Authors: Hugh Aldersey-Williams
There are in fact many differences between the skeletons of the two sexes, although they are almost entirely differences of degree rather than kind. Women tend to have slimmer bones, a narrower rib cage and a more rounded skull as well as a relatively larger, wider pelvis. (Or, in view of the above omission, we might say that the male may be identified by his heavier limbs, broader chest and more angular skull.) However, the male and female skeletons certainly may not be distinguished by their different number of ribs. The myth that women have thirteen pairs of ribs where men have twelve arises from the biblical story that Eve was created from one of Adam’s ribs. Biblical scholars have questioned the meaning of this story. In Hebrew, the word translated as rib (
) can also mean side, so God’s surgery may have been quite major when he fashioned Eve from Adam. This unusual act of creation links Christian theology to other myths, such as that of Dionysus being born out of the thigh of Zeus. Although women do not have that extra rib – thereby making
a suitably ironic title for the well-known feminist magazine – one in 200 of us does in fact have an extra rib, an evolutionary reminder that we are descended from creatures with many more sets of ribs (such as the serpent in the Garden of Eden, which would have had hundreds of them).
Another apparently obvious difference between the sexes is not reflected in the skeleton. To judge by its name, you would think that the Adam’s apple is an exclusively male appurtenance. Yet, as Genesis explains, Eve first tasted the fruit before she tempted Adam to eat from the tree of knowledge. Anatomical fact, both men and women have a feature known as the laryngeal prominence – a bulge of cartilage, not bone – that forms around the larynx. The larynx is a cavity in which air is made to vibrate using the vocal cords. It has a natural resonant frequency governed by the volume of the cavity and the size and shape of its opening. Physicists call such volumes Helmholtz resonators, after the nineteenth-century German physiologist and physicist who made such devices to help him analyse music. An empty bottle is a good example of one. If you blow across the top, it produces a tone at its resonant frequency. If you half-fill the bottle, the pitch of the tone increases. In adolescence, the cartilage around the larynx begins to bulge outward, which increases the volume of the larynx so that it can produce deeper sounds. This bulge is greater in boys than in girls, producing a typical angle of 90 degrees compared to 120 degrees, and it is this difference that explains the greater prominence as well as the deeper voice.
Bone is more advanced in its way than many artificial materials. Bones – including human bones, such as fragments of skull used as scrapers – were among humankind’s earliest tools, and today bone continues to inspire materials scientists looking for ways to combine great strength with lightweight 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 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 elbow and the skin with no muscular protection – is a consequence of our evolution into bipeds. If we still walked on all fours, the forelimb would be angled so that the elbow bent towards the back, not outward, and it would be better protected. Our knees suffer too, as we learn when we reach a certain age, and this too is a consequence of evolution, and our using two feet to bear the weight formerly borne on four. The Achilles heel, however, cannot really be counted as a weak point: anybody would succumb if shot in the heel with a poisoned arrow as Achilles was in legend. This Victorian metaphor is thought to originate instead with Samuel Taylor Coleridge’s reference to ‘Ireland, that vulnerable heel of the British Achilles’.
The physics is remarkable enough, but bone is also living tissue. It must perform its structural function at the same time as it grows with the rest of the body. Bones develop in response to stress. Tiny cracks form when they are subjected to forces during normal exercise. These cracks send chemical messages instructing new bone tissue to form. However, bone will fail if pushed only a little way beyond its normal performance limit – to about 120 per cent, compared to 200 per cent for materials like steel. ‘The body is neither over- nor under-engineered, because all bones have this 120 per cent factor,’ Chris tells me. ‘It’s actually quite natural that you become optimal.’ In other words, a bone does not become ‘too strong’ unless there is some exertion that is making it so, in which case it becomes simply strong
. Conversely, a bone does not usually weaken beyond a safe level unless through lack of use. When sportsmen speak of ‘giving it 110 per cent’ they are talking more sense than perhaps they realize.
Because of gravity, the body needs to save weight as it grows, as Haldane has explained with reference to the giants in
The Pilgrim’s Progress
. It achieves this goal in part by growing bone faster lengthways than across its width (at the price of some reduction in comparative strength in the adult bone). Something clearly guides bone to grow where it is most needed. Whatever this mechanism is – and we will come to it in a moment – it is highly dynamic and responsive to the bodily world around it. It has long been known that bones can be made to increase in size and strength if they are repeatedly stressed. The bones in the spear-carrying arm of a Roman soldier are larger than the bones in the opposite arm, and the same goes for the racquet-wielding arms of tennis players today. Especially in the case of athletic activities taken up in youth, such as ballet or gymnastics, the bones can also be shaped in response to exercise before they harden.
This process allows us to tell much about our ancestors from their surviving bones. We conceitedly believe we are taller than our ancestors because we eat so well. In fact, evidence from skeletons of
shows that they were taller than we are, owing to the strenuous work necessary to survive. From the size of the rough areas on the bones to which muscle attaches, it is known that they were proportionately fitter and heavier too. There is nothing to prevent our regaining these superhuman proportions if only we are prepared to put in the effort – our shrinking stature is not an evolutionary change, but a response to our environment.
Until recently, very little was known about this kind of bone growth. Normal bone growth during development is well understood; it involves the division of cartilage cells on fronts located at the ends of the long bones and their subsequent hardening into bone. But the way that bones respond to use or disuse during life has been something of a puzzle, despite the obvious importance of knowing more about it. Bone can lose up to a third of its mass during the short time a broken leg is in plaster, for example; fortunately, this mass is as quickly replenished when exercise is resumed. They also atrophy in the weightless conditions of space, hence the importance of modelling the behaviour of astronauts’ bodies.