CHAP, ii.] RESPIRATION. 595 



(Fig. 83), a procedure which we may speak of as positive venti- 

 lation, the result is that the inspiratory efforts are diminished, and 

 if the ventilation is continued may cease altogether. If on the 

 other hand air is repeatedly sucked out of the lungs, without 

 any corresponding inflations, negative ventilation, the inspiratory 

 efforts are increased (Fig. 84) and the increase may be such as to 

 bring the diaphragm to a state of tetanus. And in general, 

 though several complications occur which we cannot discuss here, 

 the results of inflation of the lungs on the one hand and of suction 

 or collapse of the lungs on the other hand, shew that the mere 

 inflation or perhaps rather the mere distension of the lung tends 

 to inhibit inspiratory and usher in expiratory impulses, while 

 collapse of the lung tends to inhibit expiratory and to develope 

 inspiratory impulses, the effect on the inspiratory impulses, as 

 might be expected from the dominance of the inspiratory portion 

 of the centre, being more marked than the effect on the expiratory 

 impulses. That the instrument by which these effects are produced 

 is the vagus nerve is shewn by the fact that they are no longer 

 distinctly recognizable when both vagus nerves are divided. And 

 that the results are due to the mere mechanical expansion and 

 collapse of the lung in insufflation and collapse, and not to any 

 chemical influences exerted by the larger amount or smaller 

 amount of air present in the lung in the two cases increasing or 

 diminishing the absorption of oxygen and escape of carbonic acid, 

 is shewn by the fact that the results remain in their main features 

 the same when some indifferent gas such as hydrogen is used for 

 inflation instead of air or oxygen. We infer therefore that the 

 expansion of the pulmonary alveoli in some way or other so 

 stimulates the endings in the lung of the pulmonary branches of 

 the vagus, that impulses are generated which ascending the vagus 

 trunk inhibit the inspiratory processes in the respiratory centre ; 

 and that conversely collapse of the lung similarly generates 

 impulses which are augmentative of inspiratory impulses. And, 

 assuming on the strength of analogy the existence in the vagus of 

 two sets of fibres we may say that expansion stimulates the 

 endings of the fibres which inhibit inspiration and concurrently 

 tend to augment expiration, while ctllapse stimulates the fibres 

 which inhibit expiration and augment inspiration. The respira- 

 tory pump may thus be looked upon as a self-regulating 

 mechanism : the expansion of the lungs which is the result of the 

 efferent inspiratory impulses tends to check the issue of these 

 impulses and to inaugurate the sequent expiration ; and the 

 return of the lungs in expiration tends to set going the succeeding 

 inspiration. 



The regulative influence exerted by impulses normally ascending 

 the vagus nerves is further shewn by the following striking ex- 

 periment. As we have already seen the brain above the medulla 

 may be removed without any extraordinary change in the respiration 



382 



