AIR-TUBES AND LARYNX 



367 



The primary skeletal parts are a pair of lateral cartilages, 

 situated in the walls of the vestibule on either side of the glottis ; 

 these appear to have arisen phylogenetically by a modification 

 of the vestige of a branchial arch (possibly the sixth), as is 

 indicated by the innervation of this region by a branch of the 

 vagus. The most primitive form of this lateral cartilage has 

 probably been retained in the caducibranchiate Ellipsoglossa. In 

 various other Urodeles it becomes further developed in various 

 ways, chiefly by its anterior section being differentiated into an 

 arytenoid cartilage, while its posterior section gives rise to 

 the crico-tracheal skeleton (Fig. 273). The latter gradually be- 

 comes more and more closely adapted to the walls of the air- 

 passages, and eventually extends along the whole length of the 



D 



a 



Fi<;. 273. LARYNGEAL AND TRACHEAL SKELETON OF URODELES. A, Nectnrus 

 B, Siren lacertina ; C, Ainphiiima ; D, Salamandru maculoza. 



a, the lateral cartilages (arytenoids) on either side of the glottis ; a', ridge for 

 muscles ; co, constrictor of the larnyx ; Kb, the more definite tracheal 

 cartilaginous tracts in Amphiuma and Salamandra ; K IV , fourth branchial 

 arch, from which the dilator (d) of the trachea and larynx arises : it is 

 inserted into an aponeiirosis at H ; L, L', lungs; *, the representative of 

 the cricoid cartilage ; ft, cartilages of the trachea in Siren. 



trachea, its elements passing more or less completely around the 

 windpipe, so that in some higher forms they give rise to com- 

 pletely closed tracheal rings. 



The anterior end of the crico-tracheal skeleton in Urodeles 

 gives rise to a very simple ring-shaped cricoid cartilage, which, 

 like the rest of the larynx, is much more highly differentiated in 

 Anurans. In these the larynx is regulated by a well-developed 

 series of muscles, and is provided with vocal cords, the sound pro- 

 duced by which is often intensified by the presence of vocal sacs 

 developed from the floor of the mouth. The laryngo-tracheal 

 chamber lies between the posterior cornua of the hyoid (thyro- 

 hyals), with svhich it is connected by ligaments, and is supported 



