570 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Both sets of fibres convey impulses which have their origin essentially in 

 the peripheries of the pneumogastric nerves in the lungs ; but expiratory 

 impulses may arise in the fibres of the superior and inferior laryngeal nerves, 

 especially in the former. The impulses which arise in the lungs are under 

 ordinary circumstances produced mechanically by the movements of the lungs, 

 although it is believed by some that the composition of the gases in the alveoli 

 is an important factor. According to the latter view, when the lungs are in 

 the expiratory phase the accummulation of CO 3 in the air-cells excites the 

 peripheries of the inspiratory fibres, thus giving rise to impulses which are 

 carried to the iuspiratory portion of the respiratory centre, and exciting inspi- 

 ration ; whereas the stretching of the lungs during inspiration is held to excite 

 the peripheries of the expiratory fibres, generating impuses which are conveyed 

 to the expiratory portion of the expiratory centre, causing expiration. There 

 is, however, no sufficient evidence to lead us to believe that the presence of 

 CO 2 in normal percentages influences in any way either set of fibres. On the 

 contrary, the mechanical effects of the movements of the lungs are of great 

 importance, as is apparent from the fact that inflation excites active expi- 

 ration, whereas aspiration or collapse excites inspiration; moreover, if the 

 movements of one lung be prevented by occlusion of the bronchi or by free 

 opening of the pleural sac, the effects are the same as though the vagus of the 

 same side were cut ; if now the other nerve be severed, the results are the same 

 as when both nerves are cut. The movements of the lungs therefore generate 

 alternate inspiratory and expiratory impulses, collapse causing inspiratory 

 impulses, and expansion causing expiratory impulses. The inspiratory 

 impulses, however, not only excite inspiration, but concurrently limit the 

 duration of expiration ; while the expiratory impulses excite expiration and 

 concurrently limit inspiration. 



Excitation of the superior laryngeal nerve causes expiratory stimulation, 

 and there may occur respiratory arrest in the expiratory phase. These fibres 

 are extremely sensitive; and they are of considerable physiological import- 

 ance, as is illustrated by the fact that the entrance of foreign bodies into 

 the larynx during deglutition causes an immediate arrest of inspiration, and 

 even a forced, spasmodic expiration. The foreign particles, coming in 

 contact with the keenly sensitive fibres of these nerves, generate impulses 

 which arrest inspiration, thus being prevented from being carried to the 

 lungs. 



The fibres of the glosso-pharyngeal nerves act similarly. Their excitation 

 is followed by an arrest of respiration which lasts for a period equal to that 

 occupied by about three of the preceding respiratory acts. The value of such 

 an inhibitory influence is obvious : During swallowing breathing is arrested, 

 evidently for the purpose of preventing the aspiration of food and drink into 

 the larynx. This act is purely reflex, and is due to the excitation of fibres of 

 these nerves by the fluid or the bolus of food after the act of deglutition has 

 begun. Such impulses flow to the respiratory centre, immediately arresting 

 the inspiratory discharge in whatever phase the inspiratory movement may 



