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Why Does Skin Get Pruney in the Tub?

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For a long time, scientists thought that pruning of the skin after spending time in the water was simply a matter of fingers being a little spongey. The outermost layer (the stratum corneum) of the outermost layer (the epidermis) of our skin is mostly made up of cells called corneocytes. These cells are filled with keratin, a protein that helps keep the skin hydrated by absorbing water and preventing its evaporation. When you hang out in the pool or the bathtub for a while, the keratin absorbs a lot of water, and the cells swell up. While the thin stratum corneum swells with water, the lower layers of skin that it is attached to don’t, so that outermost layer has to buckle and bend to accommodate its relatively larger size, sort of like a too-big shirt that wrinkles and bunches together when it's tucked in.

Another, more recent explanation is that the wrinkles come from vasoconstriction, or the narrowing of blood vessels. The idea is that hot water makes the blood vessels in the fingers tighten and the surrounding tissue contract, causing the skin to fold.

But the explanation might be more complicated than either one of those potential causes—especially when you consider how the phenomenon occurs in people with nerve damage.

So Unnerving

In the 1930s, two scientists examined a boy whose median nerve was severed, leaving his thumb, index, and middle fingers numb. When they soaked his hand in water, the ring and pinkie fingers wrinkled but the fingers affected by the damaged nerve stayed smooth.

And in 2001, researchers at Tel Aviv University found that nervous system malfunctions caused by Parkinson’s disease also interfered with finger wrinkling. In their study, Parkinson’s patients’ fingers wrinkled less on one side of the body than the other, and wrinkled less overall than the fingers of healthy subjects. Going by the common explanations, the wrinkles were a local phenomenon happening in very small bits of flesh. The involvement of the nervous system, though, suggests that something else is going on.

Getting a Grip

Mark Changizi, neuroscientist and the Director of Human Cognition at 2AI Labs in Boise, Idaho, thinks that the wrinkles’ neural factor is a clue that they’re adaptive. Rather than being a mere side effect of water-logged digits, he says, they’re a functional response to wet conditions: The wrinkles act like drainage networks or tire treads on our fingers and toes, channeling water away and giving skin more contact with, and a better grip on, wet surfaces.

Analyzing the wrinkles on various soaked fingers, Changizi and his team found that they all had similar shapes and characteristics—with disconnected channels that moved away from each other as they got farther from the fingertip—consistent with what is expected in a drainage network. That wasn’t much evidence for Changizi’s hypothesis, but it got the ball rolling. (Update: 11/30/2012, 1:25 pm) While that doesn't seem like much, Changizi points out that the "morphology prediction is actually very strong."

"Of the infinitely many wrinkle patterns that are possible," he says, "[the] drainage hypothesis predicts [the] actual [pattern]."

Since publishing the idea and the initial data last year, he and his team have been looking for evidence of finger-wrinkling in other primates that live in wet environments (they’d already found it happens in Japanese macaques) and are setting up experiments to directly test the wrinkles' effects on grips, While the results aren't ready to be published yet and the pilot studies so far suggest that pruney fingers do help improve grip.

(Update: 11/30/2012, 1:25 pm) Changizi has filled me in on that pilot data. The experiment was conducted Changizi and undergraduate student Joseph Palazzo. They had subjects carry out a timed task of moving objects, including bottles, stones, logs and other items, from one place on a table top to another, and back again. They did this in wet-pruney, dry-pruney (dry objects, and fingers dried after they had wrinkled), wet-nonpruney (wet fingers, but not yet wrinkled) and dry-nonpruney conditions. Wet-pruney performance was better than wet-nonpruney, with the subjects being faster and making fewer mistakes.

Changizi would like to see more behavioral studies like this carried out, and see more data from other species for further tests, but  probably won’t carry out any of these studies himself. "A more sophisticated next experiment would be version-2.0 of this sort of thing, in my mind," he says. "But not my forte." He thinks that other scientists would be much better at that kind of experiment.

"In terms of the categories of test, then," he says. "There's morphology, behavior, and phylogeny, and at this point we've done the first, poked at the second, and only wondered about the third."

