RESPIRATION 121 



removing from the body the large amount of preformed CO 2 

 which has become superfluous owing to the effect of anoxaemia 

 in lowering the threshold of CO 2 pressure. 



Figures 41 and 42 show the effects of anoxaemia combined with 

 those of the slight resistance associated with the recording ap- 

 paratus. The effects are complicated owing to the fact that with a 

 certain degree of anoxaemia, varying greatly for different indi- 

 viduals, periodic breathing is produced readily, as shown in some 

 of the tracings. Periodic breathing, or else very shallow breathing, 

 is also produced invariably after the anoxaemia, as shown in all 

 the tracings. This is of course due to the fact that so much CO 2 

 has been removed from the body by the hyperpnoea of anoxaemia, 

 just as it is removed by forced breathing. 



In Figure 41 B and Figure 42, A, C, and D, it will be seen that 

 after an initial increase in depth the breathing became progres- 

 sively shallower and more frequent just as in fatigue due to ex- 

 cessive resistance; and after a time asphyxial symptoms were 

 usually impending owing to the ineffectiveness of the shallow 

 breaths. When the experiments were made we had not investigated 

 the effects of fatigue caused by resistance, and there is now no 

 doubt that the slight resistance due to the apparatus, combined 

 with the effects of anoxaemia on the respiratory center, accounted 

 for the specially rapid failure of breathing shown in the figures. 

 When the breathing is quite free, as in a steel chamber at low 

 pressures, failure of the respiratory center does not occur nearly 

 so readily, but the difference is only one of degree; and failure 

 of the respiratory center, as shown by shallow and frequent res- 

 pirations, is the inevitable result of serious arterial anoxaemia. 

 With the increasing shallowness of the breaths the arterial an- 

 oxaemia increases, owing to causes discussed in Chapter VII. 

 This increases the failure of the respiratory center; and unless 

 relief comes the inevitable result of the vicious circle thus pro- 

 duced is death. 



We must now turn to the other symptoms and signs of want of 

 oxygen, beginning with the circulatory symptoms. Unfortunately 

 we cannot as yet measure the volume of blood circulated per 

 minute in the same easy way in which we can measure the volume 

 of air breathed. Our knowledge of the effects of want of oxygen 

 on the circulation is thus imperfect as yet. It will be discussed 

 more fully in Chaper X. When moderate symptoms of anoxaemia 

 are produced experimentally, as in a steel chamber at reduced 

 atmospheric pressure, or when air deficient in oxygen is breathed, 



