RAVEN. 71 



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 pos- 

 terior portions of the cricoid cartilage, the fibres of which 

 muscles passing over the pair of smaller muscles, just de- 

 scribed, 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 membranes, 

 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 a?sophagus as the 

 bird increases in age ; in particular parts, however, of the 

 tracheae of some birds, the bony rings are not entirely com- 

 plete at any age. Various inequalities of size occur, and 

 convolutions in diflferent parts of the same tube, in some 

 species, producing, as might be expected, a particular efl^ect 

 on the voice, to be hereafter explained and figured with the 

 species to which they belong. The length of the tube also 

 requires consideration : 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 

 Swinnmers ; but the diameter of the tube has also its in- 

 fluence, 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 

 freedom of motion, producing a corresponding variety in the 

 scale of tone. 



