490 APNCEA. [Book ii. 



augmented. But there are reasons which render such a view 

 untenable. In the first place there is no direct and satisfactory 

 proof that in apnoea the arterial blood is overloaded with oxy- 

 gen as supposed ; indeed during the course of apnoea before 

 it has come to an end the blood becomes distinctly less arterial, 

 more venous than usual. In the second place apnoea, if not 

 entirely impossible, is much more difficult to bring about when 

 both vagus nerves are divided, and if it does occur after sec- 

 tion of the vagus nerves has not the same characters as ordinary 

 apnoea. Now, when artificial respiration is being carried on 

 section of the vagus nerves can have no effect on the quantity 

 of oxygen taken up by the blood in the lungs. But the vagus 

 nerves are the channel of impulses affecting the respiratory 

 centre, and this relation of the apnoea to the vagus nerves sug- 

 gests another and different interpretation of apnoea. As we 

 have seen, expansion of the lung by acting in some way or 

 other on the pulmonary terminations of the vagus nerve sends 

 up along that nerve impulses which inhibit inspiration. And 

 it is argued that repeated forcible inflations of the lungs pro- 

 duce apnoea by generating potent inhibitory impulses, which by 

 a kind of summation of their effects in the spinal bulb stop 

 for a while the generation of respiratory impulses in the respira- 

 tory centre. This conclusion moreover is strongly supported 

 by the fact that an apnoea may be produced, so long as the 

 vagus nerves are intact, by forcible artificial respiration with 

 hydrogen instead of atmospheric air ; in other words, the in- 

 hibitory impulses generated in the vagus nerves by the inflation 

 are sufficient wholly to neutralize the development of respira- 

 tory impulses which the deficient arterialization of the blood 

 would otherwise have produced. 



§ 305. Secondary Respiratory Rhythm. Cheyne-Stokes Res- 

 piration. A remarkable abnormal rhythm of respiration, first 

 observed by Cheyne but afterwards more fully studied by 

 Stokes, and hence called by their combined names, occurs in 

 certain pathological cases. The respiratory movements grad- 

 ually decrease both in extent and rapidity until they cease 

 altogether, and a condition of apnoea, lasting it may be for sev- 

 eral seconds, ensues. This is followed by a feeble respiration, 

 succeeded in turn by a somewhat stronger one, and thus the 

 respiration returns gradually to the normal, or may even rise 

 to hyperpnoea or slight dyspnoea, after which it again declines 

 in a similar manner. A secondary rhythm of respiration is 

 thus developed, periods of normal or slightly dyspnoeic respira- 

 tion alternating by gradual transitions with periods of apnoea. 

 The cause of the phenomena is not thoroughly understood. 



