When I press a finger on a closed eye lid it means the eye lid touches eye ball, then I didn't feel any pain.
Now I open my eye lid, then I small touch(not press) the eye ball, it gives me pain, why so?
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4$\begingroup$ More of a physics thing, but I would assume that the eyelid makes sure that the force is spread over a larger area, making the pressure smaller. And also the eyelid is smoother and softer than your finger. (You probably shouldn't poke it too much, even if it is for science!) $\endgroup$– picapicaAug 5, 2016 at 12:10
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$\begingroup$ Contact-lenses also doesn't usually cause pain $\endgroup$– Always ConfusedSep 6, 2016 at 17:36
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3 Answers
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TL;DR: It's psychological.
Pain is a tricky thing, and it's certainly possible to feel pain when pressing your eyelids against your eyeballs if you press hard enough ("hard enough" in this context is generally quite a lot less hard than is required to cause damage, by the way).
However: Since your eyeballs don't have skin, they also don't have nociception; i.e. they don't have pain receptors in the same way your skin at large does. Eye pains, like headaches, are largely caused by a combination of psychological responses to perceived stimuli and pain responses in associated structures.
Basically, the pain you experience when fingering your eye isn't pain in the same sense as the pain you feel from touching a hot stove or pricking yourself with a needle, it's generated by the brain in response to novel stimulus. Your brain knows what eyelids feel like, and it's okay with them, but once you start introducing fingers into the mix, all sorts of disturbances in salinity and moisture crop up, and your brain freaks out (and rightly so).
The same phenomenon can be seen with regards to swimming in the ocean: If you try to open your eyes under water, you'll find it a painful and disconcerting experience, but if you keep at it and practice holding your eyes open under water, you'll get used to it, and the pain will go away. Similarly, contact lens wearers will soon learn to deal with these stimuli, and be able to handle eye touching.
While I wouldn't advise you to go pawing at your eyeballs, if you for whatever reason have a need to do so, acclimating yourself to the feel of it will make the associated pain go away in short order.
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$\begingroup$ Swimming in oceans? oceans contain high salt so it should cause burn. Isn't? I've experienced water in swimming pool, that was okay. But is it same for oceans? I've seen the sea-swimmers wear special type of spectacles. But is it normally possible to open the eyes in oceanic saltwater? $\endgroup$ Sep 6, 2016 at 17:43
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2$\begingroup$ @AlwaysConfused Seawater isn't particularly salty compared to the normal salt levels in tears and sweat, which gets in our eyes all the time. Also, industry divers like pearl divers have gone without goggles for… eight thousand years now? A while, anyway. $\endgroup$ Sep 6, 2016 at 18:11
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$\begingroup$ I've read about pearl collector slaves in a scifi novel 'Amphibian man' by Alexander Beliaev (translated in Bengali). But I had a doubt that whether they used any goggles etc. Thank for informing. $\endgroup$ Sep 6, 2016 at 18:18
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I think what you feel when you touch your eyeball directly is not pain as such, but you feel the uncomfortable sensation of dryness as your fingers dry out your eyeball. Your eyelids job is to continually keep the eyes moistened, and as their underside contains lacrimal fluid, you don't feel any discomfort.
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This probably isn't advisable to try, but if you lick your finger first you can touch your eyeball directly without it hurting. It would appear to be that a lack of lubrication causes the painful sensation possibly along with dirt on the skin. Someone else may have a more scientific answer.
answered Aug 5, 2016 at 14:26