252 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
the margins of the glottis. These may be tightened or relaxed, and 
by their vibration of their edges under influence of the breath the 
voice is produced. 
The larynx is scarcely more developed in reptiles. The cricoid is usually an 
incomplete ring, to which the arytenoids are attached, and the whole is placed just 
ventral to the median part of the hyoid, with which it is closely associated (fig. 259). 
In several reptiles there is a fold of the mucous membrane just in front of the glottis 
which is supposed to represent the beginnings of an epiglottis (infra), while in 
geckos and chameleons a pair of folds, running dorso-ventrally in the larynx, 
serve as vocal cords. The larynx is also rudimentary in the birds, its place as a 
vocal organ being taken by the syrinx to be described below, in connexion with the 
trachea. The arytenoids are frequently ossified in birds. 
Fic. 259. Fic. 260. 
Fic. 259.—Laryngeal apparatus of Chelone, after Goppert. a, arytenoid; b'~, first and 
second branchial arches; cr, cricoid; d, dilator laryngis muscle; g, glottis; , hyoid; he, 
hyoid cornua; sph, sphincter laryngis; tr, trachea; cartilage dotted, bone black. 
Fic. 260.—Ventral and side views of monotreme larynx, after Gegenbaur. c, cri- 
coid; h, hyoid; th, thyreoid; ér, trachea. : 
In the mammals the larynx reaches its highest development. Its 
framework is formed by the arytenoid and cricoid cartilages, homol- 
ogous with those of the lower groups, and in addition, a thyreoid 
cartilage (or cartilages) on the dorsal side anterior to the arytenoids 
and cricoids. The origin of the thyreoid is best seen in the monotremes 
where the hyoid apparatus enters into close relations with the larynx 
(fig. 260), while the second and third branchial cartilages form two 
plates, the lateral elements of the thyreoid on either side, the median 
