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i Wb wh AVES. 219 
with what are termed bastard quills. Along the base of the 
quills is a range of feathers named coverts. 
The bony tail is very short, but has a range of large quills, 
which, when spread out, assist in’ supporting the bird ; they 
are generally twelve in number, sometimes fourteen, and in 
the Gallinacez eighteen. 
The legs have a femur, a tibia and a fibula, which are con- 
nected with the femur by an articulation witha spring, which 
keeps up the extension without any effort on the part of the 
muscles. The tarsus and metatarsus are represented by one 
single bone, terminating below in three pullies. 
Most commonly there are three toes before, and a thumb 
behind ; the latter being sometimes deficient. In the Mar- 
tins it is directed forwards. In the Climbers, on the contrary, 
the external toe and the thumb are directed backwards. 
The number of articulations increases in each toe, commencing 
with the thumb, which has two, and ending with ie exter- 
.. nal toe, which has five. 
Birds are generally covered with feathers, a kind of tegu- 
ment best adapted for defending them from the rapid varia- 
tions of temperature to which their movements expose them. 
The air cavities which occupy the interior of their body, and 
even supersede the marrow in the bones, increase their 
' specific lightness. The sternal, as well as the vertebral portion 
of the ribs is ossified, in order to give more power to the di- 
“Jatation of the chest. To each rib is annexed a small bone, 
which soon becomes sgldered 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 distinguish, with 
equal facility, objects at a distance, or in its immediate vicin- 
~ ity; a vascular and plaited membrane, which stretches from 
the bottom of the globe to the edge of the crystalline, proba- 
bly assists in effecting this, by displacing that lens. ‘The ante- 
rior 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 remark- 
