RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 271 



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

 ordinated respiratory movements, which are universally acknow- 

 ledged 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 by either mechanical stimulation of the nerve- 

 endings in the lungs, due to the alternate stretching and shrinking, 

 or by chemical stimulation of these endings 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. 260) 

 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 varia- 

 tion (p. 801) is set up in the nerve when the lungs are inflated. 

 An electrical change is also observed when air is sucked out of the 

 lungs (Alcock and Seemann, Einthoven). 



When the normal excitation of the vagus fibres by expansion of 

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

 spiration, the diaphragm immediately relaxes, and a long expira- 

 tory pause ensues, broken at last by a series of inspirations much 

 deeper and more prolonged than those which were taking place 

 before occlusion. When the trachea is occluded at the end of 

 expiration, a series of deep and long-drawn inspirations occurs, the 

 first of which begins at the moment when the next normal inspira- 

 tion ought to have taken place had the windpipe been left free. 

 The most obvious explanation of these results is that the expansion 

 of the lungs sets up impulses in the vagi which cut short the in- 

 spiratory activity of the respiratory centre (inspiration-inhibiting 

 fibres), while in collapse impulses are set up which excite it to re- 

 newed inspiratory discharge (inspiration-exciting fibres). Since 

 ordinary expiration is in the main not associated with active muscular 

 contraction, the inspiration-inhibiting fibres would be at the same 

 time expiration-exciting. Clearly this would constitute a so-called 

 ' self-steering ' arrangement, each inspiration leading inevitably to 

 the succeeding expiration, and each expiration providing the neces- 

 sary stimulus for the succeeding inspiration. On this hypothesis 

 section of the vagi must necessarily be followed by slowing of the 

 respiratory movements, and we have seen that this is the case. 



A rival hypothesis is that the automatic activity of the respira- 

 tory centre leads normally to the discharge of motor impulses to 

 the inspiratory muscles, which are cut short at each expansion of 

 the lungs by the inhibitory action of the vagus, the nerve not being 

 excited during pulmonary collapse, and therefore carrying no in- 

 spiratory impulses to the centre. On this assumption, we may 

 think of the centre as being ' wound up ' like a clock, the periodic 



