Dogs have an "extra" or third eyelid that is located inside of their lower eyelid. There is a gland within this third eyelid that produces tear film and although these structures act as additional protection for the eye, VCA Animal Hospitals reports that it can also be easily irritated. When the gland in the third eyelid becomes irritated and prolapses or "pops out" of place it is called "cherry eye." It gets this name because the prolapsed gland appears as a red round mass coming from the lower inner eyelid. The mass can cover a small amount or almost the entire surface of a dog’s eye.
According to VCA, the gland of the third eyelid is typically attached to the lower inner rim of the eye, though in certain breeds including bulldogs, beagles, cocker spaniels, and shih tzus, this attachment is weak, which increases the likelihood of the animal developing cherry eye.
If an owner sees this, they should bring their dog to the veterinarian health clinic immediately. In most cases, a vet will need to surgically replace the third eyelid gland, as it produces a large percentage of the watery portion of the tear film. Without enough of the gland, a dog will become more susceptible to developing painful dry eye.