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Big Questions
Where Is the Hottest Place on Earth?
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The summer of 2017 will go down as an endurance test of sorts for the people of Phoenix, Arizona. The National Weather Service issued an extreme heat warning, and planes were grounded as a result of temperatures exceeding 120 degrees. (Heat affects air density, which in turn affects a plane’s lift.)

Despite those dire measures, Phoenix is not the hottest place on Earth. And it’s not even close.

That dubious honor was bestowed on the Lut Desert in Iran in 2005, when land temperatures were recorded at a staggering 159.3 degrees Fahrenheit. The remote area was off the grid—literally—for many years until satellites began to measure temperatures in areas that were either not well trafficked on foot or not measured with the proper instruments. Lut also measured record temperatures in 2004, 2006, 2007, and 2009.

Before satellites registered Lut as a contender, one of the hottest areas on Earth was thought to be El Azizia, Libya, where a 1922 measurement of 136 degrees stood as a record for decades. (Winds blowing from the nearby Sahara Desert contributed to the oppressive heat.)

While the World Meteorological Organization (WMO) acknowledged this reading as the hottest on record for years, they later declared that instrumentation problems and other concerns led to new doubts about the accuracy.

Naturally, declaring the hottest place on Earth might be about more than just a single isolated reading. If it’s consistency we’re after, then the appropriately-named Death Valley in California, where temperatures are consistently 90 degrees or above for roughly half the year and at least 100 degrees for 140 days annually, has to be a contender. A blistering temperature of 134 degrees was recorded there in 1913.

Both Death Valley and Libya were measured using air temperature readings, while Lut was taken from a land reading, making all three pretty valid contenders. These are not urban areas, and paving the hottest place on Earth with sidewalks would be a very, very bad idea. Temperatures as low as 95 degrees can cause blacktop and pavement to reach skin-scorching temperatures of 141 degrees.

There are always additional factors to consider beyond a temperature number, however. In 2015, Bandar Mahshahr in Iran recorded temperatures of 115 degrees but a heat index—what it feels like outside when accounting for significant humidity—of an astounding 163 degrees. That thought might be one of the few things able to cool Phoenix residents off.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
How Does Autopilot Work on an Airplane?
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How does autopilot work on an airplane?

Joe Shelton:

David Micklewhyte’s answer is a good one. There are essentially a few types of features that different autopilots have. Some autopilots only have some of these features, while the more powerful autopilots do it all.

  • Heading Hold: There’s a small indicator that the pilot can set on the desired heading and the airplane will fly that heading. This feature doesn’t take the need for wind correction to desired routing into account; that’s left to the pilot.
  • Heading and Navigation: In addition to holding a heading, this version will take an electronic navigation input (e.g. GPS or VOR) and will follow (fly) that navigation reference. It’s sort of like an automated car in that it follows the navigator’s input and the pilot monitors.
  • Altitude Hold: Again, in addition to the above, a desired altitude can be set and the aircraft will fly at that altitude. Some autopilots have the capability for the pilot to select a desired altitude and a climb or descent rate and the aircraft will automatically climb or descend to that altitude and then hold the altitude.
  • Instrument Approaches: Autopilots with this capability will fly preprogrammed instrument approaches to the point where the pilot either takes control and lands or has the autopilot execute a missed approach.

The autopilot is a powerful computer that takes input from either the pilot or a navigation device and essentially does what it is told to do. GPS navigators, for example, can have a full flight plan entered from departure to destination, and the autopilot will follow the navigator’s guidance.

These are the majority of the controls on the autopilot installed in my airplane:

HDG Knob = Heading knob (Used to set the desired heading)

AP = Autopilot (Pressing this turns the autopilot on)

FD = Flight Director (A form of navigational display that the pilot uses)

HDG = Heading (Tells the autopilot to fly the heading set by the Heading Knob)

NAV = Tells the autopilot to follow the input from the selected navigator

APR = Tells the autopilot to fly the chosen approach

ALT = Tells the autopilot to manage the altitude, controlled by the following:

VS = Vertical Speed (Tells the autopilot to climb or descend at the chosen rate)

Nose UP / Nose DN = Sets the climb/descent rate in feet per minute

FLC = Flight Level Change (An easy manual way to set the autopilot)

ALT Knob = Used to enter the desired altitude

This post originally appeared on Quora. Click here to view.

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