THE LARYNX. 



1825 



upon the cricoid cartilage devolves the whole, or nearly the whole of the movement. 

 Although the movement is generally described as rotation on a transverse axis pass- 

 ing through the two crico-thyroid joints, the articulation is of so vague a character 

 that a great deal of sliding occurs. 



The posterior crico-arytenoid muscle (Fig. 1554) is very distinct and 

 occupies the hollow on either side of the median ridge on the back of the cricoid 

 cartilage. It is triangular, with rounded angles at the base, which is at the ridge, 

 and the third sharp angle at the posterior border and upper aspect of the muscular 

 process of the arytenoid. The origin is not from the whole of the fossa on the cri- 

 coid, but chiefly from the region of the ridge whence it springs by tendinous fibres. 

 It arises also from the lower part of the cricoid, but not from the part near the 

 arytenoid. It passes over the capsule of the joint, with which it is intimately fused, 

 and is inserted as above stated, some of its fibres becoming tendinous. 



Action. It pulls the muscular process downward and inward, thus raising and 

 everting the vocal process and consequently enlarging the cleft of the glottis. 



Two occasional small muscles in the neighborhood of the inferior horn of the thyroid are 

 probably aberrant bundles of the posterior crico-arytenoid. One, the posterior crico-thyroid, 

 slightly diverging from the lower external fibres, runs from the back of the cricoid upward and 

 outward to the internal aspect of the inferior horn of the thyroid. The other, the posterior thyro- 

 arytenoid, runs from the lower horn upward to be inserted with the posterior crico-arytenoid into 

 the muscular process. 



Epiglottis 



Superior thyroid 

 cornu 



Aryepiglotticus 



Arytenoideus 



Crico-arytenoideus 



lateralis 



Crico-arytenoideus 

 posticus 



Cricoid cartilage _ 



Hyoid bone 



Laryngeal pouch 



Right thyroid ala 

 (cut) 



The lateral crico-arytenoid muscle (Fig. 1554), of an elongated triangular 

 form, arises from the upper border of the lateral part of the cricoid and from the 

 ascending edge of the plate as far as the arytenoid joint. It also may have fibres 

 springing from the crico-thyroid 



membrane. It is inserted into the FIG. 1554. 



front of the muscular process. This 

 muscle is less well defined than the 

 posterior crico-thyroid, and may 

 be more or less fused with the 

 thyro-arytenoid, on the one hand, 

 and the crico-thyroid, on the other. 



Action. It pulls the muscular 

 process forward, thereby bringing 

 the vocal cord nearer to its fellow. 



The thyro-arytenoid mus- 

 cle (Fig. 1554) arises from the 

 inner surface of the thyroid, just 

 outside the entering angle, from 

 the level of the true cord to the 

 lower border. At the side it arises 

 from a part of the crico-thyroid 

 membrane, and may there be con- 

 tinuous with the lateral crico-ary- 

 tenoid. It runs backward and is 

 inserted into the upper surface of 

 the vocal process of the arytenoid 

 and into the antero-external sur- 

 face of that cartilage. It is convenient to speak of an internal and an external part, 

 but there is no separation between them. The internal portion (m. thyreoarytae- 

 noideus vocalis) is a prismatic mass, triangular on section (Fig. 1551), forming the 

 bulk of the true cord, with one of its angles against the ligament in the free edge. 

 Ludwig taught that fibres diverged from the body of this muscle to be inserted suc- 

 cessively into the ligamentous band of the vocal cord, which thus resembled the 

 tendon of a muscle receiving oblique fibres along its side. These were supposed to 

 modify its tension indefinitely by pulling upon it at various points. This view has 

 been denied by Luschka, and the point remains undecided. Jacobson 1 found on 



1 Archiv f. mikro. Anat., Bd. xxix., 1887. 



Thyro-arytenoideus 

 extern us 



Crico-thyroideus 

 (cut) 



Trachea 



Muscles of larynx, lateral view after partial removal of right 

 thyroid ala. 



