Our hands - what’s beneath the skin?
Thus far in our journey, I've talked a lot about the philosophy of using our hands. But what are they exactly and how do they work? This week we are making a brief detour into the world of anatomy. If we are going to continue to explore the role of our hands in our lives, it's helpful to know a little bit more about exactly how they work.
The structure of the hand is made up of 27 individual bones (not including sesamoid bones which are embedded in muscles and tendons and vary between people). There are 8 carpal bones (in our wrist), 5 metacarpal bones, and 14 phalanges or finger bones (3 for each finger and 2 for the thumb). Multiply that by two and we have 54 bones in our two hands - over one quarter of the 206 total bones in an adult human body! It's worth reflecting on that for just a moment. Those two small appendages at the end of your forearms account for 25 percent of the skeletal complexity in your body. Even the foundations of their structure give you a clue to their importance.
Hand and wrist bones from Medical gallery of Blausen Medical, via Wikimedia Commons.
The bones are held together by joints and ligaments. Wherever two or more of these bones meet a joint is formed with a layer of cartilage, allowing the bones to glide smoothly against each other. First the wrist is formed where the two bones of the forearm (the radius and the ulna) meet the carpal bones. The latter are formed into two groups of four bones thus allowing the range of rotational and flex movement we see in our wrists. Next, the metacarpals are the bones in the palm of your hand connecting the wrist to the fingers. Finally, the fingers themselves have three joints each and the thumb has two.
The ligaments connect the bones and support them. They are tough, ropelike structures that stabilise the joints. The tendons are soft tissues that connect the muscles to the bones. When a muscle contracts, the tendon transfers that movement to the bone. There are over 100 ligaments and tendons in a single hand but over 4000 tendons and 900 ligaments in the whole human body. Another way to think of this is that there are approximately 4 ligaments and tendons for each bone in the human hand whereas this ratio is more like 25 per bone when averaged over the whole human body. I like to think of this as indicating that it is the complex skeletal structure that underpins the hands amazing dexterity.
Next we need to look at the muscles. There are 34 in each of our hands (and about 600 in total in our body). The muscles in our hands fall into two categories - extrinsic and intrinsic. The intrinsic muscles are small and originate in the wrist and hand. They are the muscles that help control fine motor movement, so they are working hard right now as I type this article. The extrinsic muscles, as the name suggests, originate beyond the hand in the forearm and elbow. In each of our fingers, there are three intrinsic muscles and three extrinsic muscles. These muscles enable our hands to operate in two main ways - with a precision grip for doing fine, delicate work or a power grip for grasping and lifting something heavy.
An anatomical illustration from the 1909 edition of Sobotta's Atlas and Text-book of Human Anatomy with English terminology, via Wikimedia Commons.
You can feel yourself how these muscles work as you try a simple activity. Put one hand out in front of you and rest your other hand on that forearm. Now play around with your hand movement and feel what is happening in those forearm muscles. With some hand movements, for example making a fist, you will feel the muscles in your forearm working hard. But if you now relax the hand and try just gently moving your forefinger and middle finger apart, you will feel almost no tension in those forearm muscles at all.
All day as you use your hands, there is a complex dance going on between muscles, tendons, ligaments, joints and bones. It's mind blowing if you stop to think about it! But we are only halfway though the story because we have only looked at the physical structures that support the movement of the hand. We also need to know about the nerves, the blood vessels and the skin.
The blood vessels deliver oxygen and nutrients to our body. We have two main arteries that deliver blood to our hands - the radial and the ulnar. The radial artery is the one you can feel when you take your pulse on your wrist. There are a whole network of finer blood vessels in our hands that ensure blood flow reaches all the critical components that need to work together to ensure any single movement. Our hands don't have much in the way of protection. All that component architecture is only covered by a thin layer of muscle, fat and skin. The skin on the back of the hand is thin and mobile allowing everything to move freely and thus grasp things easily. The skin on the palm is thicker, offering a bit more protection for when we are handling rough or heavy objects. But even so, this lack of protection explains why the tiniest of cuts on the hand can bleed profusely. There are blood vessels very close to the surface, so it doesn't take much to damage one.
