Spare Parts
Page 11
During the day, when you are awake and your eyes are open, they dry quickly, which is why it’s hard to keep them open for long. Try staring at your cat and see who blinks first (one trick to win the bet is to stare not at her eyes, but at her nose). Luckily, our eyelids open and shut quickly to blink and wash and lubricate the eyes with tears as many as 15 to 20 times a minute, which works out to 1,200 blinks an hour and a whopping 28,800 every single day.
That’s a lot of blinks. Science being what it is, you just know that someone somewhere decided to find out why we blink more often than seems necessary. In 2012, four investigators at Japan’s Osaka University, the Center for Information and Neural Networks (CiNet), and the National Institute of Information and Communications Technology set up a study to see what happened when a group of volunteers watched television while inside an MRI. Tracking the ebb and flow of their volunteers’ brain activity, they concluded that “[b]ecause eyeblinks tend to occur at implicit breakpoints while viewing videos, we hypothesized that eyeblinks are actively involved in the release of attention. We show that while viewing videos, cortical activity momentarily decreases in the dorsal attention network after blink onset but increases in the default-mode network implicated in internal processing. In contrast, physical blackouts of the video do not elicit such reciprocal changes in brain networks. The results suggest that eyeblinks are actively involved in the process of attentional disengagement during a cognitive behavior by momentarily activating the default-mode network while deactivating the dorsal attention network.”28, 29
In plain English, that says our eyeblinks seem to be punctuation marks. For example, as you read this paragraph, you are likely to blink at the end of a sentence as a kind of reflex that gives your brain a time-out in which to think over what you’ve just read before you go on to the next sentence. The same thing happens, say the Osaka authors, when you’re listening to a speaker and she pauses for a moment, or when you are watching a movie or a video and the action comes to a momentary halt. This nanosecond pause is so integral to our lives that we have even incorporated it into our common vocabulary like this:
“The story of life is quicker than the blink of an eye, the story of love is hello, goodbye.” Jimi Hendrix
“If you look at how long the Earth has been here, we’re living in the blink of an eye. So, whatever it is you want to do, you go out and do it.” Jamie Foxx
“Everything can change in the blink of an eye. But don’t worry; God never blinks.” Regina Brett
“I think the life we are living now is just a blink in the eye of eternity.” Keir Dullea 30
Of course, those are complete blinks, moments when the lids come together and then fly apart in, as Hendrix puts it, the blink of an eye. There are other versions of the up-and-down dance such as the hamster’s trick of blinking one eye at a time, or the human wink that says, “I was only kidding.” And then there is deliberately lowering the upper lids halfway and holding them there, a maneuver is known as “bedroom eyes” or the “come hither look” whose reputation as a romantic signal dates to the turn of the 20th century. Back then, it seemed so devastating that on September 5, 1905, the city of Houston, Texas, passed an ordinance stating that “any male person in the City of Houston who shall stare at, or make what is commonly called ‘goo-goo eyes’ at, or in any other manner look at or make remarks to or concerning, or cough or whistle at, or do any other act to attract the attention of any woman or female person upon or traveling along any of the sidewalks, streets or public ways of the City of Houston, with the intent or in a manner calculated to annoy, or to attempt to flirt with any such woman or female person, shall be deemed guilty of a misdemeanor, and, upon conviction thereof in the Corporation Court of the City of Houston, shall be fined in any sum not exceeding One Hundred Dollars.”31 Obviously, what happened in Texas stayed in Texas because movie stars from Rudolph Valentino in the 1920s to Robert Mitchum and Lizbeth Scott in the 1950s continued to employ the half-mast lid to suggest sexual intimacy right through to the moment when the Texans gave up and repealed the statute.
Today, it is entirely legal for Leonardo di Caprio, another fan of the lowered lid, to bring his sexy eyes to Houston, although they may not be as sexy as he thinks they are. In 2012, University of Michigan psychologists Daniel Kruger and Jory Piglowski showed two pictures, one of a man with eyes wide open, the other with his lids half-closed, to more than 400 male and female student volunteers.
As a rule, wide-open eyes are characteristic of all young mammals; the older the animal, the apparently smaller the eyes. No, the size of the eyeball doesn’t change; the difference lies in horizontal eyelid fissure, the opening between the upper and lower lids, which becomes slightly smaller as we grow older.32 Logically, therefore, you might conclude that those sexy lowered lids make us look more mature and therefore more trustworthy. Not a chance. The responses to the Kruger/Piglowski study, published in the journal Personality and Individual Differences, were absolutely clear and certain. Most of the girls said they’d choose the guy with the open eyes as a father for their child. More than seven of every ten boys picked the open-eye guy as a preferred partner or someone they’d trust to spend time with their girlfriends. In short, if you want to be trusted, don’t lower that lid: for both men and women, wide-open eyes translates to openly trust-able.33
Now, having done with our two eyelids, let’s consider our third. If we actually had a third eyelid. Which we did. And still do.
