Our eyes are a fundamental part of the human sensory system -- but they're complex things that can easily go wrong. Here are 23 facts about your windows to the world, including 11 things that can go horribly wrong.
Eight surprising things about eyes
- Eyebrows: Among the many potential functions of eyebrows, they keep sweat and moisture from dripping into our eyes, send information about local conditions to the nervous system via the hair follicles, detect the presence of dangerous microbes, help us convey emotion, and act as landmarks to help us identify other faces.
- Eyelashes: On mammals with eyelashes, the extensions are always one-third of the eye's width, on average. Why? Based on wind tunnel experiments, researchers believe our aerodynamic lashes help minimise airflow across the eye's surface, thereby protecting it from blowing dust and evaporation of the tear film.
- Eyelids: Each eyelid pair contains roughly 60 meibomian glands, with slightly more in the upper lid than in the lower lid. With every blink, these glands pump out droplets of oil that coat the eye's surface to prevent tear evaporation.
- Blinking: The average person blinks about 16,800 times every day. Based on observations with high-speed cameras, a blink of an eye takes a bit longer than previously thought: spontaneous blinks take about a third of a second and voluntary blinks last half a second.
- The cornea: The transparent cornea is the only body tissue with no blood vessels (which would interfere with sight). Researchers believe the cornea holds the highest concentration of nerve endings on the body's surface, however, which explains why eye pain can be so intense. In fact, injury to a single cell on the corneal surface may be enough to trigger pain.
- Corneal sensitivity: Doctors use a device called an esthesiometer to measure corneal sensitivity; the first one, built in 1894, was made of horsehair. Materials from other animals have helped us even more: shark corneas are similar enough to our own that scientists have long studied their anatomy and physiology. Because shark corneas also resist swelling and thus remain transparent under a variety of conditions, eye surgeons have successfully transplanted them into both humans and dogs.
Tears: The chemical composition of our tears changes depending on why we're crying. The Topology of Tears project by photographer Rose-Lynn Fisher includes magnified photographs of 100 tears triggered by a range of causes, from grief to onions. All are secreted by the lacrimal glands above the eyes and fall into three groups:
· Basal tears act as everyday lubricants and moisturisers and contain the antibacterial proteins lysozyme and lactoferrin.
· Reflex tears rain down in response to pain or irritation, like bright lights or tear gas. Their composition is similar to that of basal tears but has higher concentrations of lysozyme, lactoferrin and other ingredients.
· Emotional or psychic tears flow in response to emotion and can vary in their composition depending on whether we're happy or sad. Tears of sorrow, for example, contain stress-related hormones and a natural morphine-like painkiller.
- Goblet cells: Shaped like champagne flutes, these cells are scattered throughout the white part of the eye and the lining of the eyelid where they secrete the major ingredient of mucus (we also have these cells in our gut). This sticky mucus, in turn, acts as the base to which tears can adhere and spread out evenly.
Eleven eyebrow-raising conditions
- Bell's palsy is a type of facial paralysis, linked to damaged or traumatised nerves, that usually affects one side of the face. The condition was first described by a Scottish doctor in the 19th century. It can paralyse the eyelids and prevent them from blinking, potentially leading to severe dry eye symptoms and blindness.
- Conjunctivochalasis is a common age-related condition that causes the conjunctiva, the tissue that covers the white of the eye, to begin bunching up. "Like when ladies wear pantyhose and they bunch around the ankles, that's what the conjunctiva does to the bottom part of the eye," says Miami ophthalmologist Anat Galor. These excess tissue folds can block tear ducts and contribute to dry eye disease.
- Corneal ectasia refers to a group of related vision-threatening conditions, some of which are complications of procedures such as LASIK (laser in-situ keratomileusis; a type of laser eye surgery). In keratoconus, one type of ectasia, the eye's lattice of supportive collagen fibres begins to weaken, causing the cornea to bulge outward and become cone-shaped. As a result, patients can have blurry vision and light sensitivity.
- Dry eye disease is an umbrella term describing conditions that may involve disruption of the corneal surface and tear film, which covers the eye. Researchers are still debating the key contributors to the main symptom: dry-feeling eyes. Dry eye is a common symptom after LASIK. The Tear Film & Ocular Surface Society suggests that dry eye discomfort might also be an important factor in the decision to stop wearing contact lenses, made by 17 to 71 million people around the world every year, out of a total of 140 million contact lens wearers.
