I'm pretty sure all dogs have this. Rosco and Layla both have one. They're more visible when they are sleepy. I believe it is called a nictating membrane and it usually can't be seen in other breeds, but in some dogs they are visible. It's supposed to keep the eye lubricated. Check the link below for more details. Unless it's really red or irritated I wouldn't worry.
Dog Owner's Guide: The eyes have it
Dog Owner's Guide: The eyes have it