THE VERY HUMAN VERSION OF A THIRD EYELID
In 2004, having examined the eyes of eleven fetuses, two newborn babies, and one old man, a research team at the University of Lübeck (Germany) Institute of Anatomy announced that early in fetal development we humans have a skin fold on the eye that looks just like a nictitating membrane. The fold has epithelial (skin) cells top and bottom. In between are immature nerve fibers, blood vessels, and glands. At first, this fold covers most of the eye, but as our eyes and eyelids develop, the fold does not. At birth, what remains is the plica semilunaris conjunctiva, a half-moon shape fold of membrane sitting behind and extending a few millimeters out to the side of the caruncle, that small red dot or bulb at the inner corner of your eye.34
Look into a mirror—better yet, a magnifying mirror—and you may be able to see a bit of your own plica. If not, here is what it looks like in fig. 892, one of the 1,247 extraordinary anatomical drawings from the 20th American edition of English anatomist’s Henry Gray’s classic Anatomy of the Human Body (Philadelphia: Lea & Febiger, 1918):
The Human Plica Semilunaris, Henry Gray, Anatomy of the Human Body
Like your cat’s nictitating membrane, your plica semilunaris conjunctiva performs several important tasks. It separates the surface of the eye from the eyelid so the lids move smoothly rather than catching on the eyeball (if, by chance, the membrane were to be scarred, the movement of your eyeball might be affected). The lining of the plica holds goblet cells that, like the meibomium glands inside the edges of your top and bottom eyelid, secrete oily liquid that mixes with rheum, the watery liquid secreted by mucous membranes in your eyes and nose and mouth. While you’re awake, these secretions help to wash and lubricate your eyes and are then themselves swept away when you blink and send tears flowing across the eye. While you sleep, however, your rheum bundles detritus such as dust, blood cells, skin cells, and mucus into gound, the gummy yellow-y stuff sometimes known as “sleep” or in the lingua franca of the very young, “eye boogers.” These clumps of matter make your eyes feel sticky when you wake, but a warm, moist wash cloth can wipe gound away, and a pleasantly steamy shower will dissolve it. After that, your plica semilunaris can return to its basic day job, helping to keep your eye moving smoothly back and forth as you watch the world around you.
So is the plica semilunaris vestigial? Yes and no. When we take the time to look, we can see evolutionary changes all around us that produce new parts or adapt old ones or make pieces of the individual disappear, leaving nothing at all in their wake. Fo
r example, all fish have eyes, except for the blind tetras living in dark Mexican caves that didn’t need them and thus developed ways to “see” without them. Every bird currently living on earth cuts/chews/mashes its food with the beak that replaced the teeth common to the dinosaurs from which they evolved. But we neither lost nor replaced the full edge-to-edge nictitating membrane, because we never had one. True, some sources say that one or two primates such as lemurs do have a fully developed third eyelid. Others, however, insist that throughout the animal kingdom, only one species of primate, the Calabar angwantibo, a.k.a. the Calabar potto, a member of the Loris family living in the rain forests of West Africa, does. Certainly, none of the higher apes, including the orangutan with which we share so much of our DNA, do.35, 36 The closest we come to having one occurs early in our embryological life, when, as you know, membrane does cover part, but never all, of the developing eye and then pulls back into the normal human plica semilunaris present at birth.37
In the end, what we have is what we and our closest primate relatives have always had: two semiopaque eyelids that go up and down and one small but still useful transparent tissue, a simple membrane simply tailored to our standing-up-and-walking-on-dry-land-and-wearing-protective-glasses human lives.
TWO EYES. FOUR LIDS. WHAT COULD POSSIBLY GO WRONG?
Like every other body part, the eyelid is subject to its own particular problems beginning, in alphabetical order, with the birth defect ablepharia (from the Latin prefix ab- meaning away from and the Greek blephar meaning eyelid), an absent eyelid or an eyelid smaller than normal. Blepharitis is a staph infection of the lid, often at the edge where the lid meets the lashes. Blepharospasm is an involuntary twitch of the levator muscle controlling the lid. Chalazion (from the Greek khalazion meaning small lump) is a small lump. The result of an obstruction of an oil gland in either the top or bottom lid. A chalazion may resolve on its own or after treatment with warm compresses or steroid injections. If that fails, the lump may be surgically removed. Ectropion (from the Greek ek + trepein meaning turning outward) is a generally age-related outward folding of the eyelid that may cause excess tearing or hardening of the surface of the eye. Its opposite, entropion (from the Greek prefix en- meaning inward) is also usually related to advancing age, but it may also be a congenital defect or caused by a scar on the inside of the lid or the result of a chronic spasm of the eyelid. Hordeolum (from the Latin hordeum meaning barley), a.k.a. the reddish bump on the lid commonly known as a stye,38 is another staph infection, this time of a sebaceous gland. Laxity is a loosening of the lid that allows it to pull away from the eyeball. If the laxity is severe enough to pull the lid far enough away so that it creates a kind of pocket called the Fornix of Reiss, laxity may require surgery to protect the eye by tightening the lid. Ptosis (the Greek word for falling) is a sagging eyelid that may occur in one or both eyes. Like ectropion and entropion, ptosis may be linked simply to aging. Or it may be due to weakness or paralysis of the levator muscle that lifts and lowers the lid caused by any one of a variety of medical problems ranging from diabetes to Bell’s palsy to myasthenia gravis or migraines or a brain tumor. And of course, like the rest of the skin, an eyelid may develop dermatitis, redness, or a rash or other reaction to an irritant such as the shampoo, shaving cream, eye shadow, aftershave, mascara, or any of the many products we toss at our faces every day. And the lid may swell if injured or, like all your other tissues, give birth to a tumor, benign or otherwise.
