 breathing, it will be found to have no effect at all in apncea, while 

 in dyspnoea it may produce almost convulsive movements. Indeed 

 in well-marked apncea even strong stimulation of the vagus may 

 produce no effect whatever. 



The Effects of changes in the Composition of the Air breathed. 



We have already discussed the effects of such changes as are 

 produced by the act of respiration itself, viz. a deficiency of oxygen 

 and an excess of carbonic acid. We have only to add, that the 

 result of an excess of oxygen, except in the cases of extreme 

 pressure to be mentioned immediately, is simply apncea, and that 



1 It is to be regretted that this name is used by some medical authorities in a 

 sense almost identical with asphyxia. In its physiological sense, as here used, it is 

 the very opposite of asphyxia. 



