THE EXTERNAL STRUCTURE OF BIRDS. Lui) 
investment. It is beautifully adapted to harmonize with the 
rest of their organization, being extremely light, warm, and non- 
conducting. It thus serves most effectively to maintain that 
high temperature which distinguishes their class. 
Beasts are provided with hairs, but feathers are much more 
complicated and elaborate organs. They are, in fact, the most 
complicated of all the appendages of the skin which any 
animals possess. 
Whatever may be their modifications of size, colour, or tex- 
ture, they are all formed on one common plan. Each feather 
consists of a firm central axis, the base of which is the “ quill,” 
and the part above this the “rachis” or “ scapus” or “ shaft,” 
to which the web, vewillum or pogonium, is attached on either 
side. The “webs” of both sides of the “rachis,” taken to- 
gether, constitute the vane. The quill is implanted in the skin 
and has two apertures, one at either end. Into the lower 
—the umbilicus inferior—the soft vascular “ pulp” of the 
feather penetrates. The other aperture is called the wmbilicus 
superior. The “vane” consists, as before said, of the flat- 
tened expanded parts on both sides of the central axis, and each 
lateral portion of it (the fore or outer web, fig. 141, F, or the 
hind or inner web, H.V.) is made up of a number of elongated 
closely arranged lamine called ‘“ barbs; while from the 
margins of each barb much smaller processes project, called 
“barbules” or “ radii,’ and the sides of the barbules may 
also be furnished with still smaller processes or “ barbulets,” or 
barbicels, or hamuli, or hooklets. Not unfrequently a second shaft, 
called an “aftershaft,’ H.R., springs from the summit of the quill, 
and this is generally a miniature representation of the normal 
“shaft” with its “vane.” The large feathers of the wing 
and tail never have an aftershaft. They present a striking 
combination of the two generally opposite characters—strength 
and lightness—in a very high degree, as the barbules in- 
terlock and keep the whole structure remarkably firm and 
coherent. This kind of feather is called pennaceous. Certain 
feathers in which these parts are separate, and which also have 
long barbules, are very much looser in structure, and are called 
“ nlumes,” and their structure is termed plumulaceous—such as 
those of the Ostrich. 
Most Birds are provided with more or less ‘‘ down.” Down 
consists of very soft feathers, which may or may not have 
an aftershaft, and may have no rachis at all, the soft barbs 
