Fig. 3.

Vascular canals in a sheep femur (hierarchical Level 4), and winding of lamellar layers around the canals (osteons, hierarchical Level 5). (A) Resorption cavities and incomplete osteons within a juvenile sheep. (B) Less-abundant resorption canals and incomplete osteons in a mature sheep femur. Note the similarity between B and the “hatching” of osteonal canals in the human femur (from Hert et al. (Heřt et al., 1994) , with permission). (D) Spiral winding of co-oriented mineralized collagen fibrils around the central capillary of an osteon, from Wagermaier et al. (Wagermaier et al., 2006), with permission. The lamellar assembly in the whole osteon can be viewed as a series of concentric nested coils. Note the switch in handedness in the outermost layer. (E) Preceding work by Gebhardt (Gebhardt, 1906) in 1906, who empirically illustrated the stiffening effect of multi-layered coiling assemblies with varying pitch and handedness: either in tension, compression or torsion, there will always be a subset of lamellae that resist such deformation axially.