How do these liquids and nerves enable us to see? Descartes took up his ox eye and a scalpel and carefully peeled away the outer layers from the back until it was transparent. Then, he positioned the eye facing outward in a hole at a window in a darkened room. He placed a thin piece of white eggshell at the back, where he had cleared the surface. The bright scene outside was faithfully reproduced upside down and in miniature on the eggshell screen. As he wrote in his account of the experiment: ‘you will see there, not perhaps without admiration and delight, a picture [peinture] which will represent in a strictly artless way in perspective all the objects which are outside.’
This internal image is formed by optical refraction, which explains why it appears upside down. The image of ourselves that we see when we look closely into the black centre of somebody’s eye, on the other hand, is formed by reflection. This little effigy has inspired our word pupil, which derives from the Latin pupilla, meaning a little doll, as well as the charming seventeenth-century colloquialism ‘to look babies at’ somebody, which means to look adoringly into their eyes, a reference not as you might think to the urge to procreate with that person, but to the sight of this diminutive human form. This part of the eye was not called the pupil until the 1660s, however; Descartes describes it using the French word prunelle, meaning a sloe.
I decide to try to repeat the experiment. My prize-winning local butcher, Crawford White, is very tolerant of my occasional odd requests, and does not turn a hair when I ask for eyes of a bull or cow, although he explains he cannot get these for me, presumably because of the dangers of bovine spongiform encephalopathy. But, he tells me, he could have some pigs’ eyes ready for me if I come back later in the day. At home, I gingerly open the little bag I have been given and find four pairs of eyes rolling around inside. Each eye is about the size of a grape, rather smaller than a bovine eye, which I worry may make the dissection tricky. Three-quarters of the spherical surface of the eye is covered by a white layer like a large icecap. The stump of the severed optic nerve protrudes from the midst of this expanse. The front of the eye is clear with glossy black and grey depths.
I pick out one of the eyes and begin by trimming away the flesh and fat that clings to it. Then, squeezing the eye slightly between my fingers in order to give it a firm surface, I begin to cut into the white membrane that protects the clear orb within. It is very tough, and I am nervous of applying too much pressure and piercing the inner membrane with my scalpel. Almost immediately, the worst happens and a gelatinous liquid oozes out of the eye. I take a second eye and start again. The same thing happens. I change my tactics, and try to shave the white tissue away rather than cutting through it. This works better, and by the fourth attempt, I have scraped away enough at the back of the eye that I can just perceive light through the remaining film.
At last, I take the eye over to a large cardboard box that I have prepared. I have cut an eye-sized hole into the front. At the back, I have cut out the shape of an upward-pointing triangle and positioned a bright light beyond it outside the box. I position the eye in the hole ‘looking’ through the box towards the light, and then I position my own eye directly behind, looking at it. I am thrilled to see a hazy image of the triangle pointing downward projected onto the white film.
‘Now, having thus seen this picture in the eye of a dead animal, and having considered the causes, one cannot doubt that it forms a similar whole in that of a living man.’ The eye, Descartes had discovered, works like a camera obscura, projecting an inverted image of the outside world on to its back surface. In his Dioptrique, he provides a ray diagram showing how this happens. It is both clearer and more beautiful than the diagrams I find in those few of today’s anatomy textbooks that concern themselves at all with the physics of how the body actually works. In another version of the diagram, Descartes’s illustrator has drawn the small bearded head of a man in miniature looking up at the back of the eye where the inverted image is formed. He seems like an astronomer gazing at the heavens.
This idea of a homunculus standing at the back of the eye sets up a paradox. For what does this little man gaze with other than his own eyes? Does the soul have eyes as well as the man? It is, says Descartes, ‘as if there were yet other eyes in our brain’. Somehow, anyway, this image is converted into a form that can be transmitted through the brain to the soul, which Descartes locates in the pineal gland. This pea-sized organ is now known to be responsible for the release of the sleep-promoting hormone, melatonin. This gland is indeed sensitive to light, as the release of melatonin is triggered by darkness, but it is not in fact involved in visual perception.
