1090 PHYSIOLOGY 



ments become less frequent and are increased in amplitude. If now the 

 central end of one of the vagi be stimulated with an interrupted current, the 

 respiration may be quickened, or, as is more commonly the case, the in- 

 spiratory movements may be increased at the expense of the expiratory, so 

 that finally a condition of inspiratory standstill is produced, and the slip of 

 the diaphragm enters into prolonged contraction. 



With a very weak stimulus it is sometimes possible to produce augmenta- 

 tion of the expiratory movements or rather inhibition of the inspiratory, and 



this is the invariable result of passage of 

 a constant current through the vagus in 

 an ascending direction. This effect may be 

 more strikingly brought about by stimu- 

 lation of the central end of the superior 

 laryngeal nerve, which" produces first an 

 inhibition of inspiration, so that the re- 

 spiratory muscles come to a standstill in 

 the position of expiration, and then a 

 forcible contraction of the expiratory mus- 

 cles. This illustration of the presence of 

 expiratory fibres in the superior laryngeal 

 nerve is not confined to laboratory experi- 

 ence, but is constantly occurring in every- 

 day 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 inspiration. 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 alternate 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 accelerating impulses that travel 



It must not be imagined that the fibres of the superior laryngeal nerves are con- 

 cerned 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. 



FIG. 510. 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 gradu- 

 ally 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.) 



