VERTEBRATA: AVES. 591 



useless as organs of touch ; and the posterior limbs are covered 

 with horny scales or feathers. The bill, certainly, officiates as 

 an organ of touch, but it cannot possess any acute sensibility, 

 as in most birds it is encased in a rigid horny sheath. In 

 some birds, however, such as the common Duck, the texture 

 of the bill is moderately soft, and it is richly supplied with 

 filaments of the fifth nerve ; so that in these cases the bill 

 doubtless constitutes a tolerably efficient tactile organ. The 

 " cere," too, or the fleshy scale found at the base of the bill in 

 some birds, is in all probability also used as a tactile organ. 



The last anatomical peculiarity of Birds which requires 

 notice is the peculiar apparatus known as the " inferior larynx," 

 or " syrinx," by which the song of the singing birds is con- 

 ditioned. " The air-passages of birds commence by a simple 

 superior larynx, from which a long trachea extends to the an- 

 terior aperture of the thorax, where it divides into the two 

 bronchi, one for each lung. At the place of its division, there 

 exists in most birds a complicated mechanism of bones and 

 cartilages, moved by appropriate muscles, and constituting the 

 true organ of voice ; this part is termed the inferior larynx " 

 (Owen). The inferior larynx may be developed from the 

 trachea only, before the division of this tube into the bronchi ; 

 or, it may be developed wholly from the bronchi; or, last- 

 ly, and more commonly, it may be developed at the junction 

 of the trachea and bronchi and out of both. The structure 

 of the vocal apparatus is extremely complicated, and there is 

 no necessity for entering upon it here. It is to be remem- 

 bered, however, that those modifications of the voice which 

 constitute the song of birds, are produced in a special and 

 complex cavity placed at, or near, the point where the trachea 

 divides into the two bronchi, and not in a true larynx situated 

 at the summit of the windpipe. The syrinx is wanting in a 

 few birds (e.g., the RatilcB). Lastly, the trachea of birds is 

 always of considerable proportionate length, and it is often 

 twisted or dilated at intervals, this structure, doubtless, having 

 something to do with the production of vocal sounds. 



Before passing on to the consideration of the divisions of 

 Birds, a few words may be said as to the migration of birds. 

 In temperate and cold climates comparatively few birds remain 

 constantly in the same region in which they were hatched. 

 Those which do so remain, are called " permanent birds " (aves 

 manentes). Other birds, such as the Woodpeckers, wander 

 about from place to place, without having any fixed direction. 

 These are called " wandering birds " (aves erraticcz), and their 

 irregular movements are chiefly conditioned by the scarcity or 



