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 glottis is a fold of the mucous 
Fic. 261.—Dorsal and side views of larynx of opossum, Didelphys vir ginianus (Prince- 
ton 1739) cartilages dotted. a, arytenoid; ¢, cricoid; e, epiglottis; g, glottis; 4, 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. 
