304 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



inspiration would occur, which would be again cut short before it 

 had reached its full height. Not only, however, will the height of the 

 contractions be diminished by this automatic governing arrangement, 

 but each inhibitory impulse ascending the vagi will have a negative 

 after-action, tending to increase the vitality or inspiratory activity of 

 the centre. The effect of this increased inspiratory activity will be to 

 diminish the pauses between the different inspirations, so that finally 

 we get a condition in which the breathing remains rhythmic, but 

 the iuspiratory contractions are smaller and more frequent than before 

 the reinstatement of the vagi. Moreover, the centre is in a condition of 

 increased vitality, and able to respond with much greater promptitude to 

 any inspiratory stimulus. The vagi thus act like the governor of a 

 steam-engine in economising the labour expended, and in preventing 

 the centre from wearing itself out by excessive exertion. When these 

 nerves have been divided, the centre finally settles down into a 

 condition of " spendthrift activity," regulated only by the impulses 

 from the centres in the higher parts of the brain, and we have already 

 seen that, even with their aid, the aeration of the blood, the object of the 

 respiratory mechanism, becomes more and more ineffectually carried out. 



Of the other views with regard to the nature of vagus action, I need only 

 mention those of S. J. Meltzer. 1 This author, after showing that in all 

 animals excitation by means of strong currents of the central end of the divided 

 vagus produces expiration, followed after the cessation of the stimulus by a 

 strong inspiratory movement, concludes that the vagus contains two classes of 

 fibres, the inspiratory and the inhibitory or expiratory fibres. These differ in 

 the time relations of their response to stimuli, resembling in this particular 

 the accelerator and inhibitory cardiac nerves. When they are both stimulated 

 together, the primary effect is therefore expiration, the more lasting effect 

 inspiration. He imagines that the inspiratory expansion of the lungs excites 

 both sets of fibres simultaneously. During the excitation, the inhibitory 

 effects predominate over the inspiratory, so that the first result is a stoppage of 

 inspiration with relaxation of the inspiratory muscles. The expiratory 

 collapse which follows removes the stimulus, and therefore the more lasting 

 after-effect of the excitation, namely, that due to the inspiratory fibres, 

 can make itself felt, and we get a new inspiration. 



The influence of other afferent nerves on respiration. Although 

 there are hardly any afferent nerves in the body, stimulation of which may 

 not cause some change in the respiratory rhythm, yet in no case can we 

 regard their influence on respiration as equivalent to that of the vagus, 

 since their division is without effect on the respiratory rhythm. 



Superior laryngeal nerve As was first shown by Eosenthal, 

 stimulation of the central end of this nerve has a nearly pure expiratory 

 effect. Stimulation with a weak current causes an instant cessation of 

 inspiration and an expiratory pause. Stronger stimulation may evoke 

 strong contractions of the expiratory muscles. Besides this effect on 

 the respiratory rhythm, excitation of the superior laryngeal also affects 

 the swallowing centre in the medulla, and so gives rise to elevation of 

 the larynx, followed by peristaltic contractions of the oasophagus. As 

 was pointed out by Steiner 2 and by Meltzer, 3 the first phase of each 

 act of deglutition is accompanied by a short twitch of the diaphragm. 

 A tracing of the diaphragm during stimulation of this nerve shows 



1 Arch. f. physiol.. Leipzig, 1892, S. 340. 2 Ibid., 1883, S. 57. 



, S. 209. 



