324 THE VOICE. [BOOK in. 



Much difference of opinion has been expressed as to whether 

 the width of the glottis thus permanently maintained during 

 quiet breathing is identical with that assumed after death. But 

 careful laryngoscopic measurements shew that during life the 

 glottis is distinctly wider than after death ; the average width 

 during quiet breathing, is in man about 14 mm., in woman 

 about 12 mm.; after death in man 5 mm., in woman 4 mm. 

 This points to a continued ' tonic ' contraction of some or other 

 of the dilators of the glottis ; and the muscle especially con- 

 cerned in this action appears to be the posterior crico-arytenoid. 

 Whether this tonic dilator action, whose centre lies in the bulb, 

 close to or forming part of the general respiratory centre, is 

 automatic in nature or maintained in a reflex manner by afferent 

 impulses, we need not stay now to discuss ; nor need we dwell on 

 the question whether the widened glottis is the result merely of 

 the action of the dilator muscle, or whether it is the balance of a 

 struggle between antagonistic muscles ; though analogy would 

 perhaps lead us to expect the latter to be the case, the evidence 

 appears to be in favour of the former view. 



The rhythmic alternation of widening and of narrowing ob- 

 served in laboured breathing is, through the activity of the 

 bulbar 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 'cough' ( 391), to complete 

 closure of the glottis ; and further security in the act is obtained 

 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 becomes 

 narrowed, assuming what may be considered its natural dimen- 

 sions, namely, those proper to it after death, when all muscular 

 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 obvious if the breath- 

 ing 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 widening 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 



