THE RESPIRATION 293 



The nature of the gas, if non-irritant, with which this in- 

 flation is carried out is of no consequence. If on the other 

 hand the lungs be collapsed by sucking air out of them, the 

 inspiration becomes more and more powerful, and may end 

 in a spasm of the inspiratory muscles. 



This shows that with each expiration a stimulus passes up 

 the vagus which acts upon the inspiratory centre to make it 

 discharge. The vagus is thus a true excito-motor nerve, 

 making the centre act in a reflex manner. With each col- 

 lapse of the lung the vagus is thrown into action, as the 

 lungs expand it ceases to act, and, as a result, the inspiratory 

 centre stops acting, the muscles of inspiration cease to con- 

 tract, and expiration occurs. 



While ordinary respiration may thus be considered as a 

 rhythmic reflex act, it must not be forgotten that the respira- 

 tory centre can and does act rhythmically under the influence 

 of the higher centre, or a-rhythmically and spasmodically 

 when these as well as the vagi are severed from it. 



So far the pulmonary branches of the vagus alone have 

 been considered. 



But the upper part of the respiratory tract, the larynx, 

 receives its sensory fibres from this nerve. Section of the 

 superior laryngeal branch of the vagus does not alter the 

 rhythm of respiration. Stimulation of the upper end of the 

 cut nerve causes first an inhibition of inspiration and, if 

 stronger, produces forced expiratory acts. This is well illus- 

 trated by the very common experience of the effect of a 

 foreign body, such as a crumb, in the larynx. The fit of 

 coughing is a series of expiratory acts produced through 

 this nerve. 



Another set of visceral nerves having an important influ- 

 ence on the respiratory rhythm are the splanchnics. 



When these are stimulated inspiration is inhibited. Every 

 one has experienced the "loss of wind" as the result of a 

 blow on the abdomen. 



The glossopharyngeal, which supplies the back of the 

 tongue, when stimulated, as by the passage of food in the 

 act of swallowing, causes an instant arrest of the respiratory 

 movements either in inspiration or expiration. The advan- 

 tage of this in preventing the food as it is swallowed from 



