Eye Anatomy: The Tear System

Without the eye’s tear system we would have poor vision. Tears continually moisten the cornea and sclera (white part), washing away dust and debris that would otherwise irritate the eye. Irritation would cause excess tears to be produced and that would blur the vision. It would also cause burning or stinging, which would close the eyes reflexively.

Principal Tear Ingredients

Tears have three layered ingredients, each of which is produced by a different organ:

Sebum
Sebum is the top layer. It is an oily substance produced by tiny glands along the eyelid edges. They are called tarsal glands and there are about 50 along each upper eyelid and 25 along the lower lids. Sebum prevents the tears from evaporating. When you close your eyes, the sebum on the eye’s surface makes an airtight closure.
Water
Water is the middle layer and is produced by the lacrimal gland. (Lacrima is the Latin word for tear.) This roughly oval-shaped gland is situated on each side beneath the eyebrow near the temple. The water component of tears helps to spread the tears easily and helps remove bacteria from the eye surface.
Mucous
Mucous is the bottom layer, produced by microscopic cells in the conjunctiva called goblet cells. The conjunctiva is a membrane that lines the eyelids and covers the sclera like plastic wrap. The mucous bonds to the water layer and helps to distribute the tears evenly.

The Tear Drainage System

From the front eye surface the tears flow along the lower lid towards the nose. Blinking redistributes them but gradually they reach two little openings called puncta lacrimale. One is in the upper lid and one in the lower lid. They give access to drainage tubes that connect to a larger tube that takes the tears to the nose. This is why we blow our nose when we cry – because the extra tears accumulate in the nose.

Without the eye’s tear drainage system, tears would continually overflow and run down the face – as they do when we cry. Anything that causes the nervous system to stimulate extra tear production, such as strong feelings, a foreign body, onions, or a brisk wind, will overtax the drainage system so that tears spill over on to the face.

Three Types of Tears

Basal tears – the continual flow that moistens the eye surface and removes dust. Microscopically, basal tears contain many ingredients such as sodium, potassium, and glucose, as well as some that are part of the immune system and fight bacterial infection.
Reflex tears – extra basal tears that appear in response to irritation such as onion vapor, pepper spray, very bright lights, or hot spices in the mouth. They provide an extra cleaning function.
Psychic tears – caused by emotions or physical pain. When emotional, these are not necessarily tears of grief or pain – they may be tears of joy. Psychic tears contain more of certain hormones, one of which is a natural painkiller. Accompanying psychic tears there may be reddening and swelling in the face and changes in breathing.

Insufficient Tears

There are many eye conditions and general health conditions that affect the production of tears. When we go out walking on a windy day, we may come back with dry eyes, but this is temporary. When the eyes are chronically dry, the condition is known as Dry Eye Syndrome.

Dry eyes are a side effect of some diseases such as Sjogrens disease and rheumatoid arthritis. They may also be a result of aging and menopause.

Ocular rosacea can cause inflammation in the eyelid glands that produce the sebum in tears, and loss of sebum causes the tears to evaporate. The brain responds by sending messages to the lacrimal gland to produce more tear water, which then overflows from the eyes. This explains the odd phenomenon of dry eyes being too watery. The watery tears are less effective because sebum is missing, allowing the tears to continue evaporating.

If you are suffering from any tear problems and would like to speak with a qualified ophthalmologist near you, please contact us today.