PHONATION; ARTICULATE SPEECH. 579 



stationary, and to swing the quadrate plate of the cricoid and the 

 arytenoid cartilages downward and backward. This movement 

 has the result of tensing the vocal bands. The cricoid is at the same 

 time drawn backward by the action of the more longitudinally dis- 

 posed fibers. 



Nerves of the Larynx. The nerves which innervate the muscles 

 of the larynx and endow the mucous membrane wijth sensibility are 

 derived from the vagus trunk. The superior laryngeal is for the 

 most part sensor and distributed to the mucous membrane, though 

 it contains motor fibers for the crico-thyroid muscle. The inferior 

 laryngeal is purely motor and is distributed to all the muscles with 

 the exception of the crico-thyroid. 



THE MECHANISM OF PHONATION. 



Phonation, the production of vocal sounds in the larynx, is the 

 result of the vibration of the vocal bands caused by an expiratory 

 blast of air from the lungs. That a sound may arise it is essential 

 that the glottis be approximately closed and the vocal bands be made 

 more or less tense. 



The closure of the glottis the approximation of the vocal pro- 

 cesses and the vocal bands is accomplished, it will be recalled, by 

 the contraction of the lateral crico-arytenoid, the arytenoid, and the 

 thyro-arytenoid muscles. The increase in tension is accomplished 

 by the contraction of the crico-thyroid and the thyro-arytenoid 

 muscles, the former by the backward displacement of the cricoid and 

 arytenoid cartilages, the latter by converting the natural concave 

 edge of the vocal band to a straight line. The lengthening and 

 tensing of the vocal bands by the crico-thyroid muscle is regarded 

 by some investigators as a coarse means, the approximation of 

 the free edges by the thyro-arytenoid, as a finer means, of adjust- 

 ment for the production of slight changes in the pitch of sounds. 

 The extent to which the glottis is closed and the membranes tensed 

 will depend, however, on the pitch of the sound to be emitted. The 

 appearance presented by the glottis just previous to the emission 

 of a note of medium pitch, as determined by laryngologic examina- 

 tion, is shown in Fig. 267. When the foregoing conditions in 

 the glottis are realized, the air stored or collected in the lungs is 

 forced by the contraction of the expiratory muscles, through the 

 narrow" space between the bands. As a result of the resistance 

 offered by this narrow outlet and the force of the expiratory 

 muscles the air within the lungs and trachea is subjected to pressure, 

 and as soon as the pressure attains a certain level the vocal bands 

 are thrown into vibrations, which in turn impart to the column of air 

 in the upper air-passages a corresponding series of vibrations by 



