THE EXTERNAL STRUCTURE OF BIRDS. 153 
These true tail-feathers, or rectrices, have their bases covered 
and protected both above and below by feathers, which take 
their origin in the trunk, and thence project backwards over 
the quills and bases of the rectrices. They are generally small 
and softish feathers (especially those beneath the tail), which 
tend to complete the conical form of the hinder end of the 
body. They are called tectrices, 7. e. covering feathers or coverts, 
and those above and below the rectrices are respectively dis- 
tinguished as the upper and lower tail-coverts. They are 
always present, but may be very short. The upper ones 
generally extend less backwards than do the under tail-coverts. 
They may, however, take on a very great development, as, ¢.9., 
in the Peacock, the gorgeous so-called tail of which is not even 
a true feather-tail, but is formed only of elongated upper tail- 
coverts (tectrices superiores), Which when expanded are held up 
and supported by the true (though in appearance insignificant) 
rectrices beneath them. 
We have now completed our survey of the skin and its 
appendages—the exoskeleton—of the axial part of the body— 
the head, trunk, and tail. 
It remains to consider the same system of parts as developed 
on the two pairs of limbs which constitute the appendicular 
portion of the bird’s bodily frame, being appendages attached 
to its axial portion. 
Tue Limes. 
We will first consider the anterior pair of limbs called 
pectoral, from their situation beside the chest. These ‘ pectoral 
appendages ” are, of course, the wings. 
The Wings.—All birds possess wings, though in some they 
are very small. Such is the case in the Emen and the Casso- 
wary, but above all in the Apteryx. When we speak of the | 
wing of a Bird, we have mainly the feathers of the wing present 
to our imagination. This is reasonable, for the feathers are 
not only the most conspicuous objects, but the direct agents in 
effecting flight. Nevertheless, the solid structure in which 
they are implanted is of course no less necessary, it forms the 
basis and solid part of the wing. This solid structure is really 
the arm of the bird, and its distinct component parts corre- 
spond with and answer to our own. There is a part which 
answers to our upper arm and—in all existing birds—a part 

