RESPIRATION 225 



But when both the vagi and these upper paths are cut the char- 

 acter of the respiration is changed, exceedingly prolonged inspira- 

 tory spasms alternating with long periods of complete relaxation 

 of the diaphragm till the animal dies. 



From these facts it appears that the periodic automatic dis- 

 charges of the respiratory centre are being continually controlled 

 and modified by impulses passing up the vagus, and that in the 

 absence of these impulses a certain degree of control is exercised 

 by the higher paths, which, however, do not appear to be nor- 

 mally in action, at any rate to the full measure of their 

 capacity. When the vagi are severed, the control of the 

 higher paths comes into play, and is sufficient still to keep the 

 breathing regular, although it is slowed. When the higher paths 

 are cut off, the vagus of itself is able to regulate the discharge. 

 But when both are gone, the respiratory centre, freed from con- 

 trol, passes into a condition of alternate spasm and exhaustion. 

 Of the central connections of these upper paths but little is surely 

 known. The corpora quadrigemina, however, seem to contain 

 centres which can affect the respiration. Certain areas on the 

 cerebral cortex have also been described, the excitation of which 

 modifies the respiratory movements. There is no question that 

 the cortex is connected, and extensively connected, with the 

 respiratory centre, since the rate and depth of the co-ordinated 

 respiratory movements, which are universally acknowledged to 

 involve the activity of the centre, can be altered not only by 

 the will, but by the most varied psychical events. 



The rhythmical excitation of the regulating vagus fibres must 

 be brought about either by mechanical stimulation of the nerve- 

 endings in the lungs, due to the alternate stretching and shrink- 

 ing, or by chemical stimulation depending on the changes that 

 occur with each respiration in the content of oxygen and carbon 

 dioxide in the alveolar air, and therefore in their pressure 

 (p. 248) in the blood. Both views have found advocates, but 

 whatever influence the chemical changes in the blood may 

 exert, there is no doubt that the mechanical factors are the 

 more important. That the vagus is really excited is shown 

 by the fact that a negative variation (Chap. XI.) is set up in 

 the nerve when the lungs are inflated. An electrical change, 

 although not so pronounced, is also observed when air is sucked 

 out of the lungs (Alcock and Seemann). 



When the normal excitation of the vagus fibres by expansion 

 of the lungs is exaggerated by closing the trachea at the end of 

 inspiration, the diaphragm immediately relaxes, and a long 

 expiratory pause ensues, broken at last by a series of inspira- 

 tions much deeper and more prolonged than those which were 

 taking place before occlusion. When the trachea is occluded 



ID 



