RESPIRATION 233 



with the gaseous exchange in the lungs is at once followed by 

 dyspnoea.* 



Automaticity of the Respiratory Centre. The question 

 has been raised whether, in the absence of this ' natural ' 

 stimulation by the blood, and of the impulses that constantly 

 reach the centre along its afferent nerves, it would continue 

 to discharge itself, or whether it would sink into inaction. We 

 have already discussed a similar question in regard to the cardiac 

 and vaso-motor centres, and the subject must again present 

 itself when we come to examine the functions of the central 

 nervous system. In the meantime it is only necessary to say 

 that there is evidence that it is not the mere presence of carbon 

 dioxide (or other substances) in the blood circulating through the 

 respiratory centre which determines the constant excitation of 

 the centre, but rather the accumulation of carbon dioxide in the 

 centre itself when the partial pressure of that gas in the blood 

 is raised. The idea that the continuous excitation of the centre 

 is ' autochthonous ' in other words, that it is due to an internal 

 stimulating substance or substances manufactured in the centre 

 itself, as well as carried to it in the blood renders it easy to 

 understand that the discharge of the respiratory centre, although 

 modified by the quality of the blood which circulates in it, is 

 not essentially dependent on it. Indeed, in cold-blooded animals 

 whose blood has been replaced by physiological salt solution, 

 and (in frogs) even after the circulation has been stopped 

 altogether by excision of the heart, quiet, regular breathing may 

 be seen for a considerable time. Of course, blood is essential 

 for the continued nutrition of the centre and its connections, and 

 it eventually breaks down and ceases to discharge. The respira- 

 tory discharge is still less dependent for its initiation upon 

 the arrival of afferent impulses. For after section of the bulb 

 above the centre, of the cord below the origin of the phrenics, of 

 the vagi and of the posterior roots of all the upper cervical nerves, 

 the spasmodic respiration which we have already described as 

 occurring when the vagi and the higher paths have been severed 

 continues without essential modification. It has also been ob- 

 served that during resuscitation of the bulb and upper cervical 

 cord after a period of anaemia, stimulation of afferent nerves, 

 including the vagi, is entirely without influence on the respiratory 

 movements for some time after respiration has returned, presum- 

 ably because the synapses (p. 749) on the afferent paths lying 

 within the previously anaemic area are as yet unable to conduct 

 the nerve impulses. Nevertheless, the respiratory centre con- 



* The conclusion is doubtless correct, but this experiment is not decisive. 

 For the phrenic nerves themselves contain afferent fibres, the stimulation 

 of which can influence the respiration after section of the vagi. 



