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Longitudinal section of the calamus of an almost grown contour feather of a Muscovy Duck (Cairina moschata) (HE): overview
After the formation of barb ridges has ceased, the intermediate layer of the dermal collar will form the keratinized wall of the calamus and the basilar layer will form a thin, distinct corneous layer, the outer pulp membrane. The calamus remains a tubular shaped cylinder with the pulp remaining inside. When longitudinal growth of the feather has completed, the pulp regresses. Where the pulp has been resorbed, pulp caps divide the air space into pulp cap cavities. These pulp caps consist of the remaining corneous part of the inner pulp membrane and the outer pulp membrane that lines the wall of the calamus.
At this picture, longitudinal growth of the feather has completed. The pulp has regressed almost completely, only a small part still remains in the apical zone of the dermal papilla and is covered by the inner pulp membrane. Pulp caps are clearly visible in the hollow shaft of the calamus.