366 



COMPARATIVE ANATOMY 



Amphibians. The vestibule, or largngo-tracheal ckanibef, com- 

 municates with the pharynx on the one hand and with the lungs on 

 the other, and is supported by definite cartilages ; it is provided 

 with intrinsic (dilator and constrictor) and extrinsic muscles, the 

 former derived from pharyngeal muscles, and the latter from 

 trunk-muscles, as in all the higher forms. A definite trachea is 



FIG. 272. DIAGRAMMATIC TRANSVERSE SECTION ILLUSTRATING THE STRUCTURE 

 AND RELATIONS or THE DORSAL AND VENTRAL LARYNX. 



Aor, aorta : Cav.cr, cranial cavity ; Co, Co 1 , constrictor of the dorsal and ventral 

 larynx respectively ; Co.ph, constrictor of the pharynx ; Z>, dorsal larynx ; 

 Dil, Dil 1 , dilator; KB. branchial arch; Kpf.D, lumen of pharynx; Muc, 

 mucous membrane of pharynx ; SE, SE 1 , supporting elements ; * V, ventral 

 larynx ; Vay, vagus nerve. 



differentiated in Siren, Amphiuma, and the Gymnophiona only ; 

 it reaches a length of 4 to 5 or more centimetres, and its wall is 

 strengthened by a series of small, irregular cartilages, which usually 

 tend to unite into bands (Fig. 273) : only in the Gymnophiona, 

 however, do these bands begin to take on the form of half-rings, 

 and to surround the trachea more or less completely. 



