454 



THE RESPIRATORY APPARATUS IN MAMMALIA. 



Fig. 229 



POSTERO-LATERAL VIEW OF THE 

 LARYNX 



t, Epiglottis ; 2, Arytenoid cartilages ; 

 3, Thyroid cartilage ; 4, Arytenoi- 



The crico-thyroideus shortens the larynx, in bringing together the two 

 cartilages into which it is inserted. 



POSTERIOR CRICO-ARYTENOIDEUS (Fig. 

 229, 7) This is the most powerful muscle 

 in this region. Its fibres are directed for- 

 wards and outwards, and originate from 

 the bezel of the cricoid, which they cover, 

 and from the median crest of that part. 

 They all converge, in becoming more or less 

 tendinous, towards the posterior tubercle of 

 the arytenoid cartilage, on which they ter- 

 minate. Covered by the oesophagus and 

 the crico-pharyngeal muscular band, this 

 muscle is separated from that of the oppo- 

 site side by the median crest of the cricoid 

 bezel. 



The posterior crico-arytenoid muscles 

 dilate the entrance to the larynx, as well as 

 the glottis, in causing the arytenoid car- 

 tilages to rotate or swing on the cricoid 

 cartilage, and in separating them from one 

 another by their anterior and inferior bor- 

 ders. They act as a lever of the first order. 



LATERAL CRICO-ARYTENOIDEUS (Fig. 229, 

 deus muscle ; 5, Crico-arytenoideus 5). __ A triangular muscle, smaller than 



vSi^^^^Fs! the p recedin g' s;tuated between the % roid 



Crico-thyroideus; 9, Ligament be- and arytenoid cartilages, and formed of 

 tween the cricoid cartilage and fasciculi longer in front than behind ; these 

 first ring of trachea, 10; 11, In- originate on the side of the anterior border 

 fero-posterior extremities of crico- f ^ cricoid cartilage, and are directed 

 thyroid cartilages. , . , . , ., 



upwards to terminate outside the posterior 



crico-arytenoideus, on the tubercle of the arytenoid cartilage. 



It is a direct antagonist of the last muscle, and, consequently, a con- 

 strictor of the larynx. 



THYRO ARYTENOIDEUS (Fig. 229, 6). Lodged at the inner face of the 

 thyroid ala, this muscle comprises two fasciculi, separated by the ventricle 

 of the glottis. 



The anterior fasciculus is a long and pale band, originating on the 

 internal surface of the ala of the thyroid cartilage, near its receding angle, 

 and ascending to the arytenoid cartilage, bending round its external face to 

 join, on the median line, the analogous fasciculus from the opposite side, 

 mixing its fibres with those of the arytenoideus. By its inner face it covers 

 the superior vocal cord and the laryngeal mucous membrane. 



The posterior fasciculus, wider than the anterior, comports itself in a 

 somewhat similar manner. It commences from behind the same point, and 

 terminates on the external crest of the arytenoid cartilage ; but its most 

 anterior fibres pass over this crest and join the arytenoid muscle. Its 

 internal face corresponds to the vocal cord, and its posterior border is con- 

 founded with the fibres of the lateral crico-arytenoideus. 



Sometimes it happens that these fasciculi are not distinct from each 

 other, and exist only as a wide muscular band applied against the ventricle 

 of the glottis. 



It is surmised that this muscle is a constrictor of the larynx. Its 



