1082 THE VOICE. [Boox m. 



The same superior laryngeal nerve also contains motor fibres 

 for the crico-thyroid muscle ; and in this respect this muscle, 

 the chief tensor of the vocal cords, stands apart from all the 

 rest of the muscles of the larynx, for these are all supplied by 

 the recurrent laryngeal branch. These motor fibres, both of the 

 superior and of the recurrent laryngeal nerves, though running 

 in the trunk of the vagus are generally believed to belong not to 

 the vagus proper but to the spinal accessory nerve, and to the 

 division of that nerve which we have called the bulbar accessory 

 nerve ; but on this point opinions are not agreed. 



There are some reasons for thinking that the superior laryn- 

 geal contains afferent fibres not only for the crico-thyroid but 

 also for at least some of the muscles whose motor fibres come 

 from the recurrent laryngeal; and it has been suggested that 

 these afferent fibres of the superior laryngeal convey the afferent 

 impulses of the ' muscular sense ; ' but this needs further inves- 

 tigation. 



In dealing with the nervous mechanism we must now dis- 

 tinguish between the larynx as a part of the mechanism of 

 breathing and as an organ of voice. During breathing the 

 glottis is open, and at least during at all deep or laboured 

 breathing undergoes as we have previously said an increased 

 widening during inspiration followed by narrowing during the 

 succeeding expiration. In many animals this rhythmic respira- 

 tory movement is very marked ; but careful laryngoscopic obser- 

 vations shew that in man during quite quiet breathing there is 

 no appreciable change in the width of the glottis. 



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 concerned 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 bul- 



