the third eyelid in dogs

I don't generally look closely at Penny's eyes except when she rolls in the privet blossom that is everywhere in our backyard at the moment and I'm trying to brush the tiny flowers off her gorgeous long eyelashes. (I have read that dogs with hair that falls over their eyes, as Penny's will do if it's not trimmed, have longer than usual eyelashes to hold the 'fringe' away from their eyes. The Lhasa Apso has this feature and I'm convinced that somewhere in the mix of Penny's ancestry is one of these beautiful creatures.)

Anyway, back to eyes...
A friend had a scare the other day when her dog's eye seemed to be partly red and rolled back into her head. The vet said it was a problem with 'the third eyelid'. I looked around on the Net for some information and came across a series of photos of the third eyelid at The Pet Center.com In part it says:

The Third Eyelid is also known as the Nictitating Membrane. It serves as added protection for the eye through an interesting ability to close upward and over the dog's eyeball. The canine is able to retract the entire eyeball backward into the eye socket, and coupled with this retraction is the Third Eyelid's ability to slide up and over the retracted globe.

At LookD. com there's a general overview of dog anatomy and it says:

Each eye of a dog has three eyelids, the main upper and lower lids and a third lid hidden between them in the inner corner of the eye. The third eyelid can sweep across the transparent cornea of the eye and clean it like a windshield wiper.

ChinaRoad Lowchens of Australia has an article about conjunctivitis. It says this is a common problem in dogs and looks at various causes. It continues:

Regardless of the cause, a patient with conjunctivitis will often squint and/or keep the third eyelid partially covering the eyeball. Conjunctivitis is often painful, causing a dog to paw at or rub the eye against objects such as your leg or the carpet.