Review the component structures and their roles in the Primary Flight Feather.
Feather anatomy reveals a complex arrangement of Keratin-based structures that appear optimized for flying. Both the macroscopic shape of the feather and its microstructure contribute independently to its powerful ability to bear aerodynamic load. The proximal part of the feather's shaft, the Calamus, is circular, reflecting the sheath in which the feather developed. But the Distal part of the shaft, the Rachis, has a curved and ribbed rectangular cross-sectional shape that's better able to bear the stress of the bird's mass. The central shaft of the feather has a lengthy, uniform array of barbs distributed to either side. But a microscopic look reveals more intricate patterns. Between the Barbs, extending laterally from them, is a fine, double-layer meshwork of thousands of crisscrossing Barbules. These are systematically and repeatedly attached to each other by uniformly structured Hooklets that grab onto inversely shaped ridge on adjacent Barbules. Tje Barbules shape in cross section contributes to a structure that limits airflow through the feather during the bird's powerful down stroke. Yet the same structure allows air through the feather in the upstroke, thus increasing lift.
Pgs. 455-457 Fig. 13.24