Here’s something you probably haven’t given much thought to: Why do tattoos last forever?

　　這也許是你很少會思考的問題：為什么紋身能夠永久留在皮膚上呢?

　　The answer is more interesting than you might think, which is that your body thinks it is under attack when someone draws on it.

　　這個問題的答案也許比你想象的更有趣——你的身體在紋身過程中會認為它受到了襲擊。

　　It’s actually very difficult for anything to stay on the skin, The Independent reports. We shed about 40,000 skin cells per hour, or around a million per day. Anything drawn onto the skin — by a pen, for example — gradually flakes or gets washed off.

　　英國《獨立報》報道顯示，實際上物質(zhì)很難在皮膚上停留很久。每個小時我們大約會脫落4萬各皮膚細胞，或者是每天一百萬個左右。只要是用筆之類的東西畫在皮膚上的東西，都會逐漸地脫落或者被洗掉。

　　So how do tattoos avoid shedding? The answer is pretty simple — the gun (or other tool used to create the art) pushes the ink through the outer layer of the skin and into the dermis, which is a deeper layer of skin that doesn’t flake off. It’s composed of collagen fibers, nerves, blood glands, and more.

　　所以紋身是如何避免脫落的呢?其實答案非常簡單。紋身槍(或者是紋身工具)透過真皮層把顏料注入真皮層。真皮層是人體皮膚更深的一層，不會脫落，由膠原纖維、神經(jīng)、腺體等其他組織構(gòu)成。

　　Every time a needle penetrates, it causes a wound. This alerts the body to begin the inflammatory process, which sends immune system cells that try to start repairing the skin — and while this sounds like a bad thing, it’s actually what makes the tattoo last.

　　每回細針扎入皮膚，就會造成創(chuàng)傷。這就會促使身體發(fā)言，這就給免疫細胞傳遞了修復(fù)皮膚的信號。盡管這聽上去不那么美好，這的確是紋身保持永久的秘訣。

　　The cells try to “eat” the ink and transfer it away from the dermis, but there is more ink than they can break down, so the artwork stays. This is also a window in how laser removal works — the laser breaks down the ink, allowing the cells, called macrophages, to transfer the ink away.

　　細胞們試圖“吞噬”顏料，然后將其逐出真皮層。然而，真皮層中的顏料數(shù)目非常大，它們無力消化。所以紋身的圖案能夠永久保存下來。這也是激光去紋身的工作原理：激光破壞了顏料的結(jié)構(gòu)，讓這些巨噬細胞得以將顏料轉(zhuǎn)運出真皮層。