1216 



PHYSIOLOGY 



be more strikingly brought about by stimulation of the central end 

 of the superior laryngeal nerve, which produces first an inhibition of 

 inspiration, so that the respiratory muscles come to a standstill in the 

 position of expiration, and then a forcible contraction of the expiratory 

 muscles. This illustration of the presence of expiratory fibres in the 

 superior laryngeal nerve is not confined to laboratory experience, but 

 is constantly occurring in everyday life. The superior laryngeal nerve 



supplies sensory fibres to the mucous 

 membrane of the glottis, and we know 

 that the slightest irritation of these 

 fibres -the presence of a crumb or a 

 particle of mucus -causes forcible 

 expiratory spasms, with spasmodic 

 closure of the glottis, which we term 

 a cough.* 



So we see that the vagus nerve 

 contains two kinds of afferent fibres, or 

 at any rate afferent fibres with two 

 distinct functions. Stimulation of the 

 one kind stops inspiration and produces 

 expiration ; stimulation of the other 

 stops expiration and produces inspira- 

 tion. Since section of both vagi causes 

 slowing of respiration, impulses which 

 exert some influence on the respiratory 

 centre and quicken respiration must 

 travel up the vagi from the lungs. The 

 respiratory movements cause an alter- 

 nate distension and contraction of the 

 lungs, and it has long been thought 

 that it is these changes in the volume 

 of the lungs which start the accelera- 



FIG. 508. Effects of distension- 

 collapse of lung. Both curves are 

 described by a lever attached to a 

 slip of the diaphragm of a rabbit. 

 A contraction of the diaphragm 

 (inspiration) raises the lever; dur- 

 ing relaxation of the diaphragm 

 the lever falls. 



In A, the trachea is closed at x, 

 the height of inspiration ; a pause 

 follows, during which the lever 

 gradually sinks until an inspiration 

 (a very powerful one) sets in. 



In B, the trachea is closed at the 

 end of expiration, x ; there follow 

 powerful inspirations. (FOSTER.) 



ting impulses that travel up the 



vagus nerves. To test the truth of this hypothesis it is necessary to 

 study the two phases of respiration separately ; that is, to see first 

 the result on the respiratory impulses of distension of the lungs, and, 

 secondly, the result of a sudden collapse or a contraction caused by 

 sucking air out of the lungs. The effects of distension or collapse 

 of the lung may be shown by simply closing the trachea at the end 



* It must not be imagined that the fibres of the superior laryngeal nerves 

 are concerned in the reflex maintenance of the normal respiratory rhythm. 

 They are cited here merely because the result of their stimulation resembles 

 that which would be caused by stimulation of the analogous expiratory fibres 

 which run in the trunk of the vagus from the lungs to the respiratory centre. 



