204 BIRDS. 



of the chest. To each rib is annexed a small bone, which soon becomes 

 soldered to it, and is directed obliquely towards the next one, thereby 

 giving additional solidity to the thorax. 



The eye is so constructed, in Birds, as to enable them to distinguish, 

 with equal facility, objects at a distance, or near them; a vascular and 

 plaited membrane, which stretches from the bottom of the globe to the 

 edge of the crystallinelens, probably assists in effecting this, by displacing 

 that lens. The anterior surface of the ball is also strengthened by a 

 circle of bony pieces, and besides the two ordinary eye-lids, there is always 

 a third one placed at the internal angle, which, by a remarkable muscular 

 apparatus, can be drawn over the eye like a curtain. The cornea is very 

 convex, but the crystalline is very 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, how- 

 ever, does not project like that of Quadrupeds. 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 bill, usually has but 

 three cartilaginous ossa turbinata, which vary in complication ; although 

 there are no sinuses in the wall of the cranium, yet it is extremely sen- 

 sible. The breadth of 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 support- 

 ed by a bone articulated with the hyoid; in most Birds this organ is not 

 very delicate. 



The feathers, as well as the quills, which only differ in size, are com- 

 posed 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 bill is almost always corneous, and has but 

 little sensibility, 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 

 differs in its colours from that of summer ; in the greater number, the 

 female differs from the male in an inferior vividness of tints, and when 



