52 THE MYOLOGY OF THE RAVEN. 



rings, ib., c, and by approximating them, relaxes the 

 membrane occupying the elliptical interspace. These 

 membranes, projecting on each side into or below the 

 termination of the air-tube, leave a narrow chink 

 between them, through which the air passes to and 

 from the lungs ; and when, in forcible expiration, the 

 membranes are put into a sufficient state of tension, 

 they vibrate, and the vocal air is driven along the 

 trachea through the upper larynx, where some modifi- 

 cation of sound may be made. The tongue of the 

 Parrot is more fleshy than in most birds. These 

 structures, concomitant with the single glottis and pair 

 of vocal folds in the lower or true larynx, relate to the 

 faculty, so remarkable in these singular birds, of imitat- 

 ing human speech." 



Other interesting variations in these structures are 

 to be found in certain Cranes, in the peculiar Snipe 

 Rhynckcea, in certain Gallinaceous birds, and still more 

 especially in many of the Anserine fowl. 



