The incredible human hand and foot

By Dr George McGavin
Presenter, Dissected: The Incredible Human Hand and Foot

Image source, Science Photo Library

Both the human hand and foot represent a triumph of complex engineering, exquisitely evolved to perform a range of tasks.

Our arms and legs are pentadactyl limbs - they have five digits.

When the fist four-legged animals began to move onto the land from the sea around 380 million years ago some had as many as eight digits but the familiar five digit pattern soon became the standard which has since been modified in certain groups such as frogs and birds.

Few people except medical students ever get the chance to perform a human dissection.

You can learn a lot from books and lectures but it is only by dissecting the human body yourself that you truly understand how it works.

In a specially prepared room in Glasgow University, Anatomist Quentin Fogg skilfully dissected a human hand and foot - taking them apart layer by layer to reveal their innermost secrets.

The hand is one of the most complex and beautiful pieces of natural engineering in the human body.

It gives us a powerful grip but also allows us to manipulate small objects with great precision.

This versatility sets us apart from every other creature on the planet.

The hand has one of the strangest arrangements of muscles in the body.

Most of its movements are controlled by muscles that aren't located in the hand at all, but in the forearm.

The muscles of the forearm connect to the finger bones via long tendons that pass through a flexible wrist.

This remote musculature gives the fingers movement and strength that wouldn't be possible if all of the muscles had to be attached directly to them.

In effect, the hand is simply a bony puppet, lashed together by ligaments and controlled by the forearm.

But that arrangement allows us to do so much. At one extreme is the impressive strength of a climbers' hands.

Through habitual use and training even a single finger can support the entire body weight.

At the other extreme a concert pianist needs great finesse and this comes from muscles within the hand called intrinsic muscles.

Some of these muscles specifically control the thumb and little finger while others such as the lumbricals (named for their worm-like shape) are not directly attached to bones but to tendons and allow wonderful subtlety of movement.

Specialised skin

No one would doubt that thumb is the most important digit of all. It accounts for 40% of the hand's capabilities and unsurprisingly if you lose one, surgeons will happily amputate your big toe and use it to create a new thumb, sacrificing one body part for the greater good.

But which finger could you most afford to lose? I have to admit I got this wrong when hand-surgeon Donald Sammut asked me.

I thought the little finger would be dispensable but as Donald explained the little finger is actually rather important - second only to the thumb.

Oddly, the finger you can lose with minimum inconvenience is the index finger. It can be included or excluded from everything we do with our hands.

How do you know if something is smooth or rough, wet or dry, hot or cold?

The skin at the tips of our fingers is very specialised. If you slice down the middle of a fingertip you see closed cells of fat, which act as a protective cushion for the enormous number of nerve endings underneath.

There are four main types of skin receptors and these respond to light and deep pressure, touch, pain and temperature.

The fingernails play a crucial part as well. If you didn't have a rigid structure against which to press you wouldn't be able to judge how firmly to hold anything.

The hand might seem, at first glance, a more interesting bit of our anatomy but, as foot surgeon Kartik Hariharan showed, the foot is equally complex.

Containing 26 bones, 33 joints, 19 muscles and 57 ligaments, it's one of the few pieces of anatomy that can compete with the hand for sheer complexity.

Great forces

Our feet need to be strong enough deal with some of the greatest forces experienced by the body, whilst also being capable of movements as exquisite as a ballerina's pirouette.

In many ways, it's because of the foot that we've evolved such extraordinary hands.

The ability to walk upright meant early humans could cover great distances efficiently but also freed up the hands to develop their unique anatomy and capabilities.

As the dissection revealed, the skin of the foot is thick and tough and under the heel lies a pad of specialised fat, packaged up like bubble wrap to absorb shock and spread the load of our body weight.

Image source, Jacopin / BSIP/SCIENCE PHOTO LIBRARY

At the core of the foot is the arch. The metacarpals create space for a tough web of muscles and ligaments that play a vital role.

They absorb the forces created when you walk or run. But the arch also acts as a spring; storing and releasing energy when you push off with your toes.

Today we run in cushioned shoes and perhaps because of this have a tendency to land heel first.

This risks damaging our joints as the shock of the impact travels up though our legs.

Barefoot runners make contact with the balls of their feet, the arch of the foot dissipating the energy of the impact safely.

Walking exclusively on two limbs is relatively rare in the animal kingdom.

Standing still may seem a simple act but it's really not. There is a lot happening just to keep us upright.

Proprioception is the sense that tells us where the various parts of our body are in space.

Even with our eyes shut we can touch our nose with any finger. Sense organs in our muscles and joints tell the brain when and exactly where our limbs are moving and how tense our muscles need to be.

It's this continuous feedback system between muscle and brain that detects minute changes that allows us to balance.

Our hands and feet are biomechanical marvels. More than any other piece of anatomy, they are what have made us such a successful species.

They've allowed us to walk out of Africa to colonise the globe and master the natural world.

I will never look at my hands and feet in the same way again and rather than have my mortal remains rot in the ground or burn in a crematorium, I will be gifting my body to a medical school so someone can learn from the unique experience of taking me apart.

Dissected: The Incredible Human Hand is on BBC Four at 9pm on Tuesday 18 February

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