- Floppy eyelid syndrome was first described in overweight patients and is often associated with obstructive sleep apnoea. This condition refers to an upper eyelid that can easily turn inside out, leading to chronic irritation and inflammation.
- Keratitis, or inflammation of the cornea, can be caused by injury, by wearing contact lenses for too long or by a variety of infectious bacteria, viruses, fungi, amoebae and parasites. Lack of prompt treatment can lead to corneal ulcers and blindness owing to scarring.
- Meibomian gland dysfunction, believed by many ophthalmologists to be the leading cause of dry eye, results from clogged or inflamed glands that no longer produce enough oil to coat the outer surface of the tear film. This causes excessive tear evaporation that may trigger pain signals in the cornea.
- Ocular neuropathy, a term coined by Boston ophthalmologist Perry Rosenthal, refers to eye pain linked to dysfunctional nerves just below the surface of the cornea that trigger a pain-based alarm as an overreaction to perceived threats to the tear film.
- Oculofacial neuropathy, a term also coined by Rosenthal, refers to pain generated by damaged or dysfunctional pain pathways in the brain that then radiates out through the face's three-part trigeminal nerve system. Along with sharp eye pain, patients may suffer from light sensitivity, jaw pain, migraines, and other head and facial symptoms.
- Sjögren's syndrome (pronounced SHOW-grins) can accompany autoimmune disorders such as lupus or rheumatoid arthritis. It leaves patients with a dry mouth and eyes because of decreased production of saliva and tears.
- Stevens-Johnson syndrome is a severe and sometimes fatal allergic reaction to drugs or infection that can leave a patient's skin and mucous membranes in the throat, nasal cavity and even corneas covered with painful blisters.
Four eye-watering remedies
Eyesalve: A recently rediscovered tenth-century 'eyesalve' potion, recorded in a leather-bound manuscript called Bald's Leechbook that had been kept in the British Library, not only clears up styes but has also proven highly potent against the dreaded superbug MRSA.
"Absolutely dumbfounded" scientists tested it as part of the University of Nottingham's AncientBiotics Project, but home chemists might find it tough to make. The highly specific recipe calls for two species of the Allium plant (the genus that includes garlic and leeks), wine and bullocks' gall -- bile from a cow's stomach -- to be brewed in a brass vessel, aged for nine days, strained through a cloth and applied to the eye with a feather.
Unclogging blocked glands: At first glance, some treatments for clogged meibomian glands closely resemble medieval torture devices. A patient has compared one relatively crude and painful method to expressing a dog's anal glands: it involves slipping a small stainless steel spatula called a Mastrota paddle between the eye and eyelid and squeezing out the toothpaste-like secretions(doctors initially used a cotton swab). For a separate method, called a Maskin probe, a doctor manually inserts a needle into each gland or uses what look like tweezers with small rolling pins on the ends to squeeze the lids and express the glands.
North Carolina-based pharmaceutical company TearScience has developed a separate therapeutic treatment called LipiFlow. Patients wear an eyepiece that heats and massages the inner eyelids to remove the clogs. Company co-founder and optometrist Donald Korb says the 12-minute treatment has relieved the eye pain of many patients; however, the $2100 treatment often has to be repeated every year or two.
Scleral lenses: First developed in Germany and Switzerland in the 1880s and made from blown glass, scleral lenses rest on the eyes' sclera (the white tissue surrounding the far more sensitive cornea). The first models smothered the eyes and could only be worn for short periods. More sophisticated versions, made a century later from breathable acrylic, act like reservoirs to bathe the eyes in pools of oxygen-rich artificial tears and prevent any evaporation.
The Boston Scleral Lens (now called PROSE), which was created by ophthalmologist Perry Rosenthal, has helped patients who were legally blind from corneal diseases to regain their sight. It has also alleviated the chronic pain of many patients with severe dry eye symptoms, including some with post-LASIK complications and others who lost the ability to completely close their eyelids after plastic surgery.
- Moisture goggles: How-to instructions for making moisture goggles or chambers date back to 1946, but recent studies suggest that these lined glasses can minimise irritation from exposure to air and keep eyes from drying out. To reduce the stress on damaged corneas, some doctors have asked their patients to wear the goggles 24 hours a day. Given the expense of newer models, some patients have put their own spin on the old-school approach through DIY videos on how to make moisture chambers with cling film and surgical tape.
This article first appeared on Mosaic and is republished here under Creative Commons licence.
Image by Michele Catania under Creative Commons licence.