The Feathers

The exoskeleton of Birds consists almost entirely of epidermic structures in the form of horny sheaths, scales, plates, or feathers. No bird possesses dermal ossifications, unless the spurs which are developed upon the legs and wings of some species may be regarded as such.

The feathers are of various kinds. Those which exhibit the most complicated structure are called pennae, or contour feathers, because they lie on the surface and determine the contour of the body. In every penna the following parts are to be distinguished: A main stem (scapus) forming the axis of the feather, and divided into a proximal hollow cylinder, partly imbedded in a sac of the derm, called the calamus, or quill; and a distal vexillum, or vane, consisting of a four-sided solid shaft, the rachis, which extends to the extremity of the feather, and bears a number of lateral processes, the barbs. The calamus has an inferior aperture (umbilicus inferior), into which the vascular pulp penetrates; and a superior aperture (umbilicus superior) situated on the under-surface of the feather at the junction of the calamus with the scapus. The barbs are narrow plates, tapering to points at their free ends, and attached by their bases on each side of the rachis. The edges of these barbs are directed upward and downward, when the vexillum of the feather is horizontal. The interstices between the barbs are filled up by the barbules; pointed processes, which stand in the same relation to the barbs, as the barbs do to the rachis. The barbules themselves may be laterally serrated and terminated by little hooks, which interlock with the hooks of the opposed barbules. In very many birds each quill bears two vexilla; the second, called the aftershaft (hyporachis) being attached on the under side of the first close to the superior umbilicus. The aftershaft is generally much smaller than the chief vexillum; but in some birds, as the Casuaridae, the two are of equal size, or nearly so. Muscles pass from the adjacent integument to the feather sac, and by their contraction erect the feather. The other kinds of feathers differ from the pennae, in having the barbs soft and free from one another, when they constitute pennoplumae, or plumulae (down), according as the scapus is much or little developed. When the scapus is very long, and the vexillum very small or rudimentary, the feather is termed a filopluma.

The contour feathers are distributed evenly over the body only in a few birds, as the Ratitoe, the Penguins, and some others. Generally, the pennae are arranged in definitely circumscribed patches or bands, between which the integument is either bare, or covered only with down. These series of contour feathers are termed Pterylae, and their interspaces, apteria.

In some birds, such as the Herons, plumulae of a peculiar kind, the summits of which break off into a fine dust, or powder, as fast as they are formed, are developed upon certain portions of the integument, which are termed powder down patches.

The integument of birds is, for the most part, devoid of glands; but many birds have a peculiar sebaceous gland developed in the integument which covers the coccyx. This uropygial gland secretes an oily fluid, which the bird spreads over its feathers by the operation of "preening." The excretion passes out by one or two apertures, commonly situated upon an elevation, which may or may not be provided with a special circlet of feathers.

In various birds (e. g., the Turkey) the integument about the head and neck develops highly-vascular and sometimes erectile processes (combs, wattles).