AIR-TUBES AND LARYNX 



285 



cartilages a paired arytenoid, and a ring-shaped cricoid (Figs. 76 

 and 231). No considerable advance in structure is seen ; there is 

 even a reduction noticeable as regards the musculature as compared 

 with the Anura. 



One point, however, must be specially noticed, viz., the close 

 connection which obtains between the larynx and the hyoidean 



FIG. 231. LARYNX or Phyllodactylus eiLropceus. (A, skeleton, and B, 

 musculature of larynx.) 



Ar, arytenoids ; Cc, cricoid ; S, anterior median process of cricoid ; S 1 , sphincter ; 

 D, dilator ; T, trachea ; Oe, basi-hyal. 



apparatus more particularly the dorsal surface of the basi-hyal. 

 In Crocodiles and Chelonians, for instance, the larynx is firmly em- 

 bedded in a shallow depression of the latter (Fig. 76). 



A well-developed trachea, supported by cartilages, is present in 

 all Reptiles ; but the cartilages are not in all cases fused together 

 to form complete rings. The walls of the bronchi are also usually 

 provided with cartilaginous supports. 



Birds. In Birds there are two larynges, an upper and a lower. 

 The former lies in the usual position behind the tongue on the floor 

 of the pharynx, and is plainly homologous with that of other 

 Vertebrates, though it has become rudimentary and is incapable of 

 producing sound. 



The lower larynx, or syrinx, is of much greater importance ; it is 

 usually situated at the junction of the trachea and bronchi, or more 

 seldom at the lower end of the trachea alone or on the bronchi 

 alone. It functions as the organ of voice, and appears first in, and 

 is restricted to, Birds. In the most usual form (broncho-tracheal 

 syrinx), there is a movable connection between the most anterior 

 bronchial rings, with which a complicated system of muscles is con- 

 nected ; these, by their contraction, cause a stretching or relaxing of 



