Charles Darwin was no fool, nor was he averse to considering new evidence. Remembering his calm and civil approach to Richard Owen’s multiple unpleasantries regarding who first came up with the idea of natural selection (Chapter 5), were he still with us and were someone to hand him a copy of Miller’s paper, it’s a good bet you might expect to hear him say: “Pretty good for muscles I once thought to be of no use at all.”
5
Blink!
The Third Eyelid
“If you stared deep into a cat’s eyes, you would be able to see into the world of spirits.”
—English Proverb
“The nictitating membrane, or third eyelid, with its accessory muscles and other structures, is especially well developed in birds, and is of much functional importance to them, as it can be rapidly drawn across the whole eyeball. It is found in some reptiles and amphibians, and in certain fishes, as in sharks. It is fairly well developed in the two lower divisions of the mammalian series, namely, in the Monotremata and marsupials, and in some few of the higher mammals, as in the walrus. But in man, the Quadrumana, and most other mammals, it exists, as is admitted by all anatomists, as a mere rudiment, called the semilunar fold.”
—Charles Darwin, The Descent of Man, Chapter 1
WHAT RICHARD OWEN WROTE
Richard Owen (1804–1892) was not a pleasant man. By all accounts, he was not a pleasant child, either. In grammar school, his teachers called him lazy and impudent. As an adult, he was described as vain, arrogant, envious, and vindictive.1 But unpleasant does not necessarily translate to stupid, which Owen certainly was not. Entering the University of Edinburgh medical school in 1824, the 20-year-old student dismissed the standard anatomy courses as inadequate and chose instead to attend to the extramural lectures given by John Barclay, the distinguished anatomist from whom both Owen, and later Charles Darwin, learned what was there was then to learn about comparative anatomy.2
Once he left Edinburgh, Owen quickly secured a position at the Royal College of Surgeons’ Hunterian Museum in London as an assistant whose job it was to identify, label, and catalogue every single one of the more than 13,000 human and animal anatomical specimens in its collection. The job soon done, Owen was promoted to lecturer at the Museum, speaking to audiences of Victorian royalty and notables such as Darwin, who had recently come back from his journey to the Galapagos, bringing vertebrate fossils Owen was at that very moment examining.
But Owen’s climb to true eminence began when he joined the staff of The British Museum in 1856 as Superintendent of the natural history collections, which be believed deserved a home of their own someplace else.3 In a happy quirk of fate, just as Owen arrived at the Museum, Antonio Panizzi was named Principal Librarian. Panizzi loathed science in general and natural history in particular, so he enthusiastically supported Owen’s proposal to move the science exhibits to another location.4 Accordingly, in 1873 after the requisite negotiations and after Francis Fowke, the architect who designed the Royal Albert Hall and parts of the Victoria and Albert Museum, was named winner of the competition to design the new structure, ground was broken in South Kensington. The British Museum of Natural History opened in 1881; eighty-two years later it became the fully independent Natural History Museum right across Exhibition Road from the Victoria and Albert.5
Despite his endorsement of the racism of his day,6 Owen was a serious scientist. He coined the word dinosaur by combining deinos and sauros, the Greek words for terrible and lizard. He was also the first to identify and describe homology, the structural similarity of various parts of different animals such as a cat’s paw and a human hand or a bat’s wing and a human arm or a grasshopper’s leg and a human’s and so on and on and on.
Like Plato, Owen believed there was a set of blueprints that included an archetype for each and every vertebrate. The plan, he thought, spelled out each and every basic characteristic of each and every specific individual such as a fish or a mouse or a lion or one of us. But Plato’s archetype existed on a more or less spiritual realm of inaccessible perfection. Owen defined his as a vision in the mind of a God who was able to see all possible variations.
Then his vision changed. Or maybe it sharpened.
In 1860, Owen published a scathing review of On the Origin of Species in the Edinburgh Review. His aim was to demolish Darwin’s argument of evolution by natural selection. Fair enough. But soon, through a series of surprising twists and turns, Owen ended up claiming to have laid out what he called the theory of a “continuous operation of the ordained becoming of living things”—a phrase that more or less mystified all who heard or read it—ten years or so before the words natural selection arose in Darwin’s intelligent brain or tripped off his tongue.7
Well, no. Or at least not so clearly that anyone had heard him say it out loud. On the other hand, Owen could legitimately claim one important first. He was absolutely the very first to write about the nictitating membrane, which he mentioned in On the Anatomy of Vertebrates (1866), five years before Darwin’s The Descent of Man (1871).8 And how that played out offers an interesting comparison between the two men.
