REGULATION OF RESPIRATORY MOVEMENTS 



1093 



tory centre, all connections with the latter being destroyed (v. Fig. 513). Meanwhile 

 the animal carries out normal respiratory movements, which can be recorded by the 

 diaphragm slip method. While the slip is contracting regularly, the right pleura is 

 opened and the right lung allowed to collapse. The effect of this collapse, carried up by 

 the right vagus to the centre, is an extreme contraction of the diaphragm, and since the 

 onset of asphyxia is prevented by the artificial respiration carried out on the left lung, 

 the tonic standstill of the diaphragm may last over a minute. In this case therefore" 



B.C. 



L.V. (cut) 



To artif resp.app. 



R! Lung. 



L T Lung. 



FIG. 513. Diagram to illustrate Head's experiment on the effect of collapse of the 

 lung. B.C, respiratory centre ; R.v, L.V, right and left vagi. 



the effect of collapse of one lung is enormously greater than that produced by section 

 of both vagi, showing that the effect is due, not to abolition of the ordinary tonic inhibi- 

 tory stimuli, but to excitation of special inspiratory fibres in the vagus by the collapse 

 of the lung. 



By means of the string galvanometer it is possible to show definitely that a collapse 

 o" \ he lungs does set up a nervous impulse travelling up the vagus nerves. This 

 impulse must be inspiratory in character, so that there is no reason to deny the existence 

 0? both k : nds of fibres in these nerves. The effect of electrical stimulation, especially 

 with an ascending constant current, is also strong evidence in the same direction. 



After division of both vagi the total pulmonary ventilation does not as 

 a rule undergo any marked changes, and in the absence of anaesthesia the 

 aeration of the blood may be carried out almost, if not quite, as well as in 

 the intact animal. The importance of the vagus action for the organism 

 is shown, however, if we put an increased strain on the respiratory mechanism, 

 as, for instance, by increasing the percentage of carbon dioxide in the air 

 breathed. In the intact animal this procedure leads first to increased depth 

 and later to increased frequency of respiration, the total ventilation being 

 thereby augmented to such an extent as to keep the alveolar tension of carbon 

 dioxide almost constant. If the same percentage of carbon dioxide be 

 administered to an animal after section of both vagi the effect is deepening 

 of respiration, but not quickening (Fig. 514). Each inspiratory movement, 

 however, is already considerable so that the margin by which increase of 



