Cataracts and dogs..........................?

I was wondering how cataracts start to form in dogs? My dog seemed to injure his eye from a stick or bush, there is no infection and he seems to be in pretty good spirits, meaning no pain and acts like normal.

He now when he looks the other way with his eye a piece of tissue is present in both eyes I know he would have to be evaluated by a vet and he will be on friday, but i was just curious what this could be?

this is only in the corner of his eyes by the nose and is only present when he looks out ward with out moving his head. Also his eyes appear glassy a little dilated.