Anatomies: A Cultural History of the Human Body by Hugh Aldersey-Williams Page A

basics, so I turn not to an osteologist but to a structural engineer. Chris Burgoyne is a reader in concrete structures at the University of Cambridge, who has also made studies of the mechanics of bone. Like a proper engineer, he explains things best with the aid of pencil and paper, drawing simple diagrams of lines of force at lightning speed as he speaks. There are three fundamental types of lever, and the body incorporates all three. The first type has the fulcrum – the pivot point – placed between the load to be lifted and a downward applied force, like a seesaw; the other types put the fulcrum at one end of the lever, either with the force at the other end lifting an intermediate load, or with a force in the middle lifting a load at the end. When you lift your finger, you do so using muscles in your arm well above the pivot point of your knuckle. This is the seesaw case: the weight of the finger is on the other side of the fulcrum from the muscular force. Now, use your biceps to lift the length of your arm. This time, the fulcrum is at the shoulder, and the muscle applying the force is positioned between this and the centre of gravity of the arm being lifted. Finally, stand on tiptoe. Now, the upward force is provided by the Achilles tendon and muscles of the leg, the fulcrum is where the toes hinge with the rest of the foot, and the weight of the body falls between the two.
    As you will realize from your aching muscles – you may rest now – the bones are not a complete structural framework. They are one complement of what is known as the musculoskeletal system. Any functional structure must have parts that are in tension and parts that are able to withstand compression, otherwise it will either fly apart or crumble. The bones are principally used in compression. It is the muscles that provide the tension. One of Burgoyne’s studies involved a structural analysis of the human ribs. The ribs are neither constantly round in section like bars, nor flat like the pieces of a whalebone corset. Instead, they vary in cross-section from trapezoidal near where they join the spine through triangular to elliptical where they terminate at the chest. At first, this seems to make little sense in terms of their function as a protective cage around the body’s most important organs. You would simply expect the strongest cross-section to be maintained over the whole length of the bone. However, the ribs are also shaped to accommodate muscle tissue which is attached to them by means of rough ridges on some of the bone surface. This muscle tissue effectively ties the ribs together. When the muscle is taken into account as well, it emerges that the constantly changing rib shape is in fact optimized all along its length for the loads it is likely to experience.
    A discussion that begins with mechanical engineering should not omit some consideration of mechanical defects. For the skeleton is not quite so perfectly designed as William Paley and others have thought. The head can nod up and down and turn from side to side, as Paley marvelled, but it cannot, for example, turn through 360 degrees, which might on occasion be rather useful. For all their ability to resist external blows, the ribs are sometimes most at risk from the body itself. A frequent cause of rib fractures is a severe coughing fit when the pressure comes from inside the ribcage.
    One surprising advantageous feature of the skeleton is the way that the two main bones of the arm form a rigid rod by using the second bone of the forearm, the ulna, to create a lock at the elbow. By facing the palm of the hand forward, one can then carry a bulky load such as a bucket of water canted out from the body just enough that it avoids banging the knees with each step. In other respects, though, the elbow is of course a weak point, as we are reminded when we bang our funny bone. This point of weakness – where the nerves that run to the two little fingers are squeezed between the