Cherry Eye is an inflammation of the Harderian
Gland, an accessory tear gland of the conjunctiva inside the lower
eyelid. It occurs in eyes that have a poorly developed ligament holding
the gland in position. 
The
gland is contained within a fold of conjuctiva commonly called the
"third eyelid". Normally the lower pole of gland is anchored to the
connective tissue inside the bony orbital rim. In the abnormal eye, the
inferior ligament is absent or poorly developed. When the gland becomes
inflamed and swollen, it flips up from under the "third eyelid" and
herniates from under the lower eyelid, (see the figure below.)
Through repeated inflammation and herniation,
this condition is associated with the development of "dry eye" or
kerato-conjunctivitis sicca. Surgery can restore the gland to it's
proper position. Removal of the gland can precipitate "dry eye" and so
should be avoided. Relatively small, almond shaped eyes whose
structures are well supported by the infraorbital malar complex
(cheekbones) are rarely subject to cherry eye. Breeders should select
stock with healthy eye conformation, and prospective owners should look
for a small almond eye when purchasing a puppy.
Occasionally the gland will swell and herniate acutely from
allergy, injury or infection. If your dog does develop a cherry eye, it
is important to reduce it as soon as possible to avoid drying and
further injury which can turn an acute herniation into a chronic one.
You can often replace the gland into it's normal
position under the "third eyelid" membrane by a simple manual maneuver.
While stretching the eyelid in a superior lateral direction, put gentle
but firm pressure on the medial lower lid as indicated in the figure.
This maneuver will generally work on cherry eyes that have been present
for less than 24 hours, and are not very large or infected. Antibiotic
drops should be used for about 48 hours following replacement of the
gland.
