REGULATION OF THE RESPIRATORY MOVEMENTS 1143 



activity of the centre, leading to the increased inspiratory tonus already described. 

 But unchecked by any reigning impulses, the centre enters upon a career of spendthrift 

 activity. Each inspiratory contraction is maximal, but the centre, exhausted by the 

 effort, has to wait a considerable time before it can accumulate sufficient energy for 

 the next; hence the final result of section of both vagi is deepening and slowing of 

 respiration. 



Although Gad has rendered great service in emphasising the importance of 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. 526). Mean- 

 while 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 



FIG. 527. Effect of 10'6 per cent. C0 2 in a mixture containing 23'3 per cent. 2 on a 

 rabbit with both vagi divided. The gas was administered between the arrows. 

 Zero line of blood pressure is 32 mm. below bottom of tracing. Compare this 

 Figure with Fig. 518, p. 1131. (F. H. SCOTT.) 



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 

 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 

 of the 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 of both 

 kinds 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 



