6 2 6 



TEXT-BOOK OF PHYSIOLOGY. 



Nerves of the Larynx. The nerves which innervate the muscles of the 

 larynx and endow the mucous membrane with 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 approxi- 

 mately closed and the vocal bands be made more or less tense. 



The closure of the glottis the approximation of the vocal processes 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 tens- 

 ing of the vocal bands by the crico-thyroid muscle is regarded by some inves- 



FIG. 292. POSITION OF THE VOCAL 

 BANDS PREVIOUS TO THE EMISSION OF A 

 SOUND, b. Epiglottis, rs. False vocal 

 band. ri. True vocal band. ar. Ary- 

 tenoid cartilages. (Mandl.} 



FIG. 293. POSITION OF THE VOCAL 

 BANDS IN THE PRODUCTION OF NOTES 

 OF Low PITCH. /. Epiglottis, or. Glottis. 

 ns. False vocal cord. ni. True vocal cord. 

 ar. Arytenoid cartilages. (Mandl.) 



tigators as a coarse means, the approximation of the free edges by the thyro- 

 arytenoid, as a finer means, of adjustment for the producton 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 emis- 

 sion of a note of medium pitch, as determined by laryngologic examination, is 

 shown in Fig. 292. 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 

 which the laryngeal vibrations are reinforced. The degree of pressure to 



