Entomology, chemical ecology, evidence-based environmentalism and science in general. I like big bugs and I cannot lie.

Thursday, 4 December 2008

Lost in your eyes

Having seen what it’s done to Lou’s sanity I wouldn’t want to be a medic, but it’s always fun to be a tourist and I’m enjoying reading up on the eye to get some idea of what sort of volatiles I’ll be finding in tears. Until now I’d never given tears much thought, beyond them being the soggy stuff that leaks out at weddings or when someone uses a track you really like to advertise cheese strings, but the lachrymal film (to use its posh name) actually turns out to be pretty amazing.

Closest to the eye is the mucin layer, composed of jelly-like glycoproteins (protein molecules with sugar molecules stuck on, for those not in the know). This provides a nice smooth friction-free surface for the eyelids to slide over, and holds the water layer above it in place. This liquid layer is the part I’d always thought of as tears, and is what washes away any grit that gets in your eye, as well as doing something complicated to the optical properties that I’m too dense to understand. It’s apparently a mistake to think of these as two distinct layers, they shade into one another as the glycoproteins get less dense the further you go from the eyeball. So don’t.

Of greatest interest to me though is what lies above the mucino-aqueous phase (we scientists love giving things complicated names – it makes us sound intelligent and gives us an advantage at Scrabble). This is the tear lipid layer, a thin film of oils, fats and waxes that covers the mucino-aqueous layer in the same way that oil floats on vinegar when you make fancy salad dressing. Generally smelly things tend to be soluble in fats (just think of essential oils, allowing me to get in a gratuitous plug for Jeff’s sister’s company which makes very nice bath things*) so I’m guessing that whatever the flies are smelling is coming from this layer.

The tear lipid layer is itself divided into two layers (layers are so this season, darling). The inner layer is just one molecule thick but these are rather clever molecules that have one end that dissolves in water and one end that dissolves in fats. This anchors all the fats above making up the tear lipid layer (TLL to its friends) to the vertical eyeball, stopping all the oils from sliding down to the bottom of your eye and leaking out. Which would be strange.

So what’s the point of all this? The TLL reduces the rate of evaporation from the water layer, making it last ten to twenty times as long as it would otherwise. It also reduces surface tension, helping spent tears to drain better, and catches fine dust. It contains antibacterial fatty acids and its high viscosity prevents oils from your skin from getting into your eye. And as if that wasn’t enough to make you appreciate an anatomical structure you didn’t know you had ten minutes ago, it also forms a watertight seal between your eyelids when you close them, compensating for microscopic imperfections where they don’t meet perfectly and so stopping your eyes from drying out when you sleep.

The TLL is secreted by the glands of Meibomius (why is it that only people with daft names get medical discoveries named after them? Is there a duct of Smith?). These are tiny little pin-prick glands on the lip of the eyelid, invisible to the naked eye. There are more of them on the upper eyelid than on the lower, so rather more is secreted on the upper lid margin than on the lower. This is because thanks to gravity the TLL is a little thicker at the bottom than the top, so replenishing it at the top is more of a priority. The ducts of the glands run up the inside of the eyelid, so those on the upper eyelid are longer than those on the lower, helping them secrete more. This probably explains why we blink down, why our upper eyelids are longer than our lower eyelids. I always wondered about this when I was younger, but to be fair I was a bit of a weird kid.

Glands of Meibomius, from: Lozato, P. A., P. J. Pisella, et al. (2001). "Phase lipidique du film lacrymal: physiologie et pathologie." Journal Francais D Ophtalmologie 24(6): 643-658.

Blinking squeezes more of the tear lipid secretions out of the glands, and the physical action of the eyelids smoothes it evenly across the eye, so your TLL is refreshed every time you blink. Like so much else your glands of Meibomius become less efficient as you get older, so the average adult needs to blink approximately every 20 seconds to refresh their TLL but babies can go more than a minute without blinking. This probably explains why they always have such a look of bug-eyed astonishment. If you go too long without blinking the TLL will eventually break up, allowing the water layer to evaporate and so bringing the TLL down into contact with the mucins which, in scientific terminology, buggers everything up. This leads to dry, uncomfortable eyes. So blink more people.

Did I mention incidentally that oxygen can get through the whole tear film to the cells on your cornea? Oxygen can get through the whole thing to the cells on your cornea. Isn’t that cool?

Having discovered how amazing your eyes are I am now horrified that anyone wears eye makeup. Eyeliner and mascara are full of foreign fats and oils that can get incorporated into the TLL, disrupting it. Absolutely the worst thing you can do though is use eyeliner inside the lip of your eyelid, as this can block the glands of Meibomius. If this happens there’s a good chance you’d be able to clear them with massage, but if that fails there’s a procedure I read about. Actually I read the first sentence of the paragraph then squealed, jumped back and quickly turned the paper over so I couldn’t read any more. As I was on the tube at the time this resulted in a bit of extra leg room as people shuffled away from me, but I think you get the message. Down with the patriarchy ladies. Ditch the eyeliner, your tear lipid layer will thank you.

Her glands of Meibomius probably aren't very healthy.

*OK the store part of her website doesn't seem to be up yet. Link to follow

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