Optical lubrication
Evan Zabawski | TLT From the Editor August 2015
A teary-eyed tale.
When discharge dries and accumulates, we often refer to it as sleep and it feels gritty as we rub it away from our eyes.
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WHEN WE USE THE TERM “BLINK OF AN EYE,” we refer to a minuscule amount of time. In reality, the tasks accomplished with each blink suggest that the term could be used to describe efficient, multifaceted lubrication.
Blinking distributes tears on our eyes; this sufficiently hydrates and lubricates the eye to maintain comfort and proper vision. Tears are composed of water (moisture), oils (lubrication), mucus (dispersion) and antibodies and proteins (antiseptic). Healthy tears form three distinct layers.
The first layer is the mucus that is produced by the conjunctiva, a thin membrane that lines the white of the eye (sclera) and the inner surface of the eyelids. The mucus is primarily composed of mucins and inorganic salts suspended in water. Mucins provide lubricating properties that allow our eyeballs to move within the eye socket, and provide a foundation layer to help spread the second, aqueous layer.
The second layer is produced by the lacrimal glands, located at the upper, outer portion of the eye socket. This layer is commonly referred to as the tear, while the first and third layers are collectively called rheum. Not only does the second layer maintain hydration of the cornea, it also washes out dust and other foreign bodies and contains the antibacterial enzyme lysozyme.
The third layer, meibum, is composed of oils that lubricate the eyelids and prevent evaporation of the second layer. Meibum is produced by the meibomian glands, named after German physician Johann Heinrich Meibom who discovered them in 1666, which are located along the rim of the eyelids. Lower lids have approximately 25 such glands, and there are twice as many in the upper lid. Meibum prevents tear spillage onto the cheek, trapping tears between the oiled edge and the eyeball, and makes closed lids airtight.
Blinking helps push tears down the eye into ducts that eventually drain into the nasal cavity, which explains why our nose runs when we cry. However, the absence of blinking when we sleep causes the rheum to discharge at the corners of our eyes. When this discharge dries and accumulates, we often refer to it as
sleep, as in having some sleep in your eyes, and it feels gritty as we rub it away from our eyes.
This
sleep has been represented in folklore as magical sand deposited by the Sandman, who is said to use it to bring on sleep and dreams. Hans Christian Andersen wrote of him in 1841 when he said the Sandman visits children and “throws a small quantity of very fine dust in their eyes, just enough to prevent them from keeping them open, and so they do not see him.”
E.T.A. Hoffman, who wrote the darker, original version of The Nutcracker and The Mouse King from which the ballet The Nutcracker is based, wrote a somewhat different interpretation of the Sandman. In Der Sandmann, it is said he throws sand in the eyes of children who won’t sleep, causing their eyes to fall out and be collected by the Sandman, who then takes the eyes to his iron nest on the moon and uses them to feed his children. I think I prefer Andersen’s rendition.
In any event,
sleep is simply purged excess eyeball lubricant with contaminants (in the form of dead skin cells, blood cells and dust) thrown in. Gross but fascinating.
I think it is amazing that tears are able to accomplish so much in literally the blink of an eye. And I can say learning that rheum was once a lubricant has been an eye opener.
Evan Zabawski, CLS, is a senior reliability specialist in Calgary, Alberta, Canada. You can reach him at evan.zabawski@gmail.com.