The epoxy component is flexible, but weak. The composite, however, is both rigid and strong. When a glass fiber breaks because of stress and a crack starts to form, the crack runs into glue before it reaches another fiber. The glue distorts and reduces the concentration of the stress, and the adjacent fibers consequently are not exposed to the same high stress. In effect, the epoxy glue acts to spread the stress over many fibers.
Bone is constructed in a similar manner.
Within bone, the collagen is laid down along the lines of stress. No crack can penetrate far into bone without encountering myriad hydroxyapatite crystals in a collagenous matrix. Bone is more rigid than collagen, just as fiberglass is more rigid than epoxy glue. The ends and interiors of long bones are composed of an open lattice of bone called spongy bone tissue or marrow. Within this lattice framework, most of the body's red blood cells are formed.