232 General Chauacters. BIRDS. General Characters. 
forming the true shaft of the featlier, is solid, and 
composed of a white spongy substance coated wth a 
horny sheatli. It tapers gradually to the extremity ; 
its lower surface exhibits a strong groove, whilst 
its upper horny surface is usually rather convex, 
smooth, and continuous at the base, with the tubular 
portion of the feather. At the point where the 
upper horny sheath wrapping round the sides of the 
stem forms the origin of the hollow quill, there is 
usually a second small stem, also furnished with webs. 
'I'his, which is called the plumule, is not found in all 
birds, and where it does occur, is always confined to 
the soft feathers forming the clothing of the body ; the 
quill feathers of the wings and tail being destitute of it. 
The sides of the shaft are occupied by the webs, 
composed, as already stated, of numerous flattened 
fibres or barbs. These are inclined towards the apex 
of the feather. They are usually concave in front and 
convex behind ; so that they fit together very closely, 
and their mutual adhesion is provided for by tlie 
agency of a series of minute secondary fibres, or 
harhules, which spring from their margins. These 
characters are not, however, common to all the barbs 
even of the same feather. The lower barbs are usually 
'!oft and disunited, forming the substance well known 
as down, and in many of the feathers clotliing the 
body this downy portion constitutes the principal part 
of the feather, the tip only being formed by a few stiff 
barbs. The accessory plumule, where it exists, is 
always of a downy nature, and in a few species of 
birds the whole plumage shows a tendency to acquire 
a similar structure. The skins of many birds, espe- 
cially of the aquatic tribes, are also clothed, beneath 
the ordinary covering of feathers, vdth a thick coat 
of down, which, altliough evidently analogous in its 
nature to the true feathers, yet exhibits certain pecu- 
liarities deserving of special mention. The down 
consists of a multitude of minute tubes inserted into 
the skin, from the extremity of each of which there 
arises a little tuft of soft, disunited filaments. These 
ma}^ evidently be regarded as the barbs of a shaftless 
feather, and they are furnished on each side with 
numerous minute fibres representing the harhules. 
In all birds the feathers are changed once or twice 
in the course of tlie year, the old feathers falling out 
by degrees, to bo replaced by new ones. This process 
is called moulting. In it, as in the first clothing of 
the bird with its feathers, these organs are formed in 
small tubes in the skin, lined by a duplicature of the 
epidermis. A peculiar fluid secretion is produced at 
the bottom of each tube ; this is soon inclosed in a 
delicate, conical, horny sheath, with its point directed 
outwards ; and within this sheath the formation of the 
feather goes on. As the latter increases in size, the 
point of its sheath is extended towards the surface of 
the skin, from which it finally issues, and then, burst- 
ing, allows the inclosed feather to make its escape. 
The portion of the formative fluid remaining in the 
qtiill after the feather has attained its maturity, is 
gradually dried up within the cavity, where it forms 
that peculiar membranous substance which every one 
must have observed in the quills destined for use 
as pens. 
In the bats, the only forms of mammals which are 
endowed with the power of flight, the function is per- 
formed, as has already been stated, by the agency of 
broad membranes, which, when they are extended 
by the elongated fingers, constitute admirable organs 
for aerial locomotion. In the bats, also, the tall is 
usually provided with a membrane of gi'eater or less 
extent, which is of great service to these creatures in 
directing their course through the air. In birds the 
same purposes are fulfilled in a very different manner, 
but still by an extension of the tegumentary appen- 
dages. The structure of the anterior member in a 
bird, as already described (see Plate 36), is very differ- 
ent from that of the same part in a bat. For all 
practical purposes, the region of the hand may be 
regarded as reduced to a single finger, so that the 
limb forms a single series of long joints placed end to 
end. But the feathers implanted in the skin of these 
members are of large size and firm texture, and so 
arranged that when the wing is extended they spread 
out lilce the rays of a fan, so as to expose a broad 
surface to the air ; Avhilst by the mode in which they 
mutually assist each other, and their own proper 
elasticity, their power of resistance is very considerable. 
At the same time, when the Aving is closed, they pack 
together into a comparatively small compass, and are 
thus no impediment to their owner in moving about 
upon the ground or in trees. 
As the number, form, and arrangement of the quill 
feathers of the wing are of considerable importance in 
the classification of birds, ornithologists have found it 
necessary to give them different names, according to 
the region of the wing upon which they are situated. 
The longest and strongest, and consequently those 
which have the most influence upon the power of flight 
possessed by any bird, are the feathers inserted upon 
the hand ; these are called primaries. They usually 
decrease in length from the outer margin of tlie wing, 
and in this case the wing is more or less pointed in its 
outline ; ^in other cases the longest feather is the fourth 
or fifth, when the apex of the wing becomes more or 
less rounded. Their number is usually nine or ten, and 
sometimes eleven. The name of secondaries is given 
to the feathers attached to the middle division of the 
anterior limb, corresponding with the fore-arm of man ; 
these are shorter and weaker than the primaries, and 
vary far more in their number. The teriiaries are the 
feathers attached to the arm. A few small quill fea- 
thers attached to the rudimentary thumb, form what is 
called the alula, or spurious wing, and the bases of all 
the quills are concealed by numerous large but com- 
paratively soft feathers, forming the wing-coverts, Avhich 
are distinguished as primary and secondary, according 
to their position. 
The quill feathers of the tail, like those of the Avings, 
are long and stiff ; they are furnished Avith muscles, by 
Avhich they can be spread out to catch the air or con- 
tracted Avithin a small compass, and b}' the motion of 
the tail itself they may be turned in various directions. 
Hence, from their serving in some sort as a rudder for 
the bird in its aerial coui'se, they have been tei'mcd 
rectrices; the quills of the Avings being also knoAvn as 
remiges, from their being the main instruments of pro- 
