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The nature of feather construction
Feather Anatomy 101
Part of vane
with pennaceous barbs
The shaft or rachis is that portion of the feather that the barbs are attached to. It is flattened on the sides that support the barbs. It differs from the quill by being roughly rectangular in cross section. Internally it is not hollow, but rather is filled with a pithy material that contains air cells.
The barbs or rami (singular: ramus) come off the flattened sides of the shaft more toward the anterior (face) surface of the feather and in parallel rows generally opposing one another. They point outward and toward the tip of the feather. They are somewhat ovoid in cross-section (thinner side to side, wider front to back), broader near their attachment to the rachis, flattening and narrowing as they approach the tip. Barbs, like the shaft, are filled with a pithy material containing air cells. A feather may have only a couple of dozen barbs or several hundred.Barbules or radii (singular: radius) extend out from either side of the barbs. From the base to about half way to their tip, they are ribbon-like (the basal lamella). The distal half is more whip-like (the pennulum.) The barbules on the distal (upper) edge of a barb extend outward almost perpendicular to the barb. The barbules on the proximal (lower) edge of a barb lay more parallel with the barb. This is readily visible with a peacock upper tail covert feather's barbs, commonly referred to as "herl". (Herl according to dictionary definition is a barb or barbs of a feather, originating from the Middle English harle or herle which referred to fiber, hair of flax, or hemp.) Barbules extend out from a barb more proximal to the anterior (face) surface similar to barbs on a shaft. Again note the appearance of the peacock upper tail covert feather. When viewed from the anterior surface of the feather, the brightly colored eye is more dominate because the barbules (which often provide the majority of a feather's color) are attached closer to the anterior edge of the barb. When viewed from the opposing surface, the flat, rather dull color is due to the dominance of the color of the edges of the barbs as well as the location and physical shape, and in turn, light reflectance of the barbules.
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