292 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



through the resistance of the inhibitory, and an explosive discharge is 

 the result. After the discharge the inhibitory processes are in the 

 ascendant, and the stimulus from without has to continue acting for a 

 further length of time before the excitation can gather fresh strength to 

 break again through the resistance. 



This conception, in default of a better, has played a great part in the 

 speculations of physiologists on the nature of inhibition and the produc- 

 tion of rhythmic activity. It suffers, however, as was pointed out by 

 Gad, from the drawback that the molecular processes have to be con- 

 sidered as endowed with inertia. Otherwise, as soon as the excitatory 

 were in excess of the inhibitory processes, there would be a leaking dis- 

 charge, and a constant stimulus would cause a constant activity. We shall 

 have occasion to discuss further applications of this theory later on when 

 dealing with the nature of the action of the vagus on the respiratory 

 centre. I have already mentioned the opinion held by some physiologists, 

 that the respiratory centre, if separated altogether from afferent stimuli, 

 would be unable to originate any respiratory movements. How, under 

 this hypothesis, could we explain the action of oxygen-hunger on the 

 respiratory centre ? It is evident that the results of deprivation of 

 oxygen might be equally well explained by assuming that its action was 

 similar to that of strychnine, only differing from this drug in its more 

 pronounced specific effect on the medullary centres. Like strychnine, 

 asphyxia may cause convulsions of spinal origin after separation of the 

 spinal cord from the medulla oblongata. Whether we consider that the 

 activity of the respiratory centre is inaugurated by afferent stimuli, or 

 by the molecular life of the cells themselves, it is evident that a lower- 

 ing of the resistance to the passage of the impulses from the cells in all 

 directions will lead first to an increase in the respiratory movements, as 

 in dyspnoea, and later on to the general convulsions of asphyxia. 



We may conclude that respiration is dependent on the integrity of 

 a part of the central nervous system situated in the medulla oblongata, 

 and which is therefore termed the respiratory centre. This centre is 

 so arranged that it can respond to constant stimulation by intermittent 

 rhythmic discharges. Under normal conditions this constant stimulation 

 may be afforded by 



1. The sum of afferent impulses arriving at the centre. 



2. The venosity of the circulating blood. 



3. The normal metabolic changes in the nerve cells. These two 

 latter factors may probably be classed together as the oxygen-hunger of 

 the cells. 



The influence of afferent nerves on respiration. I have 

 already mentioned the fact that, on cutting off the centre as far as 

 possible from all afferent impulses, the respiratory movements, though 

 rhythmic, differ widely from the normal. They consist of a series of 

 spasms, recurring at ever-increasing intervals, which are inadequate for 

 the maintenance of life. This fact shows that, for the normal per- 

 formance of the respiratory functions, the arrival of afferent impulses 

 at the centre is necessary, and we have now to consider the paths of 

 these afferent impulses and the nature of the influence they exert on 

 the centre. 



Action of the vagus on respiration. Of all the sensory nerves 

 in the body, the vagi are the only ones whose section produces a marked 

 and permanent change in the respiratory rhythm. These, therefore, have 



