70 CORVID^E. 



before mentioned, and projecting forwards in two narrow 

 and thin parallel processes over two-thirds of the orifice 

 formed by the curved lateral portions of the cartilage un- 

 derneath : each parallel process forming a slight groove 

 on its superior surface by their edges also curving upwards. 



The glottis is closed by a pair of muscles, fig. 3, #, a, 

 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, 5, 5, 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 

 resophagus 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 

 inequalities 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 



