RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 285 



presence of carbon dioxide (or other substances) in the blood circu- 

 lating through the respiratory centre which determines the constant 

 excitation of the centre, but rather the accumulation of carbon 

 dioxide, or the increase of hydrogen-ion concentration, in the 

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

 is raised. The idea that the contimous 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 under- 

 stand that the discharge of the respiratory centre, although modified 

 by the quality of the blood which circulates in it, is not essen- 

 tially 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 blood has been replaced by physiological salt solution, and (in frogs') 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 respiratory 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 cer- 

 vical 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 

 observed that during resuscitation of the bulb and upper cervical 

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

 cluding the vagi, is entirely without influence on the respiratory 

 movements for some time after respiration has returned, presumably 

 because the synapses (p. 852) on the afferent paths lying within the 

 previously anaemic area are as yet unable to conduct the nerve 

 impulses. Nevertheless, the respiratory centre continues steadily 

 to discharge itself along the efferent paths, whose synapses are 

 situated beyond the anaemic region. Section of the bulb above 

 the level of the respiratory centre, and of the cord below the origin 

 of the phrenic nerves, in addition to the anaemia, makes no essential 

 difference in the result. The initial rate of discharge of the centre 

 thus isolated from afferent impulses is approximately constant in 

 different experiments (about four a minute in cats). 



Spinal Respiratory Centres. Although the chief respiratory centre 

 lies in the medulla oblongata, under certain conditions impulses to 

 the respiratory muscles may originate in the spinal cord. Thus, in 

 young mammals (kittens, puppies), especially when the excitability 

 of the cord has been increased by strychnine, in birds and in alli- 

 gators, movements, apparently respiratory, have been seen after 

 destruction of the brain and spinal bulb. In adult cats, when 

 the functions of the brain, medulla, and cervical cord have been 

 abolished by occlusion of their vessels, similar movements of the 



