Within the folds of the lower eyelid, a thin portion of tissue is present that is often called the "nictitans" or "third eyelid". The third eyelid is found in most domestic animals.
A gland called the "gland of the third eyelid" or the "glands nictitans" or the "haw" is located on the surface of the third eyelid which faces the cornea (clear portion on the outer surface of the globe).
The gland is a tear producing gland. This gland produces between 30 and 60% of the tears in the dog and cat. The gland is normally out of sight and held in position behind the third eyelid by a small ligament. A prolapsed gland of the third eyelid (or "cherry eye") is thought to be associated with a laxity of the ligament.

The main orbital lacrimal gland, located beneath a portion of the skull bone, produces the rest of the tears. Unfortunately, the amount of tears produced by each of these glands is variable. The longer the gland is in an abnormal position the greater risk that the gland will be damaged and not fully functional when it is tacked back into place.

In years past, when a cherry eye occurred, it would be surgically removed. It is now known that should the main orbital lacrimal gland be damaged later in life no "backup" for tear production would exist if the prolapsed gland of the third eyelid is removed.
Dogs that have had the gland of the third eyelid surgically removed have a greater risk of developing keratoconjunctivitis sicca (dry eye or KCS ) than dogs with intact third eyelid glands.
Additionally, studies have shown that leaving the gland of the third eyelid in a prolapsed position results in a higher incidence of KCS (Dry Eye) than those patient in whom the gland is surgically replaced.

Certain breeds of dogs develop KCS and cherry eye. These breeds (American Cocker Spaniel, English Bulldog, Shar Pei, Shih Tzu, Lhasa Apso, and Chow Chow) frequently develop KCS (Dry Eye) after removal of the cherry eye condition.
Dry eye is a serious eye condition that is difficult to treat, and requires life-long treatment. The chance of developing KCS (Dry Eye) is lessened by tacking the gland back into its normal position thereby keeping the gland functional. This is the most desirable way of handling "Cherry Eye". Tacking surgery performed by an experienced veterinary ophthalmologist has a failure rate of less than 5 %.

Failure means that the gland will re-prolapse and need a second surgery in about 5 cases out of 100,  animal eye specialists have had great success with the tacking procedures . The tacking surgery is certainly more expensive than surgically removing the gland. However, the cost of treating dry eye (examination and medication fees) is much higher over the life of the pet. Despite surgery, dry eye may develop later in life if damage occurs to all of the lacrimal glands. This damage is usually associated with an immune system dysfunction and its occurrence cannot be predicted.