370 COMPARATIVE ANATOMY 



The cricoid is much more sharply differentiated than in 

 Amphibians, and may give off processes ; with it the arytenoids 

 are movably connected. 1 Dilator and sphincter muscles (Fig. 275) 

 are present much as in Urodeles, and as in Mammals, are supplied 

 by two branches of the vagus an anterior (corresponding to the 

 first branchial branch of Fishes) and a posterior (recurrent), the 

 homologue of that present in Urodeles and of the fourth branchial 

 branch. 



Except in Snakes, in which considerable reductions of the 

 hyoid apparatus occur (p. 117), a close connection obtains between 

 the latter and the larynx ; in Crocodiles and Chelonians, for 

 instance, the larynx is firmly embedded in a shallow depression 

 on the dorsal surface of the basi-hyoid (Fig. 85). Moreover, the 



FIG. 277. LARYNX OF MALE DUCK. A, external, and B, internal view. 



Br, bronchus ; S, pessulus, from which a lateral outgrowth (S, between b and b) 

 extends into the tympanum, thus dividing its aperture into the trachea into 

 two portions (b, b) ; the aperture is further diminished by the circular fold 

 of mucous membrane, SF ; T, the "tympanum"; Tr, trachea; t, thin 

 region in pessulus. 



larynx becomes shifted further forwards towards the nasal air- 

 passage. On the whole, however, no considerable advance of the 

 larynx as an organ of voice is seen as compared with that of 

 Amphibians. 



Birds. In Birds 'there are two larynges, an upper (anterior) 

 and a lower (posterior). 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 

 reduced and is incapable of producing sound. 



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



1 In some Reptiles (e.g. certain Lizards and Chelonians) a structure is 

 present which to some extent recalls the epiglottis of Mammals. 



