RESPIRATION 203 



lungs excites in the vagi impulses which lead to an 

 inspiration ; distension of the lungs arouses nerve cur- 

 rents which prompt the centre to an act of expiration. 

 Divide the vagi, and the respiration becomes slower and 

 deeper ; apply a powerful stimulus to the upper end, 

 and the inspiratory movements are greatly increased. 

 This controlling action of the vagus is no doubt often 

 disturbed in disease. Irritation of the fibres of the 

 vagus, for example, appears to be the cause of the 

 rapid respiration which comes on in pneumothorax, 

 for it disappears when the nerve is divided (Traube). 

 If, again, the air passages are in any way obstructed, 

 as may happen, for example, in stenosis of the larynx, 

 the respirations though the need for air is urgent are 

 very slow, the reason being that the lung can only be 

 slowly expanded with air, which takes a long while to 

 get past the obstruction, and during all that time the 

 reflex inhibitory action of the vagus in inspiration is 

 steadily in action. 



The vagi, though the most constantly active, are by 

 no means the only afferent nerves which can influence 

 the respiratory centre. Stimulation of the nasal branches 

 of the fifth nerve produces expiratory efforts (sneezing), 

 and stimulation of the superior laryngeal has a similar 

 but even more powerful effect (coughing). Both of these 

 reflex effects upon the centre are protective in nature, 

 and lead to the expulsion of irritating foreign bodies 

 from the nose and larynx respectively. The temporary 

 arrest of respiration which is necessary during the act 

 of swallowing is secured by the action of the glosso- 

 pharyngeal, which, when stimulated in the throat, 



