2 STRUCTURE OF FEATHERS. 
plates in it, eight muscles on it, and a peculiar vascular organ inside; two 
larynges, or “ Adam’s-apples”; two bronchi; two lungs, perforated to send 
air into various airsacs and even the inside of bones; four-chambered heart, 
with perfect double blood-circulation ; tongue with several bones; two or 
three stomachs; one liver, forked to receive the heart in its cleft; gall- 
bladder or none; more or less diffuse pancreas, or “sweetbread” ; a spleen; 
intestines of much the same size throughout; cceca, or none; two lobulated, 
fixed kidneys ; two testicles fixed in the small of the back, and subject to 
periodical enlargement and decrease; one functional ovary and oviduct ; 
outlets of these last three organs in an enlargement at end of intestine, and 
their products, with refuse of digestion, all discharged through a common 
orifice. But of all these, and other characters, that come under the head 
of description rather than of definition, one is peculiarly characteristic of 
birds; for every bird has FEATHERS, and no other animal has feathers. 
Naturally, then, we look with special interest upon 
FEATHERS : 
§ 3. a. Tuerr Structure. A perfect feather consists of a main stem, 
or scape (scapus; pl. 1, fig. 7, ad), and a supplementary stem or after- 
shaft (hyporhachis; pl. 1, fig. 7, 6), each bearing two webs or vanes (vex- 
illum, pl. vewilla; pl. 1, fig. 7,¢), one on either side. The scape is divided 
into two parts; one, the tube or barrel, or “quill” proper (calamus; pl. 1, 
fig. 7, d) is hard, horny, hollow, cylindrical and semitransparent ; one end 
tapers to be inserted into the skin; the other ends, at a point marked by 
alittle pit (wmbilicus), in the shaft (rhachis), or second part of the stem; 
the rhachis is squarish, and tapers to a point; is less horny, is opaque, and 
filled with white pith; it alone bears the vexilla. The after-shaft has the 
same structure, and likewise bears vexilla; it springs from the stem, at junc- 
tion of calamus and rhachis, close by the umbilicus. It is generally very 
small compared with the rest of the feather; but in a few birds is quite as 
large; it is wanting in many; and is never developed on the principal wing 
and tail feathers. The vane consists of a series of appressed, flat, narrowly 
lance-shaped or linear laminz, set obliquely on the rhachis, and divarica- - 
ting outward from it at a varying angle; each lamina is called a barb - 
(barba; pl. 1, fig. 6, a, a). Now just as the rhachis bears barbs, so does 
each barb bear its vanes (barbules; pl. 1, fig. 6, 6, 6, c) ; it is these last 
that make a vane truly a wed, that is, they connect the barbs together, so 
that some force is required to pull them apart. They are to the barbs ex- 
actly what the barbs are to the shaft, and are similarly given off on both 
sides of the barbs, from the upper edge of the latter. They are variously 
shaped, but generally flat sideways, with upper and lower border at base, 
rapidly tapering to a slender thread-like end; and are long enough to reach 
over several barbules of the next barb, crossing the latter obliquely. All 
the foregoing structures are seen with the eye or a simple pocket lens, but 
the next two require a microscope; they are barbicels (or cilia; pl. 1, fig. 
