EVERYONE has their quirks, but for some people it's an extra body part.
And it's not unusual in some cases for people to be completely unaware of their existence.
3 The fabella is a sesamoid bone, similar to the kneecap, and grows in the tendon of a muscle
Scientists recently discovered that a bone that was thought to have been near extinction is making a comeback.
The little bone, known as the fabella, is found at the back of the knee – if it is found at all.
It was only present in 11.2 per cent of people's knees globally back in 1918 and scientists thought it served little or no purpose in our bodies.
But new research carried out at Imperial College London has found it can now be found in 39 per cent of adults around the world.
And people with osteoarthritis of the knee were twice as likely to have one as those without the condition.
However its exact purpose remains a mystery, while its increase has left scientists baffled.
But the fabella is not the only variation in human anatomy – they can occur as a result of genetics, environmental factors, mistimings in embryological development, or simply a failure of structures to disappear as part of normal development.
Most are benign and don’t cause disease.
Here Adam Taylor, director of the Clinical Anatomy Learning Centre, shares with The Conversation the ones that are best known to anatomists…
People have 20 primary teeth, known as "milk teeth”, which are lost and replaced by 32 permanent teeth.
But up to two per cent of people have extra teeth.
Most of these people have one or two extra, known as supernumerary, teeth, but there are medical reports of people with many more extra teeth, with one female having 19 supernumerary teeth.
3 This is an X-ray of a person with supernumerary teeth
Males and females have nipples because, early on in development, before the sex of the fetus has been determined, two ridges of tissue form, running from the front of the armpits to the groin.
These ridges are known as mammary ridges.
Over time, both disappear to leave a single area where the mammary gland and nipple develop.
It is possible for people to have supernumerary nipples, known as polythelia, along these lines, not in the middle of the chest between the existing nipples, as depicted in some TV and film shows.
There are reports of people with seven nipples.
Most people have ten fingers and ten toes, but many people are born with extra digits.
They are most commonly seen on the hands and are usually associated with disorders, such as Down syndrome.
Some ethnic groups are more likely to have extra digits than others.
African-Americans have a much higher presence of an ulnar polydactyly – a digit on the little finger side of the hand.
Caucasians have a higher presence of an additional digit on the radial (thumb) side of the hand, known as radial polydactyly, but this is less common.
While most people with extra digits have one or two, there are reports of people with 31 and even 34 digits.
Cplbeaudoin/Shutterstock, CC BY-SA3 Woman with an extra digit on the pinky side of her hand, known aspreaxial polydactyly
Muscles can also vary from one person to another. One of the easiest to observe – or observe its absence – is a muscle called palmaris longus.
The best way to see if you have this muscle is to put your thumb and ring finger together and then bend your hand towards you.
If you have this muscle, you should see a tendon pop up out of the wrist, running from the forearm and into the hand.
This muscle can be in one or both arms. In some people, it is absent in both. It is absent in both arms in about 10 per cent of Caucasians and absent in one arm in 16 per cent.
There are suggestions of an evolutionary loss of this muscle, with mammals such as orangutans, who use their arms for walking, having this muscle, but higher apes, such as gorillas and chimpanzees, showing an absence.
The good news for those of us who don’t have it is that it doesn’t make our grip strength weaker compared with those who have it.
Although those who do have it may find it useful if surgeons ever need to repair a tendon, as the palmaris tendon is easily accessible and can be harvested for grafting.
There is a similar muscle in the lower leg called plantaris, which is believed to be absent in 7-20 per cent of limbs.
This muscle cannot be seen without using imaging, such as ultrasound, as it lies deep in the calf region of the leg.
But like its variably present compatriot in the arm, it can be used for tendon grafting if needed.
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Some variations only come to light as people age, such as men born with a uterus.
This developmental anomaly may only become manifest during puberty, with blood appearing in the urine.
It is actually the menstrual cycle exiting through the urinary system.
All of this goes to show that human anatomy is not as clear-cut as school textbooks might suggest.
We’re as variable as snowflakes. Something to be celebrated, surely.
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