CHAP, vii.] SPECIAL MUSCULAR MECHANISMS. 1083 



bar nervous mechanism, brought about by the various muscles 

 spoken of above, the sphincter group being especially used for 

 narrowing. When occasion requires, a powerful action of this 

 group leads, as in the first step of a c cough,' to complete 

 closure of the glottis; and further security in the act is ob- 

 tained by the narrowing of the vestibule or space above the 

 vocal cords, the ventricular bands being brought together by the 

 thyro-ary-epiglottic assisted by other muscles. 



Both the continued patency and the rhythmic changes are 

 carried out by means of the recurrent laryngeal nerves. When 

 in a living animal both these nerves are divided, the glottis be- 

 comes narrowed, assuming what may be considered its natural 

 dimensions, namely, those proper to it after death, when all mus- 

 cular contractions have ceased. Owing to the narrowing the 

 entrance and exit of air into and out of the lungs is less easy 

 than before, and a certain amount of dyspnoea, especially ob- 

 vious if the breathing be hurried, may result ; but the extent to 

 which this occurs differs much in different kinds of animals and 

 indeed in different individuals. It need hardly be said that 

 when both the recurrent nerves are divided the rhythmic widen- 

 ing and narrowing wholly cease, the glottis remaining immobile ; 

 the voice also is lost. When the nerve is divided on one side 

 only, the glottis becomes deformed ; when an attempt to utter 

 voice is made, the vocal cord on that side remains farther away 

 from the middle line than its fellow, owing to the failure of the 

 adductor muscles on that side, and no voice is produced, since 

 the approximation and parallelism of the vocal cords can no 

 longer be effected. On the other hand during a deep inspira- 

 tion the glottis is deformed by the vocal cord on that side being 

 nearer the middle line than its fellow, owing to the failure of the 

 posterior crico-arytenoid on that side. When the peripheral por- 

 tion of one recurrent nerve is stimulated, the vocal cord of the 

 same side is approximated to the middle line ; when both nerves 

 are stimulated, the vocal cords are brought together and the glot- 

 tis is narrowed ; though the nerve is distributed to both dilating 

 and constricting, to abductor and adductor muscles, the latter 

 overcome the former when the nerve is artificially stimulated. 

 But this is true only when the stimulus is adequately strong ; if 

 the stimulus be weak, the abductors alone are thrown into con- 

 traction and the glottis is widened. We may, in this connec- 

 tion, add the remark that ether paralyzes the adductors before 

 the abductors, and has this effect, even after division of both 

 recurrent nerves ; the more general respiratory function of the 

 larynx, the maintenance of a wide passage by means of the 

 abductors, is preserved, while the more special function of pho- 

 nation, the narrowing of the glottis by the adductors, is lost. 

 A like differentiation of the two functions is shewn, in a re- 

 verse way, by the clinical experience that while functional ner- 



