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 r.eptiles. 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. 



P h 



or 



FIG. 259. FIG. 260. 



FIG. 259. Laryngeal apparatus of Chelone, after Goppert. a, arytenoid; b 1 - 2 , first and 

 second branchial arches; cr, cricoid; d, dilator laryngis muscle; g, glottis; h, hyoid; he, 

 hyoid cornua; sph, sphincter laryngis; tr, trachea; cartilage dotted, bone black. 



FIG. 260. Ventral and side views of monotreme larynx, after Gegenbaur. c, cri- 

 coid; h, hyoid; th, thyreoid; tr, 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 



