Chap, ii.] RESPIRATION. 483 



ventilation, the result is that the inspiratory efforts are dimin- 

 ished, 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. 98) 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 de- 

 velop inspiratory impulses, the effect on the inspiratory impulses, 

 as might be expected from the dominance of the inspiratory por- 

 tion 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 increas- 

 ing 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 there- 

 fore 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 as- 

 cending 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 inspira- 

 tory 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 

 collapse stimulates the fibres which inhibit expiration and aug- 

 ment inspiration. The respiratory 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. 



§ 298. The double or alternate respiratory action of the 

 vagus nerves on which we have dwelt above may be taken as 

 in a general way illustrative of the manner in which other 

 afferent nerves and various parts of the cerebrum are enabled 

 to influence respiration. As we have already said, and indeed 