If Owen was not pleasant, Darwin appears to have been just the opposite, a steady, even-tempered man sometimes characterized as a “plodder” who thoroughly examined each thought and fact before leaping on to the next. He was also a warm and loving father and family man. In Victorian England, where children were handed over to governesses and fathers were generally remote, Darwin’s relationship with his six boys and four girls was warm and loving. “To all of us,” one of his daughters later wrote, “he was the most delightful play-fellow, and the most perfect sympathizer…. He cared for all our pursuits and interests, and lived our lives with us in a way that very few fathers do. Whatever there was best in us came out in the sunshine of his presence.”9
He was equally accommodating with his persistently difficult critic.
In his autobiography, Darwin wrote that Owen’s enmity blossomed after the publication of On the Origin of Species, “not owing to any quarrel between us, but as far as I could judge out of jealousy at its success.” That notwithstanding, The Descent of Man generously and accurately credits Owen’s Anatomy of Vertebrates in multiple citations throughout the book, including Owen’s comments on the third eyelid: “The nictitating membrane, or third eyelid, with its accessory muscles and other structures, is especially well developed in birds, and is of much functional importance to them, as it can be rapidly drawn across the whole eyeball,” the entry in Origin reads. “It is found in some reptiles and amphibians, and in certain fishes, as in sharks. It is fairly well developed in the two lower divisions of the mammalian series, namely, in the Monotremata10 and marsupials, and in some few of the higher mammals, as in the walrus. But in man, the Quadrumana,11 and most other mammals, it exists, as is admitted by all anatomists, as a mere rudiment, called the semilunar fold.”12
Darwin’s generosity had absolutely no effect at all on Owen, who remained unappeased and unpleasant. That was a pity because he and Darwin were equally right in their observation of the animals and equally not-quite-right about the human eye.
LOOKING THROUGH THE LID
Dogs have masters. Cats have staff. And as every cat staff person knows, cats blink to express love or, if you stare them down, submission.13 We blink, too, but not as efficiently because while they have three complete eyelids, we have only two and a much smaller version of a third. If we did have eyes like cats and dogs and camels and polar bears and seals and, yes, aardvarks, companies marketing “tearless” shampoos would be out of business because what those animals have that we don’t is a nictitating membrane (from the Latin verb nictare meaning to blink).14
This third lid is a transparent fold of tissue covered with conjunctival (mucous) membrane and lying between the surface of the eye and the up-and-down eyelids. Like the semiopaque lids, this third lid has glands and small ducts through which it releases liquids onto the eye. For some animals, this pathway may provide a
s much as half of all the tears that moisten the eye and flush it with naturally occurring substances that protect against the equally naturally occurring microbes found on all body surfaces. When deployed, the nictitating membrane slides horizontally across from the corner of the eye nearest the nose to cover the entire eyeball without interfering with vision. This makes it possible for amphibious animals such as frogs and diving animals such as sea lions to see as clearly underwater as on dry land. In the air, the third eyelid keeps floating debris from crashing into the sky-diving predator’s eyes while ensuring the keen vision that enables an owl or a falcon to spot a moving mouse as much as two hundred feet away. On land, the third lid appears to cushion a woodpecker’s retina when the bird bangs its beak against a tree, keep sand out of a camel’s eyes, and filter ultraviolet (UV) light, thus reducing a polar bear’s risk of snow blindness.15, 16, 17
Some animals can voluntarily control the movement of their nictitating membrane. But that cat curled up on your pillow or that dog barking to be taken out for a walk? Not so much, because their nictitating membranes have fewer muscles than you’d find, for example, in an eagle’s. As a result, your pet is unlikely to favor you with a glimpse of his or her third eyelid unless you force the issue by blowing a puff of air at the animal’s eye or your vet presses back the lid during a medical examination. Early in the 20th century, some veterinarians recommended removing the third eyelid to make it easier to examine the cat. Bad idea, says veterinarian Paul E. Miller, a Diplomate of the American College of Veterinary Ophthalmologists (ACVO). Losing the third lid—to an injury or when treated for a malignant tumor—often leads to chronic irritation of the cornea or the ophthalmological problem known as cherry eye, a swelling and/or inflammation of the lid itself. In fact, Miller says, the nictitating membrane is so important and so common that the real question is not “‘[w]hy do cats [and dogs] have a third eyelid?’ but ‘Why don’t people?’”18
Actually, we do. It’s called the plica semilunaris conjunctiva, and, like the nictitating membrane, it is valuable indeed.
