BIRDS. 203 
enlarged by a salient process in its middle. It is originally composed of 
five pieces: a middle one, of which this salient lamina makes a part; two 
triangular, anterior, lateral ones, for the articulations of the ribs, and two 
posterior, which are lateral and bifurcated, to increase its surface. The 
greater or less degree of the ossification of the notches of these last, and 
the interval they leave between them and the principal bone, determine 
more or less the flight in birds. The diurnal Birds of prey, the Swallows 
and the Humming-birds, lose, as they grow old, all traces of these un- 
ossified spaces. 
The fourchette produced by the junction of the two clavicles, and the 
two powerful stretchers formed by the coracoid apophyses, keep the 
shoulders apart, notwithstanding the efforts requisite for flight, that act 
in an opposite direction; the greater the power of flight, the more open 
and vigorous is the fourchette. The wing, supported by the humerus, 
fore-arm, and by the hand, which is elongated, and has one finger and the 
vestiges of two others, is furnished throughout its length with a range of 
elastic quills, which greatly extends the surface that resists the air. Those 
quills which belong to the hand are termed primaries, and there are always 
ten of them; those attached to the fore-arm are called secondaries, but 
their number varies; weaker feathers appended to the humerus are called 
scapulars ; the bone, which is analogous to the thumb, is also furnished 
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 Gallinace eighteen. 
The legs have a femur, a tibia, and a fibula, which are connected with 
the femur by an articulation with a 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 Martins 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 the external 
toe, which has five. 
Birds are generally covered with feathers, a kind of tegument best 
adapted for defending them from the rapid variations of temperature to 
which their movements expose them. The air cavities which occupy the 
interior of their body, and hold in the bones, the place of the marrow, 
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 dilatation 
