86 GENERAL ORNITHOLOGY. 
them. Another feature is, that they are usualgg@Mdividually moved by subcutaneous muscles, 
of which there may be several to one feather, passing to be attached to the sheath of the tube, 
inside the skin, in which the stem is inserted. These muscles may be plainly seen under the 
skin of a goose, and every one has observed their operation when a hen shakes herself after a 
sand bath, or any bird erects its top-knot. 2. Down-feathers, plumule, are characterized by 
a downy structure throughout. They more or less completely invest the body, but are almost 
always hidden beneath the contour-feathers, like padding about the bases of the latter; occa- 
sionally they come to light, as in the fleecy ruff about the neck of the condor, and then usually 
replace contour-feathers ; they have an after-shaft, or none ; and sometimes no rhachis at all, 
the barbs then being sessile in a tuft at the end of the quill. They often stand in a regular quin- 
eunx (+!) between four contour-feathers. 3. Semiplumes, semiplume, may be said to unite 
the characters of the last two, possessing the pennaceous stem of the former, and the plumula- 
ceous vanes of the latter; they are with or without after-shaft. They stand among penne, as 
the plumule do, about the edges of patches of the former, or in parcels by themselves, but are 
always covered by contour-feathers. 4. Filoplumes, filoplume, or thread-feathers, have an 
extremely slender, almost invisible stem, not well distinguished into barrel and shaft, and 
usually no vane, unless a terminal tuft of barbs may be held for such. Long as they are, 
they are usually hidden by the contour-feathers, close to which they stand as accessories, 
one or more seeming to issue out of the very sacs in which the larger feathers are implanted. 
These are the nearest approach to hairs that birds have; they are very well shown on domestic 
poultry, being what a good cook finds it necessary to singe off after plucking a fowl for the 
table. 5. Certain down-feathers are remarkable for continuing to grow indefinitely, and with 
this unlimited growth is associated a continual breaking down of the ends of the barbs. Such 
plumule, from being always dusted over with dry, scurfy exfoliation, are called powder-down ; 
they may be entitled to rank as a fifth kind, or pulviplumes. They occur in the hawk, parrot, 
and gallinaceous tribes, and especially in the herons and their allies. They are always present 
in the latter, where they may be readily seen as at least two large patches of greasy or dusty, 
whitish feathers, atted over the hips and on the breast. The design is unknown. 
Feather Oil Gland. — Birds do not perspire, and cutaneous glands, corresponding to the 
sweat-glands and sebaceous follicles so common in Mammalia, are little known among them. 
But their ‘‘oil-can” is a kind of sebaceous follicle, which may be noticed here in connection 
with other tegumentary appendages. This is a two-lobed or rather heart-shaped gland, sad- 
dled upon the ‘‘ pope’s nose,” at the root of the tail, and hence sometimes called the wropygial 
(Lat. wopygium, rump), or rump-gland. If there be no single word to name it, it may be 
called the eleodochon (Gr. édavodoxos, elaiodochos, containing oil). It is composed of numerous 
slender tubes or follicles which secrete the greasy fluid, the ducts of which, uniting successively 
in larger tubes, finally open by one or more pores, commonly upon a little nipple-like elevation. 
Birds press out a drop of oil with the beak and dress the feathers with it, in the well-known 
operation called ‘‘ preening.” The gland is large and always present in aquatic birds, which 
have need of waterproof plumage; smaller in land-birds, as a rule, and wanting in some. The 
presence or absence of this singular structure, and whether or not it is surmounted by a particu- 
lar cirelet of feathers, distinguishes certain groups of birds, and has come to be made much use 
of in classification. 
Pterylography. 
Feathered Tracts and Unfeathered Spaces. — Excepting certain 
birds having obviously naked spaces, as about the head or feet, all would be taken to be 
fully feathered. So they are all covered with feathers, but it does not follow that feathers are 
everywhere implanted upon the skin. On the contrary, a uniform and continuous pterylosis 
is the rarest of all kinds of feathering; though such occurs, almost or quite perfectly, among 
