220 



AVES. 



able muscular apparatus^, can be drawn over the eye like a 

 curtain. The cornea is very convex, but the crystalline is 

 flat, and the vitreous humour small. 



The ear has but a single small bone, formed of one -branch 

 that adheres to the tympanum, and of another terminating in 

 a plate that rests upon the fenestra ovalis ; the cochlea is a 

 slightly arcuated cone ; but the semi-circular canals are 

 large, and lodged in a part of the cranium where they are 

 completely surrounded by air cavities, which communicate 

 with the area. Nocturnal Birds alone have a long external 

 conch, which, however, does not project like that of Quadru- 

 peds. The external meatus is generally covered with feathers, 

 whose barbs are more fringed than the others. 



The organ of smell, concealed in the base of the beak, usu- 

 ally has but three cartilaginous ossa turbinata, which vary in 

 complication ; although there are no sinuses within the pari- 

 etes of the cranium, it is extremely sensible. The breadth of j 

 the osseous openings of the nostrils determines the strength of 

 the beak ; and the cartilages, membranes, feathers and other 

 teguments which narrow down those apertures, influence the 

 power of smell, and the nature of the food. 



There is but little muscular substance in the tongue, which 

 is supported by a bone articulated with the hyoid ; in most 

 Birds this organ is not very delicate. 



The feathers, as well as the quills, whicli only differ in size, 

 are composed of a stem, hollow at base, and of barbs, which 

 are themselves furnished with smaller ones ; their tissue, lustre, 

 strength, and general form vary infinitely. The touch must 

 be feeble in all such parts as are covered with them, and as 

 the beak is almost always corneous, and has but little sensi- 

 bility, and the toes are invested with scales above, and a callous 

 skin underneath, that sense can have but little activity in this 

 class of animals. 



Birds moult twice a year. In certain species, the winter 

 plumage diflers in its colours from that of summer; in the 

 greater number, the female differs from the male in an infe- 

 rior vividness of tints, and when this is the case, the young of 



