8 



CLASS AVES. 



the upper side, and with a callous skin under- 

 neath ; this sense must be but little efficacious in 

 birds. 



The feathers fall sometimes twice a year. In some 

 species, the winter plumage differs from that of sum- 

 mer. In general, the female differs from the male, 

 by colours less bright, and the young of both sexes 

 resemble the female. When the adult male and 

 female are of the same colour, the young have a dress 

 peculiar to themselves. 



The brain of birds has the same character as that 

 of other vertebrated oviparous animals ; but is dis- 

 tinguished by a size in proportion very considerable, 

 often exceeding that of the same organ in the mam- 

 malia. It is principally to tubercles, analogous to the 

 corpora striata, that the volume is referrible, and 

 not to the hemispheres, which are very narrow, and 

 without circumvolutions. The cerebellum is largej 

 almost destitute of lateral lobes ; and almost entirely 

 formed by the vermiform process. 



The trachea of birds has its annulations entire ; at 

 its bifurcation is a glottis, generally furnished with 

 peculiar muscles, and named the lower larynx : it is 

 there that is formed the voice of birds ; the enor- 

 mous volume of air contained in the air-vessels con- 

 tributes to the force of their voice, and the trachea, 

 by its various form and movements, to the modifica- 

 tion of the voice. The upper larynx, very simple, 

 has but little to do with this. 



The face, or upper beak of birds, formed princi- 

 pally by the intermaxillaries, is prolonged backwards 



