Most of us are familiar with calluses.

　　These extra-tough patches of skin are generally quite useful–we might even say handy–because they act like a kind of natural armor, protecting areas of skin that get an unusual amount of wear and tear. Have you ever wondered exactly what a callus is, or how it forms on your skin?

　　Callus formation is triggered by pressure or abrasion. The heel of your foot as it rubs inside your shoe, or the palm of your hand if you’re doing a lot of manual labor are good examples of this.

　　While it might feel like a callus is something extra, added to your skin, it’s actually just a build-up of what’s already there.

　　Here’s How It Works

　　Your skin has a number of layers of different types of cells.

　　The outermost is a layer of hardened, dead cells. This top layer is usually about twenty-five cells thick, and it constantly replaces itself as the outermost cells flake off to be replaced by new hardened, dead cells underneath.

　　All About Friction

　　If your skin is subject to an unusual amount of friction, this layer of dead cells increases. New dead cells are added faster than the old ones slough away. This can build up the outer layer from twenty-five cells thick to over a hundred cells thick. You’ve grown a callus.

　　While calluses are generally useful, if the process continues unchecked a callus can thicken into a cone-shaped structure called a corn. These super calluses can be quite painful, and may require the attention of a doctor.

　　译文

　　对于“老茧”，我们都很熟悉。

　　这些皮肤上突出的硬皮是很有用的，也可以说是方便，因为他们扮演着保护常磨损皮肤的天然盔甲的角色。你是否思考过什么是老茧?他又是怎样形成的?

　　老茧是由压力和摩擦而形成。脚后跟的茧是由其在鞋里摩擦而形成，正如手茧是在大量的劳动过程中形成。

　　尽管看起来似乎是皮肤上多出的部分，然而事实是已经存在的皮肤角质的累积。

　　形成过程

　　皮肤是由若干层不同类型的皮肤构成。

　　表层是一些坏死的硬细胞，通常有25个细胞那么厚，表层细胞不断的脱落，取而代之的是下层硬的坏死细胞，由此而完成自我更替过程。

　　一切皆由摩擦引起

　　假如皮肤受到不寻常量的摩擦，这层坏死细胞将会增加，新的坏死细胞增长速度比旧细胞脱落的速度快，这就致使皮肤表层的厚度增加到100个细胞那么厚，由此，老茧便形成。

　　虽然一般来讲，茧是有益的，但如不控制其过程，老茧厚度就会增加到被称作鸡眼的锥形结构，那样就会很疼，甚至需要去看医生。