Descartes’s picture of the eye was incomplete as well as flawed – it offered no explanation of our ability to judge the size of things that we gain from having our two eyes spaced apart, for example. But it was revolutionary because it appeared to bring sight – the most mysterious, even mystical, of our senses, linked after all with ‘visions’ as well as straightforward seeing – within the scope of a mechanistic view of the body. Touch, taste and smell involve our physical interaction with the substance of the world. Even hearing, through the time it takes for a sound to reach us, is easy to imagine as some thing arriving at our ears from a distant source. Now sight could be comprehended in a similar way.
As I have pigs’ eyes to spare, I decide to round off my experiment by trying to view an eye in cross-section, producing in reality the diagram that Descartes warns cannot be seen, by taking a bold slice through its equator. I approach the task with trepidation and some sense of horror. Luis Buñuel’s image of a man slicing into a woman’s eye with a cut-throat razor flashes through my head, even though I have never seen the surrealist film from which it comes. (Like Descartes, Buñuel actually used a calf’s eye, as is all too apparent in the shot when I finally see it.) In the moment before I wield the scalpel, I understand why it is that organ donors are often more reluctant to let go of their eyes than they are even their hearts.
Yet when I actually cut into the eye, my perception changes. My knife is not sharp enough, and I cannot help squashing the eye out of shape as I depress the blade, spilling the contents. The worst over, my horror dissipates, to be replaced by fascination. Although they have not held in their correct positions, I can see that there are three distinct transparent liquids: a small quantity of a watery liquid, a larger quantity of liquid like jelly that has not quite set, and, slipping out from between them, a clear bead about the size of a pea. Though soft, it holds a definite shape that is oblate, and flatter on one side than the other. These are the aqueous humour, the vitreous humour and the lens, whose different refractive indices allow us to focus images of the outside world. Animal viscera are revealed as pure Cartesian mechanism. What began as an anatomical investigation has ended as a physics experiment.
Eyes are an important element of our identity. They are said to be the window on the soul. Even in fables of werewolves, the transformed man retains his human eyes. Yet what is it about them that conveys individuality? Colour is their most distinctive attribute. Eye colour was a feature of Alphonse Bertillon’s system of identification for the Paris police, and has been routinely included in official identity records since the introduction of standard passports, where it supplements the likeness offered by a black-and-white photograph. The popular idea that eye colour is important seems likely to be reinforced once again with the introduction of iris-scanning technology to replace document checks.
This is doubly ironic because colour is not what is scanned. Iris scanners in fact use infrared light to detect unique patterns in the iris. And, although the iris is named after the Greek word for rainbow, it may come as a surprise to learn that there is no distinctive colour present in the eye in any case. The colours that we perceive do not arise from different pigments, but are what is known as ‘structural colour’ – an illusion of colour produced by an effect of interference between light rays that is also found in butterfly wings and iridescent bird feathers. All eyes contain a certai
n amount of one pigment, melanin. I found dark flecks of this pigment floating in the humours when I cut into my pig’s eye. It is the variation in the levels of this pigment, together with the light-interference effect, that gives rise to the entire range of eye colours that we cherish. With progressively less melanin present, the eye can appear dark or light brown, hazel, green, grey or blue.
Francis Galton was curious to learn what eye colour had to say about heredity. He built himself a travelling case with sixteen numbered glass eyes of different colours. The eyes were set into a sheet of metal moulded in such a way as to give each of them eyelids and an eyebrow, an alarming surrealist touch when you first open up the case. Galton needed to be sure that the colour labels he chose from among the ‘great variety of terms’ used by compilers of family records were the ones important in nature. He didn’t choose brown or blue, as we usually do, but categories of light and dark, splitting those with ‘hazel’ eyes into both camps. He then compared children with their parents and grandparents, whipping up his usual storm of statistics, but finding nothing more noteworthy to say at the end of it than that both blue eyes and brown eyes are observed to persist down the generations.
