74 CORVID^E. 



extending from the upper portion of the cricoid cartilage 

 along the two branches of the arytenoid cartilages, upon 

 each outer edge of which they are inserted; and it is 

 opened by a pair of muscles, fig. 2, b, b, arising from the 

 lateral and posterior portions of the cricoid cartilage, the 

 fibres of which muscles passing over the pair of smaller 

 muscles, just described, are inserted upon the inner edge 

 of each arytenoid cartilage. The obvious use of these two 

 pair of muscles is to govern the size of the aperture. 



The tube of the windpipe is composed of two mem- 

 branes, enclosing between them numerous cartilaginous, or 

 bony rings, forming a cylinder more or less perfect from 

 end to end. Ossification appears to commence in these 

 rings at the front of the trachea, from which point the 

 bone gradually extends equally on both sides towards the 

 oesophagus as the bird increases in age ; in particular 

 parts, however, of the tracheae of some birds, the bony 

 rings are not entirely complete at any age. Various in- 

 equalities of size occur, and convolutions in different parts 

 of the same tube, in some species, producing, as might be 

 expected, a particular effect on the voice, to be hereafter 

 explained and figured with the species to which they 

 belong. The length of the tube also requires conside- 

 ration : thus shrill notes are produced by short tubes, and 

 vice versa; the first are possessed by the Singing Birds, 

 and the reverse by some of the Waders and Swimmers ; 

 but the diameter of the tube has also its influence, large 

 tubes producing notes low in the scale of tones, and 

 vice versa. The substance of the tube itself has also to be 

 considered, though some anomalies present themselves. 

 Those birds possessing strong and broad cartilages, or bony 

 rings, have monotonous and loud voices ; while the more 

 slender rings, with enlarged spaces between them, allow a 



