DISSECTION OP THE LARYNX. 



235 



Fig. 30. 

 Larynx, Side View. 



1. Glossal Process of Hyoid ; 2. Small Cornu ; 

 3. Great Cornu ; 4. Arytenoid Cartilage ; 5. Tliyro- 

 Hyoideus ; 6. Insertion of Sterno-Thyroid ; 7. Crico- 

 Thyroidens ; S. Crico-arytenoideus Posticus ; 9. Ist 

 Ring of Trachea ; 10. Thyroid Body. 



The Hyo-epiglottideus Muscle (Fig. 31) takes origin from the upper 

 face of the body of the hyoid bone ; and, passing backwards in the 

 middle line, it is inserted into the anterior surface of the epiglottis at 

 its lower part. Its fibres are 

 mixed with a quantity of fatty- 

 elastic tissue. 



Action. — To assist the 

 natural elasticity of the epig- 

 lottis in restoring the cartilage 

 to the erect position at the 

 close of the act of deglutition. 



Intrinsic Group. — This in- 

 cludes four pairs of muscles, 

 viz. , the crico - thyroid, the 

 thyro-arytenoid, the posterior 

 crico-arytenoid, and the lateral 

 crico-arytenoid ; and a single 

 muscle — the arytenoideus. 



The Crioo-thyroid Muscle 

 (Fig. 30) arises from the side 

 of the cricoid cartilage ; and 

 its fibres, passing obliquely 

 upwards and backwards, are inserted into the lower edge of the thyroid 

 wing. 



Action. — This muscle acts on the crico-thyroid joint, increasing the 

 tension of the vocal cord by increasing the distance between the fore- 

 part of the thyroid and the base of the arytenoid cartilage. In this 

 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 crico-thyroid joint, and in- 

 cising the ala a little behind the body of the thyroid, after the 

 manner of Fig. 31. 



The Thyro-arytenoid Muscle (Fig. 31) consists of two parallel 

 bundles, between which the mucous membrane 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 inserted 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, diminishing 

 the tension of the vocal cord by acting on the crico-thyroid joint. 



