RESPIRATORY SYSTEM OF BIRDS. 



225 



state of tension, they vibrate, and the vocal air is driven along 

 the trachea through the tipper larynx, where some modification 

 of sound may be made. The tongue of the Parrot is more fleshy 



106 



Lower larynx : A, right side ; B, left side. Mergus serrator. cccxx. 



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 207 



in these singular birds, of imitating 

 human speech. 



In the males of the Mergansers 

 and of most Ducks a certain number 

 of the terminal rings of the trachea 

 are welded together and expanded 

 into an irregular bony case, divided 

 into two unequal cavities. In the 

 Mergus serrator , fig. 106, the broad 

 ' pessulus,'/, leaves a passage at its 

 upper part, h, by Avhich the air from 

 the right bronchus, y, can pass to and 

 from the trachea, e : part of the outer 

 wall of the right laryngeal chamber 

 is formed by membrane, li : tliis 

 chamber is extended by the osseous 

 cavity, g. A similar but somewhat 

 more complex lower larynx exists in the male Anas clangula. 

 These modifications relate to the power rather than to the variety 

 of the voice. 



VOL. II. Q 



Lower larynx. Mergus Merganser, cccxx. 



