228 



THE ANATOMY OF THE HORSE. 



elastic fibres connecting it to the thyroid ; but in the horse this action 

 is assisted by the hyo-epiglottideus muscle. 



THE MUSCLES OF THE LAEYNX. 



Extrinsic Group. — This includes the sterno- thyroid, the thyi'O-hyoid, 

 and the hyo-epiglottideus. The last of these is a single muscle ; the 

 other two are double. 



The Steeno- THYROID Muscle (Fig. 26). See page 146. 

 The Thyro-hyoid Muscle (Figs. 26 and 27). This is a dark-coloured, 

 fleshy nuiscle taking origin from the thyroid cornu (heel process) of the 

 hyoid bone, and inserted into an oblique line on the outer surface of the 

 thyroid wing. 



Action. — Acting alone, the thyro-hyoid muscles would elevate the 

 larynx between the thyroid cornua of the hyoid bone ; but when they 

 act in concert with the sterno-thyroid, the thyroid cartilage will be 

 steadied, and will serve as the fixed point for the crico-thyroid and thyro- 

 arytenoid muscles. 



The Hyo-epiglottideus Muscle (Fig. 27) takes migin from the 



upper face of the body of the 

 hyoid bone ; and passing back- 

 wards in the middle line, it is 

 inserted into the anterior surface 

 of the epiglottis at its lower part. 

 Its fibres are mixed with a quan- 

 tity of fatty-elastic tissue. 



Action. — To assist the natural 

 elasticity of the epiglottis in 

 restoring the cartilage to the 

 erect position at the close of the 

 act of deglutition. 



Intrinsic Group. — This includes 

 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 arytenoid- 

 eus. 

 The Ceico-thyroid Muscle (Fig. 26) arises from the side of the 

 cricoid cartilage ; and its fibres, passing obliquely upwards and back- 

 wards, 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 



Fig. 26. 



Larynx, Side View. 



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

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

 Hyoideus ; 6. Insertion of Stemo-Thyroid ; 7. Crico-' 

 Thyroideus ; 8. Crico-arytenoideus Posticus ; 9. 1st 

 Eing of Trachea ; 10. Thyroid Body. 



