CHAP, vii.] SPECIAL MUSCULAR MECHANISMS. 325 



during a deep inspiration 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 portion 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 glottis is narrowed ; though the nerve is distri- 



o o o 



biited to both dilating and constricting, to abductor and adductor 

 muscles, the latter overcome the former when the nerve is arti- 

 ficially stimulated. But this is true only when the stimulus is 

 adequately strong ; if the stimulus be weak, the abductors alone 

 are thrown into contraction and the glottis is widened. We may, 

 in this connection, add the remark that ether paralyses 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 phonation, the narrowing of the glottis by the adductors, 

 is lost. A like differentiation of the two functions is shewn, in 

 a reverse way, by the clinical experience that while functional 

 nervous disorders, such as hysteria, are marked by failure of the 

 adductors alone, the characteristic loss of voice being due to this, 

 the first effect of structural changes in the bulb or other parts 

 of the nervous mechanism is to bring about failure of the abductors ; 

 indeed the condition of the larynx as shewn by the laryngoscope 

 may be used as an aid to the diagnosis of commencing organic 

 disease. 



909. When the larynx is used for voice the recurrent 

 laryngeals are brought into play in order to produce the essential 

 condition of voice, the approximation of the vocal cords. The 

 vocal cords having been adequately approximated, low notes 

 may be uttered without any further change in the larynx ; in 

 their natural position of rest the vocal cords are sufficiently tense 

 to permit their being thrown into vibrations when brought near 

 enough together and subjected to a sufficient blast. In order 

 however to utter notes at all high, the tension of the cords must 

 be increased ; and this as we have said is brought about chiefly 

 by means of the superior laryngeal nerves and the crico-thyroid 

 muscles. Hence when these nerves are divided or fail through 

 disease, high notes can no longer be uttered ; and the division or 

 failure of the nerve even on one side only will bring about this 

 result. 



When in using the voice a change has to be made from a 

 higher to a lower note, while the action of the crico-thyroid ceases 

 or is lowered, that of the antagonistic thyro-arytenoid comes into 

 play, and the recurrent laryngeal nerve is again used. 



910. Utterance of the voice is a conspicuously voluntary act 

 and in the vast majority of cases an eminently skilled act. Hence 



