378 



RESPIRATION. 



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 gen- 

 erates 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 collapse stimulates the fibres which inhibit expiration and augment 

 inspiration. The respiratory pump may thus be looked upon as a self-regu- 



Effects of Repeated Suctions of the Lungs. Negative Ventilation. (Head.) The curve corre- 

 sponds exactly to Fig. 105, except that the lungs are subjected to repeated suctions without 

 corresponding inflations. The result is that the inspirations are repeated in such a way as to 

 lead almost to an inspiratory tetanus of the diaphragm. 



lating 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 expi- 

 ration tends to set going the succeeding inspiration. 



The regulative influence exerted by impulses normally ascending the 

 vagus nerves is further shown by the following striking experiment : As we 

 have already seen, the brain above the medulla may be removed without 

 any extraordinary change in the respiration taking place. We have also 

 seen that when both vagus nerves are divided the respiration is slower and 

 deeper, but is otherwise regular. If, however, after the removal of the brain 

 above the medulla both vagus nerves are divided, if the respiratory centre 

 be cut off at one and the same time from impulses passing down from the 

 higher parts of the brain and from impulses ascending the vagus nerves, 

 the result is that the respirations take on the form of a series of long con- 

 tinued inspiratory spasms. It would seem as if there were a tendency in the 

 respiratory centre to go off into tetanic inspiratory explosions, that this ten- 

 dency is held in check by impulses from the brain when the vagus nerves 

 are divided, and by impulses along the vagus nerves when the brain is re- 

 moved, but meets with no adequate checks when impulses from both sources 

 are cut off at the same time. 



310. Hypotheses have been put forward to explain the changes in the 

 respiratory centre which lead to the rhythmic discharge of inspiratory and 

 expiratory impulses and the further changes which result from the advent 

 of augmenting and inhibitory impulses; but these as yet remain mere hy- 

 potheses, and it would not be profitable to discuss them here. We may add 



