Figure 5.

A schematic view of the three major structural components of the feather rachis. (a) (i) superficial layers of *fibres, the ultimate size-class in the hierarchy of feather keratin filaments (approx. 6 µm diameter), wound circumferentially round the rachis. (ii) The majority of the fibres extending parallel to the rachidial axis and through the depth of the cortex. Part of the section is peeled back to show why the fibres and even megafibrils are not usually recognized in histological sectioning, but rather only fibrils lower down the hierarchy (based on the electronic supplementary material, figure S2c). Any longitudinal section along the line of the arrows or at any point along the height of the fibre other than at the fibre surface (arrowheads) will fail to show the fibre. (iii) It shows the medulloid pith comprising gas-filled polyhedral structures (based on SEM images, electronic supplementary material, figures S5 and S6). Inset, part of a steel rebar with nodes, used in engineering technology to reinforce high-rise structures, analogous to rachidial fibres. (b) Schematic cross section of fibres and biodegraded ‘matrix’: (i) fibres; (ii) residual cytosol of keratinocytes presumably housing effete organelles and perhaps cytoskeletal elements—all degraded along with corneous envelope; (iii) interdigitating plasma membrane of the original keratinocytes with associated corneous envelope proteins. (c) A schematic three-dimensional cross section of the rachis showing approximate thickness (based on SEMs) of the three keratin layers comprising, (i) circumferential and (ii) longitudinal fibres of the cortex and (iii) polyhedra of medulloid pith. Asterisk denotes homologous with syncitial barbules.