[00:00.00]Listen to part of a lecture in a biology class.
[00:04.21]Professor: I think the lens of the eyes just about the most fascinating part of the body. [00:09.52]For one, it's the only, I repeat only transparent cellular tissue in the human body. [00:16.39]In fact, truly transparent tissue is very uncommon in nature. [00:21.56]There is some sort of clear creatures that live in oceans and rivers, like the jellyfish. [00:26.97]They're quasi transparency allows them to blend in with the water to escape predators. [00:32.77]I say quasi, because jellyfish are translucent. [00:36.51]They're not totally see through like the lenses. [00:39.68]So you have to wonder why the lenses is. Um, how how is transparent tissue possible? Marlin?
[00:47.18]Female student: There's no direct blood supply to that part of the eye.
[00:50.71]Professor: Good. Exactly. Marlin, the blood and blood vessels would color the lens would probably end up looking pinkish, but that's only part of the explanation. And what else?
[01:02.43]Male Student: Well, there was something about crystals and fibers in the chapter we read. [01:07.52]It said that the lens is sometimes called a biological crystal. [01:12.69]Not a true crystal like a diamond, but It has a very regular arrangement of cells. [01:18.19]The cells are exactly aligned in tightly compacted.
[01:21.65]Professor: Right. It's made up of layers of cells, but the cells are all of one type, a rigid elongated cell. That's called a lens fiber because it's all one kind of cell and because they're align so regularly, you don't have all the angles you get when different types of cells are fitted together, which refract light bended as it passes through. [01:45.13]And that contributes to its transparency. [01:48.12]But there's something missing in a lens fiber. What doesn't a lens fiber have?
[01:54.14]Male Student: Well, we said blood vessels, right?
[01:56.23]Professor: The lens itself, right? Is that it? Anybody? No? [02:02.50]This is very important. The lens fibers don't have organelles. [02:08.95]Organelles are parts that function, like miniature organs, specialized parts of a cell, like the nucleus that stores the DNA or mitochondria, which are the powerhouse of a cell. Okay? [02:22.82]Now what's interesting here is that lens fibers as their first forming, they start out as cells with organelles. [02:30.60]But as the eye develops, the cells lose all of them, including the nuclei. [02:35.75]And without organelles, the lens fiber is nice and clear. [02:39.58]And all those lens fibers packed together make a nifty crystal-clear lens. [02:44.46]But that clarity comes at a cost. So, marlin, what's the downside of this?
[02:51.35]Female Student: Actually, well, if it doesn't have organelles, I mean DNA or whatever. Then what happens if the lens is damaged or something?
[03:00.40]Professor: And that's the problem, the price of clear vision of transparent lenses is that the lens fibers can't regenerate or repair themselves, as other cells do, which leads me to this. [03:13.31]As we get older,the lens can get cloudy or yellow. It can be cataracts, or happen just from aging. [03:20.40]There are lots of possible causes of that, but when you get down to it, the fact remains that the lens fibers just can't repair themselves. [03:28.23]So when the lens gets discolored, what do you think happens to your sight?
[03:33.00]Male Student: Well, would change your color perception wouldn't it?
[03:36.32]Professor: Exactly. Discolored lenses absorb certain wavelengths of light, so some light rays won't reach the retina, so some wavelengths aren't processed, which will affect your color vision. [03:48.09]I just read an article about the French impressionist painter Claude Monet. [03:53.05]And it was really interesting because it showed examples of his paintings, and you could see from his later paintings what he was seeing, how his eyes with his eyesight had changed. [04:04.32]As he got older, certain colors began to dominate his work, yellows and reds and browns. [04:10.53]He was mostly seeing those colors because his lenses had yellowed, load and weren't transparent anymore.
托福14天冲刺微信战团