The ultimate answer to Galton’s question about heredity came in 2008, when a team of (largely blue-eyed) researchers at the University of Copenhagen discovered a mutation of a particular gene that regulates a protein needed to produce melanin. Babies are often blue-eyed at first, even when born to brown-eyed parents, because this protein is yet to be released to its full extent. According to Hans Eiberg, who led the research, his genetic discovery suggests that all blue-eyed individuals alive today can trace their ancestry to one original Ol’ Blue Eyes, who was the first to undergo this mutation, between 6,000 and 10,000 years ago.
A mere accident in nature, maybe eye colour doesn’t have quite the significance we thought in culture either. Becky Sharp in Vanity Fair has green eyes, Anna Karenina has grey, James Bond, blue. The worse the novel, it seems, the more important it is to be exact in description. To wit, Judith Krantz’s Princess Daisy has ‘dark eyes, not quite black, but the colour of the innermost heart of a giant purple pansy’. But many of the most famous characters in fiction prove surprisingly elusive when it comes to eye colour. Mr D’Arcy merely thinks Elizabeth Bennet has ‘fine eyes’ in Pride and Prejudice. Julian Barnes devotes large portions of his novel Flaubert’s Parrot to the matter of Emma Bovary’s eyes, berating a (real-life) critic who had triumphantly spotted Flaubert’s supposed sloppiness in describing her eyes variously as blue, black and brown. It doesn’t matter, Barnes suggests; or rather it does, but not in the sense that we must know the colour of her eyes in order to identify, or identify with, the heroine. Emma’s eyes are whatever colour Flaubert chooses to make them for reasons of his own at that point in the narrative. In Tess of the D’Urbervilles, Thomas Hardy also dodges the question of the colour of his heroine’s eyes, which are ‘neither black nor blue nor grey nor violet; rather all those shades together, and a hundred others, which could be seen if one looked into their irises – shade behind shade – tint beyond tint – round depths that had no bottom; an almost typical woman’. If an author wants us to believe that his character is everywoman, then being vague about eye colour is a good way to start.
It seems that our sense of sight has grown in importance during human evolution, and this growth may be at the expense of other senses. For example, we have many genes involved in processing smells, but they are underused in comparison with the relatively few we have dedicated to vision. As sight has become more important to us, it is the brain’s ability to process visual signals that has developed fastest. Our eyes themselves have not kept pace with our thirst for visual information, which may help to explain why, in a world where visual communication is increasingly important, so many of us nevertheless need to wear glasses.
In order to understand the extent to which vision is realized in the brain rather than the eye itself, and to which it overlaps with other sensory information, I pay a visit to the Cross-Modal Research Laboratory at the University of Oxford. The tiny laboratory resembles something between a toy store and a corner shop, stacked with odd gadgets and familiar food brands. Its director is Charles Spence, a professor of experimental psychology. He is wearing his trademark red trousers when I meet him, and speaks with an unnerving staccato delivery. The senses – the familiar five of sight, hearing, touch, taste and smell, though many more according to some beliefs – are usually considered in isolation, he explains. But we use them in concert. This leads to some very odd perceptions with disturbing implications. For example, Charles tells me, an interviewer is more likely to regard a job applicant as a serious candidate if the interviewer is holding a heavy file on his lap than if he is holding a lighter one. The weight of the file counts for more than what he sees and hears. ‘Never mind the quality, feel the width,’ it seems, is not just a desperate sales pitch, but an axiom in nature too.
Our unconscious mixing of sense signals can easily mislead us. It can also be exploited in order to alter our behaviour. Much of Charles’s work is for product manufacturers who can make good use of multisensory discoveries such as the fact that the sound you hear as your teeth crunch on a crisp, and even just the rustling of the packet, is a significant factor in your perception of its flavour. ‘We’re interested in the interaction of the senses, both at the level of the single cell and how it comes together in the brain. Can you “taste the weight”, for example? Or, how does the fragrance somebody is wearing affect your estimation of their age?’