WHAT YOU SEE IS WHAT WE’VE GOT
If our eyes are the window to the soul, our eyelids are the curtains we open and shut at will to keep our inner selves secret. Less poetically, our eyelids are simply very thin pieces of skin that along with the lashes keep the eye free of dust and other floaters although they exist mainly to keep our eyes moist.
The top-most layer of your eyelid is the thinnest skin on your body, 0.05 mm thick, which means about 30 times thinner than the protective skin on the bottom of your feet. Next up, or down, is the orbital septum, a membrane overlying the orbital fat pad and the three muscles (levator palpebrae superioris, levator aponeurosis, Muller’s muscle) that move the top lid up and down. After that, you find the tarsal plate, closely woven connective tissue that gives the eyelid its structure and strength. Inside this, aligned with the follicles from which your lashes grow, are sebaceous glands, a.k.a. meibomian or tarsal glands, which secrete lipids (fats) that mix with watery fluid that runs down from tear glands above your eye through tiny ducts in the lid to the surface of your eye. The upper lid has four nerves—the infratrochlear, supratrochlear, supraorbital, and lacrimal—each sending sensory signals such as temperature and touch. The lower lid makes do with branches of the trigeminal nerve, the one that sends sensation to your entire face. Blood flows to both lids from arterioles (small branches of the carotid artery) to even smaller capillaries through which oxygen and nutrients pass into the eyelid tissues and waste products are eliminated. Finally, there’s the palpebral (the Latin noun for eyelid ) conjunctiva, the slick membrane that lines the back of your eyelid and covers the white part of your eyeball.
THE HUMAN EYE UNMASKED
Experts may differ by a few days one way or the other on how your eyelids become eyelids, but in general, this is how it goes in the weeks and months after conception:
3 weeks after conception: To begin the process, a gene called PAX6 tells epithelial cells that build your skin and connective tissues to start to construct your eyes.
4 weeks after conception: You now have two small cavities called optic vesicles, small sacs filled with fluid, one on each side of the front part of your developing brain. These optic vesicles will eventually become the optic cups, the frame that holds a retina, an iris, and a ciliary body (muscles and cells that secrete the thick-ish transparent fluid called aqueous humor).
5 weeks after conception: The lenses of your eyes begin to form at the front of the optic cup.
7–8 weeks after conception: Your eyes now look pretty much like eyes. Very small folds at the top and bottom are the first small signs of lids. They will stretch out and grow as you develop mesenchymal tissue, an embryonic form of the connective tissue that joins, supports, binds, or separates all the other tissues or organs in your body.
9 weeks after conception: Your eyelids, which until now have been separate, fuse together at the edges to enable the formation of interior structures such as the tarsal plates (tightly woven connective tissue) and the follicles from which your lashes will emerge.
12–18 weeks after conception: The muscles that move the lids, the meibomium glands, tarsal plates, eyelash follicles, and the fat pads that support the eye and fill the lid, as well as the connections to blood vessels that will bring blood into the eye, make their appearance and begin to grow.
20 weeks after conception: Your eyelids separate, and your open eyes are clearly visible on an ultrasound picture.
22 weeks after conception: Your eyelashes are beginning to poke out of their follicles.
24 weeks after conception: The development of the tarsal plate and tarsal glands continues, and the muscles that will control the eyelids begin to establish connections.
27–30 weeks after conception: You have visible eyelashes and eyebrows, and your eyelids can open and close.
