CHAP, ii.] RESPIRATION. 357 



of the artificial respiration so long as the vagi are intact; when 

 these are divided the movements of the nostrils exhibit a rhythm 

 independent of those of the chest. From this it is inferred that the 

 mere mechanical expansion of the lungs transmits along the vagus 

 an impulse tending to inhibit inspiration and to generate an ex- 

 piration, and the mechanical contraction of the lungs an impulse 

 tending to inhibit expiration and to generate an inspiration. That 

 is to say the very expansion of the lungs, which is the natural 

 effect of an inspiration, tends of itself to cut short that inspiration 

 and to inaugurate the sequent expiration, and similarly the 

 contraction of an expiration promotes the following inspiration. 

 The lungs in fact may be spoken of as being so far self-regulating. 



The influence of the vagus is further shewn by the following 

 experiment. If the medulla oblongata be carefully divided in the 

 middle line respiration may continue to go on in quite a normal 

 fashion, indicating that the centre is composed of two lateral 

 halves placed one on each side of the median line. If however 

 one vagus be then divided, the respiratory movements both costal 

 and diaphragmatic, on the side of the body on which division of 

 the vagus has taken place, become slower than those on the other 

 side, so that the two sides are no longer synchronous. Obviously 

 the vagus influences primarily the respiratory centre of its own side ; 

 though under normal conditions the two halves of the centre work 

 in harmony and synchronism. 



When after division of both vagi, the medulla being intact, the 

 central stump of one vagus is stimulated with a gentle interrupted 

 current, the respiration, which from the division of the nerves had 

 become slow, is quickened again; and with care, by a proper 

 application of the stimulus, the normal respiratory rhythm may 

 for a time be restored. Upon the cessation of the stimulus, the 

 slower rhythm returns. If the current be increased in strength, 

 the rhythm may in some cases be so accelerated that at last the 

 diaphragm is brought into a condition of prolonged tetanus, and 

 a standstill of respiration in an extreme inspiratory phase is the 

 result. 



If the central end of the superior laryngeal branch of the vagus 

 be stimulated, whether the main trunk of the nerve be severed or 

 not, a slowing of the respiration takes place, and this may by 

 proper stimulation be earned so far that a complete standstill of 

 respiration in the phase of rest is brought about, i.e. the respiratory 

 apparatus remains in the condition which obtains at the close of 

 an ordinary expiration, the diaphragm being completely relaxed. 

 In other words, the superior laryngeal nerve contains fibres, the 

 stimulation of which produces afferent impulses whose effect is to 

 inhibit the action of the respiratory centre; while the main trunk 

 of the vagus contains fibres, the stimulation of which produces 

 afferent impulses whose effect is to accelerate or augment the 

 action of the respiratory centre. In some cases stimulation of the 



