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Things people know: why do fingers wrinkle in water?

AB asks "Why are there wrinkles in the fingers after a long stay in the water?"

Wrinkled fingers. Photo: Brenderous, Wikimedia

We usually notice the wrinkled finger when leaving the pool, but if you follow the fingers from the moment they get wet, you will discover a similar process in all of them: a few minutes after the immersion, a vertical wrinkle appears in the center of the upper part of the finger (towards the palm) and gradually more parallel wrinkles appear on its sides. The wrinkles deepen and finally horizontal wrinkles also appear at the end of the finger which sometimes connect the longitudinal lines. The wrinkles are denser in the pinky where about one millimeter separates the wrinkles and more spaced in the thumb.

Why do the fingers insist on wrinkling? There is a standard answer to this: the skin absorbs water, expands, therefore after about half an hour of wetness there is too much skin on those fingers. If you wear a garment that is several sizes bigger than you, it will look wrinkled and this is what happens to the finger.

A simple and elegant explanation, but there is a small problem with it: it is not true. Already many years ago, a strange fact was discovered: when the nerve connection to the skin is cut off, the fingers do not wrinkle. The skin is the same skin, the water is the same water but the fingers leave the pool as smooth as they entered. This is also true for those whose nerves in their skin have been damaged by a disease, for example in leprosy patients (more precisely - Hansen's disease, whose connection to biblical leprosy is absolutely coincidental) or even those who apply local numbing cream before baptism.
It turns out that wrinkling is not the result of passive absorption of water in the outer part made up of dead skin cells, but an active process that requires a reaction of the nervous system.

As a response to moisture, the cells of the inner lining (epithelium) of the blood vessels shrink. The blood supply decreases by almost 30% and the result is that there is "less finger" under the skin. In the person whose finger was amputated and reattached, the opposite phenomenon was observed: the blood supply actually increased following immersion in water so that the transplanted finger remained smooth while its companion on the other hand wrinkled as usual.
Because the skin is connected to the soft tissue underneath, compressive forces act on it, and when this force passes a critical threshold, a wrinkle appears. Since the force acts on the skin from the outside in (pulling to the inner layer that has suddenly shrunk), the wrinkles will appear in the longitudinal direction - the same circumference of skin that covered the finger in the dry state as a round ring twists to surround a shrunken finger. At the upper end, the skin does not cover a cylinder, but a kind of hemisphere where the forces act downwards and the wrinkles change direction.
But if we dipped the whole body, why do only the fingers wrinkle?
It turns out that the shape of the wrinkles, their dimensions and the order of their appearance can be calculated from a simple physical model: a thin coating layer on a cylinder rounded at the end. The geometric result of the forces acting on the coating (representing the skin) depends, according to the calculations, on the ratio between the thickness of the coating and the radius of the cylinder. The calculation revealed that in order for the contraction of the inside of the finger to cause a wrinkle, the upper stratum corneum of the skin is required to be quite thick in relation to the radius of the finger. When the skin coating is thin, the force will often be less than the threshold required to create the crease, and even if wrinkles are formed, they will be too shallow and dense to be noticed. This layer is 10-20 microns (thousandths of a millimeter) thick in most of the body, but in the palm of the hand it is about half a millimeter thick - thick enough for a child to be able to impress their friends by passing a needle under this coating. The finger has the optimal combination for wrinkles: a thick epidermal coating on a thin roll, so you can easily see the wrinkles when you get out of the pool. In people who engage in physical work and develop thick skin on their hands, the distance between the wrinkles increases accordingly.

But why would the nervous system bother to respond to moisture like that?
Recently, it has been hypothesized that these wrinkles play a role in adapting the palm to grip in wet conditions. The ability of the hand to grip branches or tools was essential for the survival of our ancestors in the eras without the towel when natural selection acted on us. Even when they are wet, our ancestors had to quickly climb a tree dripping with water or grip a wet and slippery stone when an enemy appeared in front of them. The bent toe is adapted to this just as a grooved tire is adapted to grip the road even on rainy days. The function of the folds in the finger, like the function of the grooves in the tire, is to allow water to drain from the grip surface and thus prevent the creation of a slippery shielding layer. Toe creases, according to this approach, represent a more sophisticated design than the soles of boots or tires that are grooved for the same purpose because they are flexible and erased under pressure. The vertical wrinkles allow water to flow from the finger which is the first point of contact we create downwards without getting trapped between the skin and the surface and a little later when we tighten the hand around the bone (for example a branch we hang on to while climbing a tree) the pressure aligns the ridges and thus the necessary grip surface is preserved. Although the researchers have not yet proven that it is really easier to climb a wet tree with a wrinkled finger, it is still tempting to think that there is indeed an evolutionary purpose to the strange wrinkles.

Thanks to Dr. Xi Chen for his help.

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One response

  1. Water transfers heat many times more than air. It is possible that the reaction is accidental because the real goal is to reduce the exposure of the blood to the skin in the case of a wet city and maintain the body's heat balance.
    The argument that it is designed to allow the holding of smooth surfaces from getting wet is not satisfactory - even if you go for a walk in the rain, the wet hands do not show wrinkles. You really have to dip them in water for a few minutes.
    In addition, anyone who has tried to work with wet hands knows how sensitive the skin is to damage and cuts in this situation.

    It is true that today it often happens in the bath when we adjust the water temperature, but in nature most encounters with moisture are also from bodies of water at a lower temperature than the environment. The body draws the blood from the lower layer of the skin and the contraction is a side effect.

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