当你半夜醒来 发现黑暗中有双发光的眼睛盯着你 你一定会被吓到
It can be terrifying to wake up in the middle of the night to a pair of glowing eyes staring at you from the darkness,
only to realize that it’s just your cat.
… Or is it?
很多动物 像鳄鱼 果蝠和狗 眼睛里都有发光结构 这个可以增强夜视能力
Lots of animals, including alligators, fruit bats, and dogs, all have a shiny structure in their eyes that gives them better night vision,
which also causes this creepy glow.
The layer of tissue at the back of your eyes is called the retina, and it’s made up of special light-sensitive cells.
Light that hits those cells gets turned into an electrical signal that’s sent along the optic nerve, which sits behind the retina.
That signal travels all the way to the brain.
如果光线接触不到这些细胞 就不能产生信号 也就无法被大脑察觉
And light that misses those cells isn’t turned into a signal, so your brain can’t detect it.
从爬行类到鸟类再到其他哺乳类动物 比如说猫 动物界的多数物种都拥有相同的眼睛结构
Many species across the animal kingdom have this same basic eye structure, from reptiles to birds to other mammals like cats.
One way animals have evolved to see better is to have more of those light-sensitive cells in each eye.
Humans, for example, have over ninety-five million of them per retina.
But a lot of animals—especially nocturnal animals whose eyes have to work well at night—have another way of detecting more light.
它们眼睛内部有一种叫做反光膜的结构 这个词（tapetum lucidum）在拉丁语中的意思是”明亮的挂毯”
They have a structure called the tapetum lucidum, which is Latin for “bright tapestry.”
It’s a layer of tissue that sits behind the retina and acts like a mirror.
The tapetum lucidum reflects light that gets through the retina back at those light-sensitive cells
giving them another chance to detect it.
But some of the reflected light flies back out the animals’ pupils,
which is what makes it seem like their eyes are glowing.
而光的颜色 也叫做眼耀 取决于反光膜的组成物质
The color of that glow, also called eyeshine, depends on what the tapetum lucidum is made of.
And different critters have different highly ordered molecules or fibers that create a reflective surface.
Some fish eyes use guanine—one of the chemical building blocks in DNA—and have white eyeshine.
Sheep have collagen, which also provides structure to muscles and skin in lots of animals,
which leads to a blue or green glow.
And cats use riboflavin and zinc, among other molecules.
锌的含量决定猫的眼耀是黄色 绿色 或是蓝色
And the amount of zinc does a lot to determine how yellow or green or even blue their eyeshine is.
So your kitty’s eyes aren’t actually like little flashlights;
they’re just reflecting some of the ambient light in your room.
But if they start glowing red and shooting laser beams, you should probably run.
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