Your teeth have three main layers.

你的牙齿主要有三层。

If you look at a simplified (oversimplified) picture of the brain, there are roughly three main layers.

如果你看一张简化（过度简化）的大脑图片，大致有三个主要层次。

But of course it's much more than just a storage tank — it's lined with the same four main layers found through most of the GI tract — the mucosa, submucosa, muscularis externa, and serosa — but it's got a few special modifications.

当然，它不仅仅是一个储存罐。它内衬着与大部分胃肠道相同的四个主要层——粘膜、粘膜下层、外肌层和浆膜——但它有一些特殊的修改。