MUSCLES OF THE LARYNX. 



489 



without cilia. There are no mucous glands in the membrane covering either 

 the superior or the inferior chords. The inferior vocal chords alone are con- 

 cerned in the production of the voice. 



Muscles of the Larynx. The muscles of the larynx are classified as ex- 

 trinsic and intrinsic. The extrinsic muscles are attached to the outer surface 

 of the larynx and to adjacent organs, such as the hyoid bone and the sternum. 

 They are concerned chiefly in the movements of elevation and depression of 

 the larynx. The intrinsic muscles are attached to the different parts of the 

 larynx itself, and by their action upon the articulating cartilages, are capable 

 of modifying the condition of the vocal chords. 



The vocal chords can be rendered tense or loose by muscular action. 

 Their fixed point is in front, where their extremities, attached to the thyroid 

 cartilage, are nearly or quite in contact 

 with each other. The arytenoid cartilages, 

 to which they are attached posteriorly, 

 present a movable articulation with the 

 cricoid cartilage ; and the cricoid, which 

 is narrow in front, and is wide behind, 

 where the arytenoid cartilages are attached, 

 presents a movable articulation with the 

 thyroid cartilage. It is evident, therefore, 

 that muscles acting upon the cricoid car- 

 tilage can cause it to swing upon its two 

 points of articulation with the inferior 

 cornua of the thyroid, raising the anterior 

 portion and approximating it to the lower 

 edge of the thyroid ; and as a consequence, 

 the posterior portion, which carries the 

 arytenoid cartilages and the posterior at- 

 tachments of the vocal chords, is depressed. 



This action would, of course, increase the FIG ier Posterior view of the muscles of 

 distance between the arytenoid cartilages i,^5SXS3KL*-. 4*4, 

 and the anterior portion of the thyroid, ^^^S^S^^^^T" 

 elongate the vocal chords, and subject 



them to a certain degree of tension. Experiments have shown that such an 

 effect is produced by the contraction of the crico-thyroid muscles. 



The articulations of the different parts of the larynx are such that the 

 arytenoid cartilages may be approximated to each other posteriorly, thus 

 diminishing the interval between the posterior attachments of the vocal 

 chords. This action can be effected by contraction of the single muscle of 

 the larynx (the arytenoid) and also by the lateral crico-arytenoid muscles. 

 The thyro-arytenoid muscles, the most complicated of all the intrinsic mus- 

 cles in their attachments and the direction of their fibres, are important in 

 regulating the tension and capacity of vibration of the vocal chords. 



The posterior crico-arytenoid muscles, arising from each lateral half of 

 the posterior surface of the cricoid cartilage and passing upward and outward 



