RESPIRATORY ORGANS. 253 



element of the hyoid forming a copula. In the higher mammals the 

 association of hyoid and larynx is not so intimate, even in the embryo, 

 but the thyreoid shows its double origin in its development. 



In the higher mammals the thyreoid cartilage forms a half ring on 

 the ventral side of the anterior end of the larynx, its anterior dorsal 

 angles being produced into cornua connected by ligament with the 

 hyoid (fig. 261). Dorsal to the thyreoid is the glottis with the aryte- 

 noids in its walls. Posterior to it is the ring-shaped cricoid, following 

 which is the trachea. Anterior to the glot'tis is a fold of the mucous 



FIG. 261. Dorsal and side views of larynx of opossum, Didelphys virginianus (Prince- 

 ton 1739) cartilages dotted, a, arytenoid; c, cricoid; e, epiglottis; g, glottis; h, hyoid; 

 t, trachea; th, thyreoid. 



membrane of the pharynx, the epiglottis, supported by an internal car- 

 tilage (possibly the fourth branchial arch) which articulates with the 

 anterior margin of the thyreoid. The epiglottis usually stands erect, 

 leaving the glottis open for respiration, but during deglutition it folds 

 back over the glottis, thus preventing the entrance of food into the 

 trachea. 



Internally the cavity of the larynx bears a vocal cord on either side. 

 These are folds of the mucous membrane, extending from the thyreoid 

 to the arytenoids, and by movements of these latter cartilages they can 

 be tightened or relaxed, thus altering the pitch of the note caused by 

 their vibration. Anterior to these cords is a pocket, the laryngeal 

 ventricle (sinus of Morgagni) on either side, small in most mammals, 

 but developed in the anthropoid apes to large vocal sacs (in some 

 there is a median vocal sac in addition), which act as resonators, 

 adding to the strength of the voice. 



