Take a femur for example. Most everyone knows that long skeletal bones are hollow, with sp

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Take a femur for example. Most everyone knows that long skeletal bones are hollow, with spongy ends and hard shafts. The hard shaft is not simply solid material, like concrete; it's composed of many (hundreds? thousands?) tiny cylinders called osteons. The osteons have several layers; each is composed of a slanted spiral of collagen fibers (elastic) with the spaces filled up by minerals (the hard, brittle part). In adjacent layers, the slanted collagen spirals are oriented _in diffent directions_, so that each osteon is very resistant to stress coming from a single direction. The hard shaft is made up of osteons stuck together with more minerals and connective tissue (like the collagen, but not as elastic). The construction is remarkably like a fiberglass/resin or graphite/resin composite, but each of the fibers is microstructured to be flexible, yet stress resistant! The hard part of the bone (shaft) is a tube of this stuff. The ends are thin shells of this, but full of spongy bone. The inner, spongy part has many stuctures in it called trabeculae. These are similar to (and derived from) osteons, and perform a "guy wire" function. They are located along tension stress lines in the bone (no fooling- I saw a diagram of predicted stress lines next to an actual femur cross section- remarkable correlation). The trabeculae act like cables to, e.g., keep the head of the femur from breaking off. (It's at an angle at the top of the shaft, you know.) So, a bone is composed of a hard, elastic shell which is itself composed of myriad tiny cylinders all built in such a way as to resist stress from all directions simultaneously. If that weren't enough, guy wires inside the hollow center transfer tension stress to the strong shaft. A remarkable piece of engineering! Almost makes one believe in divine guidance!

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