104 RESPIRATION 



Some years ago it was discovered by Pembrey and Allen 16 that 

 the well-known pathological form of periodic breathing named 

 after Drs. Cheyne and Stokes, who described it (though it was 

 previously described by John Hunter), is abolished by giving the 

 patient pure oxygen to breathe. This observation indicates with 

 great certainty that ordinary pathological Cheyne-Stokes breath- 

 ing is caused also by want of oxygen participating in the excita- 

 tion of the center. Pathological periodic breathing and that of 

 hibernating animals will be discussed later. 



The normal pressure of oxygen in the alveolar air is about 

 100 mm. or 13.1 per cent of an atmosphere. On looking at the 

 dissociation curve of oxyhaemoglobin in human blood (Figure 

 20) it will be seen that a fall of 4 per cent of an atmosphere, or 

 30 mm., makes very little difference to the saturation of the haemo- 

 globin. Nor has such a fall any appreciable influence on the rest- 

 ing breathing at the time. It is thus evident that, although there 

 is no appreciable store of readily available oxygen in the liquids 

 of the body outside the red corpuscles and certain muscles which 

 contain a little haemoglobin, there is a store of oxygen, available 

 without any inconvenience, in the air of the lungs. If the breathing 

 is temporarily stopped during some occupation this store is drawn 

 on. Thus if the breath is held for half a minute the oxygen runs 

 down by about 4 per cent in the alveolar air during rest ; but under 

 normal conditions it is quite impossible to hold the breath long 

 enough to imperil seriously the oxygen supply to the tissues. In 

 spite of the gradual manner in which, as we have just seen, CO 2 

 acts on the respiratory center, there is never, except under very 

 artificial conditions, any considerable oxygen want. The com- 

 paratively large volume of air which is always in the lungs gives 

 sufficient oxygen storage to guard against the temporary want of 

 oxygen. Were this amount of air much less the danger would be 

 always present, and, as we shall see later, this danger or incon- 

 venience is present at high altitudes, when the mass of oxygen in 

 the lungs is greatly diminished. At a high altitude one cannot 

 hold the breath for more than a few seconds without feeling an 

 imperative desire to breathe, and such operations as shaving, or 

 reading a barometer, are thus rendered troublesome. Nature sees 

 to it that ordinary mortals who live under a pressure of about one 

 atmosphere carry about sufficient oxygen in their lungs to pre- 

 vent oxygen want; and there seems to be some evidence that 



16 Pembrey and Allen, Journ. of Physiol., XXXII, Proc. Physiol. Soc., p. xviii, 

 1905 ; also Medico- Chirurg. Trans., XI, p. 49, 1907. 



