278 RESPIRATION 



the curious phenomenon that in resuscitation of the bulb after a 

 period of anaemia the natural respiration, when it returns, may 

 have for a short time exactly the same rhythm as the artificial 

 respiration which has just been stopped. 



That the blood when the gaseous exchange in the lungs is inter- 

 fered with produces dyspnoea by acting on some portion of the brain 

 may be shown in an interesting manner by establishing what is 

 called a cross-circulation in two rabbits or dogs. The vertebral 

 arteries and one carotid are tied in both animals; the remaining 

 carotids are divided and connected crosswise by glass tubes, or, 

 what is better, as it avoids the risk of clotting, they are crossed by 

 suturing the cut ends, so that the brain of each is supplied by blood 

 from the other. When the respiration is artificially hindered or 

 stopped in one of the animals, it shows no dyspnoea; it is in the 

 other, whose brain is being fed with improperly ventilated blood, 

 that the respiratory movements become exaggerated. The point 

 of attack of the ' venous ' blood has been further localized in the 

 spinal bulb by the observation that when the brain has been cut 

 away above it, the cord severed below the origin of the phrenics, 

 and all other nerves connected with the region between the two 

 planes of section divided, any interference with the gaseous ex- 

 change 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 circu- 

 lating 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 pf 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) 



* 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. 



