单选题
About myopia—if you have it, be happy. As shown by numerous scientific studies, nearsighted men and women boast a higher average intelligence than their nonmyopic cohorts. The precise mechanism of this association remains unknown, but it is tempting to postulate an effect of myopia on early childhood development. Most nearsighted kids wander around undiagnosed for years, and during this formative period—unable to see the baseballs, Frisbees, and rocks thrown at them by their playmates—they spend a lot of time indoors. The nonathletic myopias who take up reading to while away the hours get high scores on the SATs, while those who take up eating tweak our claustrophobia by overflowing the adjacent seat on airplanes. Myopia also exerts a compelling influence on career choice: the great majority of my fellow ophthalmologists wear either contact lenses or thick myopic spectacles. Pathology breeds preoccupations. However beautiful the human eye, it serves a more important purpose than romantic allure. Forty percent of the brain is devoted to vision, which provides us with more information than our other four senses combined. Our optic nerves transmit millions of impulses to the brain every second, impulses that specify the location, color and intensity of light for all the points in our visual space. Better yet, thanks to a mysterious algorithm that fuses the slightly disparate images from each of our eyes, our visual cortex, via a neurological miracle known as depth perception, shows us the world in three dimensions. An impressive feat since a video camera, arguably the benchmark of modern technology, can muster only two dimensions. Certain ocular tissue stands on the pinnacle of evolution. How does nature, so crude in claw and fang, create a surface that brings light into a pinpoint focus? This surface must be perfectly curved, perfectly transparent, perfectly smooth. It must be—water! Which is to say, the cornea owes its optical perfection to a tear film whose dissolved salts, lipids, and proteins maintain a flawless wetted surface. A man who has no tears stands on the brink of blindness. Worse yet, that man will writhe in agony: a dry cornea, thanks to the most exquisite pain threshold in the human body, responds to each blink with a tormenting jolt. Dry eye victims compare the sensation to that caused by rubbing the eyeball with shards of glass.