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 arca. 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 
