1140 



PHYSIOLOGY 



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 



FIG. 523. Effects of distension and 

 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 tne presence of a crumb or a particle of 



the lever falls. mucus causes forcible expiratory spasms, 



In A, the trachea ia closed at x, ^^ spasmodic closure of the glottis, which 



the height of inspiration; a pause 



follows, during which the lever we term a COUgh. 1 



?^ U ^uUne) n B etr ti0n S ^ * vagus nerve con- 

 In B, the trachea is closed at the tains two kinds of afferent fibres, or at 



end of expiration, x; there follow anv _ ftt _ o-ffprpnt fihrpq with two rH<d-inH- 

 powerful inspirations. (FOSTER.) an ^ . e 



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 re- 

 spiratory centre and quicken respiration must travel up the vagi from the 

 lungs. The respiratory movements cause an alternate distension and con- 

 traction 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 

 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 of inspiration or of expiration . The 

 results of such an experiment are shown in Fig. 523. 



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

 ccriK-d 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. 