Vision is surprisingly easy to fool, perhaps because our brains are so biased in favour of this sense. One famous experiment is known as the rubber-hand illusion. A subject’s hand is positioned out of their sight, while an artificial hand (a rubber glove will do) is placed in the line of sight where they might normally expect their real hand to be. The experimenter then touches both the invisible real hand and the visible artificial hand with a synchronized stroking action. After a while, the subject begins to feel that the fake hand is really their own. A cruel extension of the experiment involves bringing a hammer down on the artificial hand: the subject cannot help flinching. In these situations, the brain is prioritizing visual information over weaker signals sent from receptors under the skin of proprioception, our sense of our position in space. The hand must be a reasonable likeness for the trick to work: a left glove for a right hand will not produce the effect. However, since a bright yellow rubber glove works perfectly well, it seems that skin colour for once doesn’t matter.
A still more dramatic illustration comes from the psychologist Richard Gregory, who witnessed the recovery of a man who had been blind from birth until he was given a corneal graft. Gregory took the man to various stimulating venues in London, including the zoo and museums. At the Science Museum, he was shown a lathe, as he had always been interested in machinery. In its glass case, he was unable to recognize it. But once he had run his hands over it, he understood it fully. As Gregory tells it, ‘he stood back a little and opened his eyes and said: “Now that I’ve felt it I can see.”’ The moment explained why, on the journey to London, the subject had been entirely nonplussed by the sights streaming by the car window. The fact that he remained effectively blind to objects until he had touched them indicates that neural pathways concerned with vision had been taken over in his blindness by touch, and that his brain was only now beginning to rewire itself.
Understanding how our senses overlap in the brain can lead to better treatments for sensory loss. For example, therapeutic procedures using mirrors can help amputees who experience pain associated with their lost ‘phantom’ limb and stroke victims who have lost motor control on one side of the body by enabling them to compare sensory feedback obtained by proprioception with what they see in the mirror. One sense can even begin to replace another on a permanent basis. Blind people who use part of their brain normally dedicated to vision to interpret the letters of Braille sometimes find that the tac
tile sensitivity of the fingers is increased, giving them better spatial discrimination. In 1969, Paul Bach-y-Rita at the University of Wisconsin in Madison scaled up this idea to create prosthetic ‘eyes’ using arrays of vibrating pins acting like pixels to create crude images of scenes recorded by a camera. The device, called BrainPort, was initially designed as a vest to be strapped to the stomach, where the large expanse of skin would serve as a touch-sensitive screen. Later versions were miniaturized to fit on the surface of the tongue, which is much more touch-sensitive. Bach-y-Rita’s subsequent innovations show that other senses may be recreated in the same way, such as balance in subjects who have suffered damage to the part of the ear normally responsible for providing this sense. After a short period using the BrainPort, modified to detect tilt, some patients even found some restoration of ‘balance memory’ that lasted for several hours after the device was removed. People learn to use such equipment by a laborious process of conscious sensory translation, but as they become more familiar with it, the brain’s neural pathways adapt so that the substitute sense is experienced more like the sense that has been lost.
We are inherently multisensory beings. We see and hear together. We use our senses of smell and taste together. Combined sense signals often amount to more than the sum of their parts, and are more memorable. I am sure I would not recall a particular occasion when I was listening to the gods’ entry into Valhalla from Wagner’s opera Das Rheingold on the car radio if I had not been driving across the Severn bridge at that very moment, for example. It is only when he actually smells and tastes the famous madeleine that Marcel Proust’s memories of lost time are unleashed; the sight of it alone is not enough to do this. The converse is true, too: take away one sense, perhaps one we don’t even realize we are using at the time, and our perception is disproportionately impaired. A loss of sense of smell takes away much of the enjoyment of food, because so much of what we think of as taste is in fact linked to smell. Or, as Charles Spence’s tests have shown, it may be important that a warning signal on a car dashboard is delivered by visible and audible means together, such as a flashing light with an intermittent tone. The brain may miss either of these signals on its own, but has a much better chance of registering the correlated event.
Anatomies: A Cultural History of the Human Body Page 20