CHAP, vii.] SPECIAL MUSCULAR MECHANISMS. 1461 



to the thyroid cartilage, and so shortening the distance between 

 the processus vocales and that body. 



These several acts, however, the widening or narrowing of the 

 glottis, the tightening or slackening of the vocal cords, are only 

 the gross acts, so to speak, of the movements of the larynx. When 

 a voice of any kind has to be uttered the cords must be approxi- 

 mated and to a certain extent tightened ; and for the carrying 

 out of even these gross acts not one muscle only but more than 

 one, and often several are brought into play; the movements 

 which give rise to any kind of voice are combined and coordinated 

 movements. But, as we shall see presently, when this or that 

 particular kind of voice is being uttered or when changes in the 

 voice are being effected, the above words, widening, tightening 

 and the like, very imperfectly describe what is taking place in the 

 larynx ; changes of a very complex nature are brought about, and 

 for these the greatest nicety of combination is necessary. 



908. We may now turn to the nervous mechanisms of the 

 larynx. Fibres of the superior laryngeal branch of the vagus 

 nerve are distributed to the mucous membrane of the larynx, and 

 serve as the afferent channels by which impulses from the ex- 

 quisitely sensitive surface pass upwards to the central nervous 

 system. 



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 ( 617) called the bulbar accessory nerve; 

 but on this point opinions are not agreed. 



There are some reasons for thinking that the superior laryngeal 

 contains afferent fibres not only for the crico-thyroid but also for at 

 least some of the muscles whose motor fibres come from the re- 

 current 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 investigation. 



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 ( 336) an increased widen- 

 ing during inspiration followed by narrowing during the succeed- 

 ing expiration. In many animals this rhythmic respiratory 

 movement is very marked ; but careful laryngoscopic observations 

 shew that in man during quite quiet breathing there is no 

 appreciable change in the width of the glottis. 



932 