28 weeks after conception: The tarsal gland now stretches the length of the tarsal plate, and the ciliary process has become part of the layer underneath the skin of the upper lid, while your eye muscles attach to tendons to complete the circuit that will allow them to move the eye.
32 weeks after conception: More stretching and connecting to complete the lids.
36 weeks after conception: With all their parts in place and functioning, your eyelids are ready to open on your very own brave new world.19, 20
Of course, nothing human ever proceeds in a totally straight line, so you will not be surprised to read that as eyelids develop, some differences do, as well. For example, on average the shape of a Black African’s eye is slightly more rectangular than a Caucasian’s. The African person’s eye itself—the round globe—is likely to protrude out farther from its boney orbit. The intercanthal distance (the space between the inner corner of the left eye and the inner corner of the right) is longer, and one study suggests that the “normal Mexican” (no racial characteristics given) eyeball protrudes less than does the “normal” Black or Caucasian one.21, 22
The most visible structural difference among human eyelids is the epicanthic fold (epi from the Greek, meaning on, over, above, and canthus from the Greek kanthos meaning the corner of the eye), also known as the plica palpebronasalis (from the Latin plica meaning fold and palpebra meaning eyelid). Unlike the more common “double eyelid,” with its horizontal crease between eye and eyebrow, the epicanthic “single” eyelid is smooth all the way down to the lashes. This smooth lid is common among fetuses of all ethnic persuasions, but most develop the eyelid crease by the sixth month of pregnancy. Some babies are born with the smooth eyelid, but again most soon develop the crease in the middle. The exceptions are those whose epicanthic fold remains through life, primarily people from Asia. But an epicanthic fold is also natural and common among Native Americans, the Inuit, some Arabs, and a few scattered Europeans, most likely Scandinavians and Poles. Epicanthic folds are also linked to several medical conditions. Perhaps six in ten people with Down syndrome have the smooth eyelid, which is why John Lan
gdon Down, who identified the condition, described them with the now-discredited term “mongoloid.” The epicanthic fold may also occur with Zellweger syndrome, a genetic defect that causes a reduction in myelin, the material that sheathes nerve cells; Turner syndrome, a missing or incomplete X chromosome in women; phenylketonuria, an inherited heightened level of the protein phenylalanine that may lead to intellectual disability; and last but by no means least, fetal alcohol syndrome.23
For healthy people, the epicanthic fold is no physical problem, just one more interesting human difference. Some people, more likely female than male, do consider it a cosmetic problem to be remedied by epicanthoplasty, surgery to produce a double lid fold. A less invasive solution is either “eyelid glue” or a very thin transparent double-sided tape that holds the lid in place when you fold it back into what the Japanese manufacturer of Koji Eye Talk Tuck in Tape promises what will be “a perfect and very natural eyelid with a crease that others will not be able to see.”24 Given the history of cosmetics, including the lethal dusting of lead powder once used to whiten ancient Roman and Renaissance European faces, this seems unnecessary but relatively harmless.
Smooth or folded, our multitasking eyelids are not thick enough to block out all light when you shut them. The evolution-minded among us will note that fish that see light scattered under water have one transparent lid.25 Frogs that live a combination life on land and in water do just fine with one transparent lid and one semiopaque one. Clearly, as we humans moved up onto land we developed our two sort-of-opaque lids to protect against, but not entirely block, bright sunlight.26 Early on, these lids worked quite well, allowing some light in to wake us up at daybreak, ready to make good use of all the light-filled working hours. Then the alarm clock replaced the rising sun, and 24/7 electric light extended our waking hours. But neither triggered the evolution of newer, more solidly opaque eyelids although they do block light to let you slumber on, and in doing so they do protect your sleeping eyes. Your cat or, pace Charles Darwin, your dog may sit right there in front of you snoring away with her (or his) eyes wide open because she (or he) can deploy the nictitating membrane to cover the eye, moisten it, and keep out dirt and dust, which is what your closed eyelids do all night, just as your pet fish, with no eyelids at all, can sink to sleep at the bottom of his tank. But the one in ten of us humans who can actually sleep with his or her eyes open risk facing the dawn with dry, irritated eyes, not to mention a night of constant interruption due to light that may prevent soothing, refreshing rest.27
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