Friday Finger Fun facts
Remote control fingers
We work our fingers by remote control. Of course, in one sense, we work all of our moving body parts by remote control – the control center is our brain. However, the fingers are special, because there are no muscles inside the fingers. The muscles which bend the finger joints are located in the palm and up in the mid forearm, and are connected to the finger bones by tendons, which pull on and move the fingers like the strings of a marionette.
When the hand is kept wet, the skin of the palm wrinkles. Why? The exact mechanism is not known, but it is clearly controlled by nerves. When the nerve which supplies feeling to an area of skin on the palm is cut, that area of skin not only becomes numb, loses its ability to wrinkle when wet. It also loses the the ability to sweat.
The wrinkles on the back of the finger knuckles are actually dimples, and mark areas where the skin is attached to the tendon beneath the skin.
Finger joints only have wrinkles and creases if the joint moves. If a finger joint stops moving, the creases eventually flatten out.
The muscles which power the fingers are strong – strong enough for some people to climb vertical surfaces supporting their entire weight at times by a few fingertips. The muscles which accomplish this feat are stronger than you might imagine, for the biomechanics of the hand require that the force generated by the muscles which bend the fingertips must be at least four times the pressure which is produced at the fingertips.
The thumb is controlled by
o 9 individual muscles, which are controlled by
o all 3 major hand nerves
And moves in such a complex fashion that there are 6 separate descriptive terms just for particular directions of movement of one thumb joint – the basal joint, at the base of the thumb.
Do fingernails have feeling? No, but the fingernail extends deep beneath and behind the skin of the cuticle, and nerves on the back of the finger around the cuticle sense forces transmitted from the tip of the fingernail. The brain integrates the sensations from the nerves of both the fingertip pad and cuticle to give a complex enhanced perception of pressure and shear at the fingertips. Loss of a fingernail changes the feeling on the palm side of the fingertip.
Hand and Brain
About a quarter of the motor cortex in the human brain (the part of the brain which controls all movement in the body) is devoted to the muscles of the hands. This is usually illustrated with a drawing of a human figure draped over the side of the brain, body parts sized proportional to the amount of brain devoted to their movement, referred to as a homunculus – as illustrated in this drawing from Dr. Wilder Penfield’s monograph “The Cerebral Cortex of Man.”:
Hair and nails
Structurally, fingernails are modified hairs.
The skin of the palm
The skin on the palm side of the hand and fingers is unique for these reasons and more:
* No hair (the medical term is glabrous).
* Usually neither color nor the ability to tan.
* Tough and durable, yet sensitive.
* Anchored down to the bones beneath through an intermediate layer of fascia. This arrangement keeps the skin of the palm from sliding around like a rubber glove when we use our hands to grip and twist. In some people, this layer of fascia shrinks and thickens, leading to Dupuytren’s disease.
* “If you can move your finger, it isn’t broken” False.
* “Cold hands, warm heart” Well, it really depends on why ones hands are cold…
* “Computer use causes carpal tunnel syndrome” Probably false.
* “Eating gelatin makes your fingernails stronger” False – no evidence to support this.
* “Cracking your knuckles gives you arthritis” False – no evidence to support this.
During the Gallic wars, Julius Caesar ordered the thumbs of captured warriers amputated so that when they returned to their country, they would serve as examples and be unable to bear arms again. This practice was later used in a number of wars and in the slave trade.