The Mysterious Power of Human Sight

The Mysterious Power of Human Sight

By SIR CHARLES SCOTT SHERRINGTON

How does a pin’s-head ball of cells in the course of so many weeks become a child?  Consider the story of the making of just one individual part: the eye.

The many cells, which make the human eye, have first executed correctly a multitudinous dance engaging millions of performers in hundreds of sequences of different steps.  To picture the complexity and the precession of this performance beggars any imagery I have.  It suggests purposive behavior—not only by individual cells but also by colonies of cells.  And the impression of concerted endeavor comes; it is no exaggeration to say, with the force of a clef-evident truth.

The eyeball is a little camera.  Its smallness is part of its perfection.  But this is a spheroid camera, which focuses itself automatically, according to the distance of the picture interesting it.  It turns itself in the direction of the view required.  Indeed it is two cameras finished to one standard so that the mind can read their two pictures as one.  And it is contrived as though with forethought of self-preservation.  Should danger therein, in a trice its skin shutters close, protecting its transparent window.

If a craftsman sought to construct a camera, he would turn for his materials to wood and metal and glass.  If told to use instead of some albumen, salt and water, he certainly would not even begin.  Yet this is what that staring embryo, a little pin’s-head bud of multiplying cells, not one ten-thousandth part the size of the eyeball, proceeds to do.  The whole structure, with its prescience and all its efficiency, is produced by out of specks of granular skin cells arranging themselves as of their own accord in sheets and layers, and acting damningly on an agreed plan.  The magic juices that make the eye go by the chemical names protein, sugar, fat, salts, water.  Of them 80 percent water.  It all sounds like an unskillful, over salted tale that challenges belief.  But to faithful observation so it is.  There is more yet.

The biconvex lens is made up of cells like those of the skin but modified to be glass-clear.  It is delicately slung with accurate centering across the path of the light, which will some months later enter the eye.  It is a preparing in darkness for use in light.  In front of it a circular screen controls, like the iris stop of a camera, the width of the beam and is adjustable, so that in a poor light more is taken for the image.  In a camera, the observer working the instrument makes this adjustment.  The observer supplies the actual motor power.  In the eye this adjustment is automatic, worked by the image itself!

Not only must the lens be glass-clear but also its shape must be optically right.  Its two curved surfaces, back and front must be curved to the right degree, so that light brought to a focus on the retina and gives there a shaped image.  The optician obtains glass of the desired refractive index and skillfully grinds its curvature in accordance with mathematical formulae.  With the lens of the eye, a batch of granular skin cells are told to travel from the skin, to which they strictly belong, and to settle down in the mouth of the optic cup and arrange themselves in a compact and suitable ball.  Next they are told to turn into transparent fibers, and to make themselves into a sub sphere—a lens of the right size, set at the right distance between the transparent window of the eye in front and the sensitive seeing screen of the retina behind.  In short, they behave as if fairly possessed.

Furthermore, the lens of the eye, compassing what no glass lens can, changes its curvature to focus near objects as well as distant when wanted—for instance, when we read.  And not merely the lens but the pupil—the camera stop—is self-adjusting.  All this without our having even to wish it; without even our knowing anything about it, beyond that we are seeing satisfactorily.

The lens and screen cut the chamber of the eye into a front half and a back half, both filled with clear humor, practically water, kept under a certain pressure to maintain the eyeball’s right shape.  The front chamber is completed by a layer of skin specialized to be glass-clear, and free from blood vessels which, if present, would throw shadows within the eye.  This living renewed and which has the special chemical power of killing germs.  The skin above and below this window grows into movable flaps, dry outside like ordinary skin, but moist inside, which wipe the window clean every minute or so by painting fresh tear water over it.

The eye’s key structure is the light-sensitive screen at the back.  It receives, takes and records a continually changing moving picture, lifelong, without change of ‘plate,’ through very walking day.  And it signals its shifting exposures to the brain.  It is nine fold layer of great complexity.  It is, strictly speaking, a piece of the brain lying within the eyeball.

The cells that are at the bottom of the cup become a photosensitive layer—the sensitive film of the camera.  The nerve lines connecting the photosensitive layer with the brain are not simple.  The human eye has about 137 million separate ‘seeing’ elements spread out in the sheet of retina.  The number of nerve lines leading from them to the brain gradually condenses down to a little over a million.  They are in series of relays, each resembling a little brain, and each so shaped and connected as to transmit duly to the right points of the brain itself each light picture momentarily formed and ‘taken.’  On the sense-cell layer the image has, picture like, two dimensions.  But the step from this to the mental experience is a mystery.  For it is the mind which ads the third dimension when interpreting the two-dimensional picture!  And it is the mind, which adds color.

The chief wonder of all we have not touched on yet.  The eye sends into the cell-and-fiber forest of the brain throughout the walking day continual rhythmic streams of tiny, individually evanescent electrical potentials.  This throbbing, streaming crowd of electrified shifting points in the sponge work of the brain bears no obvious semblance in space pattern to the tiny two-dimensional upside-down picture of the outside world which the eyeball paints on the beginnings of its nerve fibers to the brain.  But that little picture sets up an electrical storm.  And that electrical storm affects a whole population of brain cells.  Electrical charges have in themselves not the faintest elements of the visual—have nothing of “distance,” nor “vertical,” nor “horizontal,” nor “color,” nor “near, nor “far,” nor visual anything—yet they conjure up all these.  A shower of little electrical leaks conjures up for me, when I look, the landscape, the castle on the height of my friend’s face and how distant he is from me.

How explain the building and shaping of the eyeball, and the establishing of its nerve connections with the right points of the brain?  And how explain not the eye but the “seeing” by the brain behind the eye?  This is the wonder of wonders, familiar even no boredom.  So much with us that we forget it all our time.