Birds have this third eyelash called the nictitating membrane.

鸟有第三层睫毛，叫做瞬膜。

But the nictitating membrane itself is completely to transparent.

但瞬膜本身是完全透明的。

That's the nictitating membrane and this little fold is the remnant.

那是瞬膜，这个小褶皱是残余的。

To see underwater they have a nictitating membrane.

为了在水下观察，它们有瞬膜。

The eye could pull back and blink, and then the nictitating membrane come across as well to protect the eye.

约翰：眼睛可以向后拉并眨眼，然后瞬膜也会出现以保护眼睛。

They protect their eyes with their  handy-dandy nictitating membrane, which clears debris and tears from their eyes.

他们用得心应手的瞬膜来保护眼睛，这种膜可以清除眼睛中的碎屑和眼泪。

One which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane.

我活着的一个肯定是在这种情况下， 正如解剖中出现的那样， 原因是蜴膜发炎。