EYELIDS OF THE WORLD
Animals with no eyelids
Insects
Animals with one transparent eyelid
Snakes*
Fish
Animals with two eyelids
Humans**
Primates***
Some lizards****
Animals with three eyelids
Aardvarks
Bald eagles
Beavers
Camels
Cats (including the Big Ones)
Cormorants
Crocodilians*****
Dogs
Ducks
Frogs
Lizards
Owls
Polar bears
Seals
Salamanders
Sharks******
* The snake’s one eyelid, a layer of transparent skin, is called a brille or ocular eye cap.
** Not counting the plica semilunaris conjunctiva because it does not cover the entire eye.
*** Other than some lemurs and the lorisoid Calabar angwantibo.
**** One clear lid to shield an open eye; one opaque lid to close the eye.
***** Alligators, caimans, and crocodiles.
****** Some sharks have nictitating membranes; others, such as the great white, protect the eyes simply by rolling them back into the head.
[from Stephens, Christina, “What animal has three eyelids?” Pawnation.com, http://animals.pawnation.com/animal-three-eyelids-1300.html & “Ten most incredible eyes in the animal kingdom,” Scribo, http://scribol.com/environment/10-most-incredible-eyes-in-the-animal-kingdom/2]
6
Pearly Whites
Wisdom Teeth
“Every tooth in a man’s head is more valuable than a diamond.”
—Miguel de Cervantes, Don Quixote (1605)
“It appears as if the posterior molar or wisdom-teeth were tending to become rudimentary in the more civilised races of man. These teeth are rather smaller than the other molars, as is likewise the case with the corresponding teeth in the chimpanzee and orang; and they have only two separate fangs. They do not cut through the gums till about the seventeenth year, and I have been assured that they are much more liable to decay, and are earlier lost than the other teeth; but this is denied by some eminent dentists. They are also much more liable to vary, both in structure and in the period of their development, than the other teeth. In the Melanian races [people with dark skin], on the other hand, the wisdom-teeth are usually furnished with three separate fangs, and are generally sound; they also differ from the other molars in size, less than in the Caucasian races. [German anatomist and anthropologist Hermann] Schaaffhausen accounts for this difference between the races by ‘the posterior dental portion of the jaw being always shortened’ in those that are civilised, and this shortening may, I presume, be attributed to civilised men habitually feeding on soft, cooked food, and thus using their jaws less. I am informed that it is becoming quite a common practice in the United States to remove some of the molar teeth of children, as the jaw does not grow large enough for the perfect development of the normal number.”
—Charles Darwin, The Descent of Man
FISH TEETH & DARWIN’S FINCH
Ask the first ten biologists you meet on the street where life on earth began, and nine are likely to say, “The sea.” But if the tenth man you run into is University of Oregon paleontologist and geologist Greg Retallack, he just might mention the serious possibility that life on earth began on earth, that is, on land. His proof? The Ediacara biota.
Biota is the collective name for all the plants and animals in a specific region such as the European continent or geological time period such as ours, the Cenozoic Era. The biosphere, which OxfordDictionaries.com defines as “the regions of the surface, atmosphere, and hydrosphere of the earth (or analogous parts of other planets) occupied by living organisms,” includes all the biotas on the planet. These animals (and plants) come in two basic varieties: Prokaryotes and eukaryotes. The first group, comprising cells without a membrane-enclosed nucleus, includes the entire world of bacteria from the good ones that turn milk into yogurt to the bad ones such as E. coli, which famously causes foodborne illness. Members of the second group—everything else, even single-cell organisms such as yeasts and amoebas—have cells with a membrane-enclosed nucleus that holds genetic material and organelles, tiny specialized structures that perform specific functions such as metabolizing proteins or absorbing energy from light. The appearance of multicelled organisms was a significant moment that d
ramatically altered life on earth by producing complex entities whose cells worked together to complete complex processes rather than simply absorbing and using energy to stay alive. In a fascinating article on Astrobio.net, science reporter Charles Q. Choi notes that understanding the startling evolution from prokaryote to eukaryote and from single to multiple cells “could shed light on how complex extraterrestrial life might evolve on alien worlds.”1
Back here on earth, the Smithsonian National Museum of Natural History explains that “[t]he oldest known possible multicellular eukaryote is Grypania spiralis, a coiled, ribbon-like fossil two millimeters wide and over ten centimeters long. It looks very much like a coiled multicellular alga and has been described from banded iron formations in Michigan 2.1 billion years old. Grypania may not be a eukaryote, but another, unrelated colonial eukaryote, Horodyskia, is known from sedimentary rocks dated at 1.5 billion years in western North America and from rocks more than 1 billion years old in Western Australia. The earliest-known occurrence of multicellular animals is the Ediacaran fauna.”2