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

performance. As you might expect for a substance that spends most of its time supporting our weight, bone is somewhat stronger in compression than it is in tension. A bone can typically resist a load of a tonne and a half per square centimetre before it breaks. The bones of a child’s arm are easily strong enough to support the weight of a family car, for example. Its tensile strength is nearly as great, comparable with that of metals such as copper and cast iron. Only in torsion is bone relatively weak, which explains why most fractures are the consequence of severe twisting forces.
    Most bones, especially the long bones of the limbs, tend to be relatively straight. This is not so much in order that they can extend as far as possible for a minimum outlay of material, but because a straight bone has far greater strength than a curved one. The structural columns that support buildings are straight for the same reason. Many of the larger bones are basically tubular in shape. If you cut through them (ask your butcher), you will see an interior structure like a sponge, full of holes. It is clear that this makes the bone lighter than it would be if it were solid. But there is more to it than this. In fact, this is no sponge, but a precisely engineered microstructure providing a network of tiny struts placed just where the bone is most likely to experience forces upon it. Today, furniture designers are beginning to make chairs and tables according to the same minimal principles, using computer-generated force diagrams to tell them where best to place the structural fabric of the object.
    It is not any single bone that really inspires; it is how they all work in concert. As the spiritual ‘Dem Bones’ reminds us (a little incorrectly), each bone is connected to at least one other. To a first approximation, the body is simply an assemblage of straight, rigid beams hinged in various ways at the ends to the next such beam to make up an articulated whole. Few studies were made of the human body as a mechanical system until the launch of the American space programme, when it became important to know how it would respond to the absence of gravity. However, two forerunners in the field were Christian Braune and his student Otto Fischer in Leipzig. Their research during the 1880s arose from early studies of human gait, in turn prompted by the investigations of men such as Etienne-Jules Marey and Eadweard Muybridge into human and animal movement using early methods of high-speed photography. It was a logical extension of this work to want to establish the body’s centre of gravity, which Braune and Fischer did by carefully balancing frozen cadavers. They also identified the centres of gravity of major components of the body by cutting them off the cadavers and performing the same balancing tests. Calculations made today – for example, to estimate the extent of whiplash in car accidents – still rely on data from very few original studies like these.
    The crude approximation involved in this work hardly does justice to the elegant complexity so admired by Paley. The human skeleton has to perform a huge variety of tasks, including locomotion, balance, resistance and manipulation, all of which expose the bones to high stresses. Normal walking involves fractional adjustments in the position of many individual bones. The gait has half a dozen component actions, for instance, from the pelvic rotation that allows the body to pivot around the stance leg so that the free leg can swing forward until the heel strikes the ground, to subsequent adjustments that transfer the body’s weight from the old stance leg to the new forward leg. Many subtle flexions of the knee, ankle and foot ensure that the foot meets and leaves the ground smoothly with each step. The forces that result from all of this complicated activity are equivalent to as much as eight times body weight.
    It’s all very involved and interdependent. I feel I need to go back to