REGULATION OF RESPIRATORY MOVEMENTS 1219 



the inhibitory or expiratory impulses which ascend the vagi, there is no doubt 

 that he went too far in denying the existence of inspiratory fibres in the vagus. 

 This is shown by the following experiment of Head. According to Gad's view, 

 collapse of both lungs implies simply a removal of the normal inhibitory impulses 

 ascending the vagi, and is therefore equivalent to division of these two nerves. 

 If in the rabbit the left vagus be divided, a tube can be introduced into the 

 left bronchus, and artificial respiration can be performed by alternate inflation 

 and collapse of the left lung, without in any way affecting the respiratory centre, 

 all connections with the latter being destroyed (v. Fig.- 511). 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 



RT Lung. 



Lung. 



FIG. 511. 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. 



of the diaphragm, and since the onset of asphyxia is prevented by the arti- 

 ficial respiration carried out on the left lung, the tonic standstill of the dia- 

 phragm may last over a minute. In this case therefore 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 inhibitory 

 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 t 

 collapse of the lungs does set up a nervous impulse travelling up the vagu 

 nerves. This impulse must be inspiratory in character, so that there is no reason 

 to deny the existence of both kinds of fibres in these nerves, 

 electrical stimulation, especially with an ascending constant current, is als 

 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, 



