The Usefulness of Man’s Hand
The hand is one of the most intricate and useful mechanisms of the entire human body; it is a prehensile (appendage for grasping) that humans share with chimpanzees, lemurs, and monkeys; even Koalas have opposable thumbs that are very similar to the thumbprints of the hands of humans. We humans absolutely have the ability to “think” with our hands; when we consider their connection to the brain we find the hand contributes to our thoughts and feelings. Fingers contain some of the most dense nerve endings on the entire body. The hand is greatest source for tactile feedback on the body and has the greatest impact on the sense of “touch”.
The hand has an intricate connection with the eyes and brain partially because they have the greatest mobility of any part of the human body. Each hand is paired with a dominant opposite side of the brain in the same fashion as the eyes. This “crisscrossing” of neuronal passageways occurs throughout the nervous system. The primary motor cortex is responsible for movement in the hands and body and executes movements in concert with the rest of the motor cortex.
There are 27 bones in the hand. 14 of which are in the fingers. There are 24 muscles groups innervated by various motor and sensory pathways that comprise 3 nerves: the radial, ulnar, and median nerves. These cascade to form 2500 nerve receptors per square centimeter on the surface of each hand.
Bones of the Human Hand
Lets start by looking at the bones. Each finger has three sections of bone: distal (fingertip), middle, and proximal; the thumb has two, the
Ligaments and Tendons of the Hand and Wrist
In the hand, there are 18 ligaments that are separated into four groups:
- The ligaments of the wrist proper which unite the ulna and radius with the carpus: the ulnar and radial collateral ligaments; the palmar and dorsal radiocarpal ligaments; and the palmar ulnocarpal ligament.
- The ligaments of the intercarpal articulations which unite the
carpal bones with one another: the radiate carpal ligament; the dorsal, palmar, and interosseous intercarpal ligaments; and the pisohamate ligament. (Shown in red in the figure.) - The ligaments of the carpometacarpal articulations which unite the carpal bones with the metacarpal bones: the pisometacarpal ligament and the palmar and dorsal carpometacarpal ligaments. (Shown in green in the figure.)
- The ligaments of the intermetacarpal articulations which unite the metacarpal bones: the dorsal, interosseous, and palmar metacarpal ligaments. (Shown in yellow in the figure.)
In the image below,
The Hand’s Muscles Groups
I could probably write an article on each of the finger muscles exclusively. Bear with me as we go through these muscles groups. The muscles of the hand are some of the most sensitive and finely tuned muscles in the body. They are normally separated into two categories: extrinsic and intrinsic. Extrinsic muscles have their muscle belly (the majority of muscles fibers) on the forearm.
The intrinsic muscle groups are the thenar (thumb: Abductor pollicis brevis abducts, Flexor pollicis brevis, Opponens pollicis) and hypothenar (little finger) muscles; the interossei muscles originating
The fingers have two long flexors located on the underside of the forearm. The deep flexor attached to the distal phalanx (farthest) and the superficial flexor attaches to the middle phalanx. These are what allows your fingers to bend. The thumb also has two flexors, one long and one short and these work together with the thenar muscles to allow the thumb to grasp. The thumb is quite a complex mechanism in and of itself; kinda makes me want to write an article on it.
The extensors on the top of the forearm arrange in an even more complex way. The tendons unite with the lumbrical and interrossus muscles to form the extensorhood mechanism. The extensors straighten the digits. The thumb has two extensors on the forearm which form the anatomical snuff-box, or the triad at the base of your thumb. The pointer finger and little finger both have an extra extensor for pointing.
The Skin of the Hand
The skin of the hairless side of the hand (palm) is very thick and can be bent easily while maintaining connection with the muscles and bones of the hand. Palm skin is usually lighter because of inhibited melanin (skin pigment) production and therefore don’t tan. Fingerprints, or the papillary ridges exist to increase friction when the hand is grasping an object. The skin of the top of the hand is soft and pliable to allow the fingers to recoil quickly.
Conclusion
The hand is complicated, especially in terms of muscular innervation, but we are still learning enormous amounts about how they have evolved into their current state. Comparative physiology is very useful for this and we are constantly exploring more about ourselves through animals and our genetic ancestors. If you have any requests for articles, or interesting additions to this one, please ask. Feel free to add anything that I have missed, or to ask any questions in the comments.
sources (besides Wikipedia):
1. http://www.oandplibrary.org/al/pdf/1955_02_022.pdf (Craig L. Taylor PHD & Robert J Schwartz, MD)
2. http://www.aofas.org/footcaremd/conditions/ailments-of-the-big-toe/Pages/Sesamoiditis.aspx
3. https://ispub.com/IJFS/1/2/9047#sthash.lchtoImt.dpbs