DISSECTION OP THE LARYNX. 



229 



action either the cricoid or thyroid attachment may be the fixed point 

 of the fibres. 



Directions. — The thyro-arytenoid and lateral crico-arytenoid muscles 

 lie under cover of the thyroid wing, which must therefore be removed 

 on one side. This is to be done by removing the thyro-hyoid and crico- 

 thyroid muscles, disarticulating the cri co-thyroid joint, and incising the 

 ala a little behind the body of the thyroid, after the manner of Fig. 27. 



The Thyro-arytbnoid Muscle (Fig. 27) consists of two parallel 

 bundles, between which the mucous inembrane of the ventricle of the 

 larynx protrudes as a pouch. Its fibres arise from the inner surface of 

 the thyroid wing near its junction with the body, and from the crico- 

 thyroid membrane. The lower fibres are {■^serted into the outer surface 

 of the arytenoid cartilage, while its higher fibres join those of the 

 arytenoideus muscle. 



Action. — The muscle is antagonistic to the crico-thyroid, dimin- 

 ishing the tension of the vocal cord by acting on the crico-thyroid 

 joint. 



The Posterior Crico-ary- 

 tenoid Muscle (Figs. 26 and 

 28). This is the most powerful 

 of the intrinsic muscles. Its 

 muscular tissue is dark red, 

 and mixed with tendinous 

 tissue. Its fibres take origin 

 from the outer surface of the 

 cricoid bezel, and are inserted 

 into the prominent tubercle 

 on the external angle of the 

 arytenoid cai'tilage. 



Action. — To swing out- 

 wards the arytenoid carti- 

 lage, and thus to separate 

 the vocal coi-ds and dilate 

 the glottis. 



The Lateral Crico-ary- 

 tenoid Muscle (Fig. 27). 

 This muscle is placed below 

 the thyro-arytenoid, under 

 concealment of the thyroid 

 wing. Its fibres arise from the upper border of the side of the cricoid 

 cartilage ; and passing backwards and upwards, they become inserted 

 into the same tubercle on the base of the arytenoid as the posterior 

 muscle, and into the outer surface of the arytenoid in front of that 

 tubercle. 



Larynx, Side View (thyroid ala iikmoved). 



1. Glossal Process of Hyoid ; 2. Cut Base of Thyroid 

 Cornu ; 3. Small Cornu ; 4. Great Cornu ; 5. Epiglottis ; 

 6. Arytenoid Cartilage ; 7. Cut Wing of Thyroid Carti- 

 lage ; 8. Facet on Cricoid for Articulation with Thyroid 

 Cartilage ; 0. Pouch of Mucous Membrane from Ven- 

 tricle of Larynx ; 10. and 11. Upper and Lower Bundles 

 of Thyro -Arytenoideus ; 12. Crico-Arytenoideus Lateralis; 

 13. Crico-Arytenoideus Posticus ; 14. Thyro-Hyoideus ; 

 15. Hyo-Epiglottideus ; IG. Thyroid Body ; 17. 1st Ring 

 of Trachea. 



