500 PHYSIOLOGY CHAP. 



]>aivtic state of the vagus, determined by the altitude. In this 

 ra,se the eupnoea is no longer perfect, and the inspiratory acts are no 

 longer determined by the nervous vibrations that ascend by the 

 afferent paths of the vagus to the centres, but by a certain degree 

 of venosity acquired by the blood circulating in them during the 

 expiratory pause. 



While the intermittent excitations ci the centripetal nerve- 

 paths determine particularly the frequency of rhythm, i.e. the distri- 

 bution of the total air that passes through the lungs iu the time 

 unit, the chemical excitation produced by the venosity of the blood 

 especially determines the inti'iisity of the rhythm, i.e. the total 

 quantity of air breathed in the time unit. The theory of dyspnoea 

 agrees perfectly with this conclusion. In dyspnoea, in general, and 

 particularly in that determined by the increased production of 

 carbonic acid, the reflex excitability of the centres comes into 

 play, and causes automatic excitability to become latent by 

 adapting the respiratory rhythm to the chemical needs of the 



organism. 



As in apnoea and dyspnoea the factor of the central excitations 

 determined by the amount of the external stimuli predominates, 

 so in tachypnoea and apnoea the factor of increased, or diminished, 

 excitability predominates, owing to which the centres become more 

 sensitive, or refractory, to the action of the said stimuli. 



But the excitability of the centres (whether automatic or 

 reflex) is an oscillating quantity, which follows closely, and is, so 

 to speak, modelled on all the vicissitudes of the intimate metabolic 

 processes. Each explosion of energy that accompanies a katabolic 

 disruption determines a relative degree of resistance in the 

 centres to external and internal stimuli ; each accumulation of 

 energy, determined by an anabolic construction, increases their 

 susceptibility to the same. Further, these respiratory tracings 

 show not merely the rhythmic and alternate activity of the 

 antagonistic respiratory muscles, but they sometimes, particularly 

 after the action of certain poisons, exhibit slow positive and 

 negative oscillations in the tone of the said muscles, which recall 

 the oscillations of tone in the auricles, as described by Fano, and 

 in the vessels, as described by Schiff and by Trail be and Ilering. 

 This is well illustrated in the tracing of Fig. 230, registered 

 by Mosso from a rabbit intoxicated with piridine. Under normal 

 conditions these oscillations in the tone of the respiratory muscles 

 are absent, but occasionally they become visible in sleep as in 

 Fig. 237. These slow oscillations in the tone of the muscles 

 are the external expression of the corresponding oscillations of 

 excitability (automatic and reflex) in the respiratory centres. 



As soon as these oscillations of excitability of the centres are 

 exaggerated, the phenomenon of Cheyne-Stokes breathing sets in. 

 The pauses depend essentially upon the depression of excitability 