The nerves carry the electrical signals that communicate with our brain. There are three main nerves - the ulnar, the median and the radial - all of which originate at the shoulder. These work together to send messages to our hand to signal that a particular movement is required. They also collect information from thousands of touch receptors in our hand to process sensations of touch, pain, temperature and vibration. A single fingertip has about 3000 of these receptors making it one of the most touch sensitive places in the human body (along with the lips and mouth). Think about how running your hand across finely woven silk feels different to finely woven cotton. Or how you can feel a single human hair between your fingers. Or how different it feels to mould freshly made warm playdough compared with when it's cold, straight from the fridge. All of these sensations are caused by the nervous system processing thousands of tiny messages from different types of touch receptors. That's amazingly cool when you stop to think about it!
The complexity of the hands’ nerve story arises depending on whether the touch receptors are found on hairy or hairless skin (we have both on our hands); whether their function is related to touch, pain, temperature, vibration, or movement; and whether they are slow or fast to fire. It's a fascinating rabbit hole which is way beyond the scope of this post. But it is really interesting that researchers continue to make breakthroughs in a field which still has so much that needs to be understood. For example, the 2021 Nobel Prize for Physiology or Medicine went to David Julius and Ardem Patapoutian for their research identifying the different sensors on nerve cells that detect heat, cold and pressure. Knowing more about exactly how these receptors work has important implications for pain management for example.
Your hands are an exquisitely developed masterpiece of engineering. In 1883, Sir Charles Bell expressed it beautifully.
"The human hand is so beautifully formed, it has so fine a sensitivity, the sensibility governs its motions so correctly, every effort of the will is answered so instantly, as if the hand itself were the seat of that will... We use the limbs without being conscious, or at least, without any conception of the thousand parts which must conform to a single act."
Mug of tea anyone?
As we go further along this journey, I'll be sharing more about how this wonderful anatomic structure at the end of each arm is linked to how we live and learn. But today I'm content to just take a little extra notice of what my hands are doing. To be aware of the exquisite dance taking place between my brain and the bones, muscles, tendons, ligaments, joints, nerves, touch receptors, and blood vessels encased in skin that is starting to show distinct signs of middle age. I'll be incredibly grateful for the millions of movements they have already performed during my life, not least the thousands and thousands of stitches they have allowed me to make. And I'll be happy that today their complex interaction happens almost without conscious direction as I drink a warm mug of tea, tap out a text to a friend, pack some delicious silk threads for my customers, and reach out to stroke my daughter's hair. I hope that you will find that same sense of awe and wonder in your own hands.
References
Bell, Sir C, 1833 (1979). “The Hand: It’s Mechanism and Vital Endowments as Evincing Design”, Pilgrim Press.
Hancock, E. “The Handy Guide to Touch” - https://pages.jh.edu/jhumag/495web/touch.html.
iKnowledge, 2015. "Wrist and Hand" - https://clinicalgate.com/wrist-and-hand-3/.
InformedHealth.org, 2010 (updated 2018). "How do hands work?" -https://www.ncbi.nlm.nih.gov/books/NBK279362/.
Napier, John (Revised by Russell H. Tuttle), 1993. "Hands", Princeton University Press.
Nguyen JD and Duong H, 2021 "Anatomy, Shoulder and Upper Limb, Hand Arteries" - https://www.ncbi.nlm.nih.gov/books/NBK546583/.
“Nobel Prizes 2021” - https://www.nobelprize.org/all-nobel-prizes-2021/
Reynoso, H. “Fingertip sensory nerve endings”- https://carta.anthropogeny.org/moca/topics/fingertip-sensory-nerve-endings.
How to comment
I love receiving your comments and hearing your ideas! Making a comment is easy. Just type it into the box below. If you test it with “Preview” first, the name field will show “Not Logged In” (which is a system thing I can’t change). Never fear though! When you click on “Post Comment”, a dialogue box will come up asking for your Name, Email, and Website URL. The only field that must be completed is “Name”. It can be anything you choose - full name, first name, initials, nickname. So go ahead. Let me know what you think :)
