The Tearing Mechanism in dogs

Each eye has two lacrimal glands: one beneath the bony ridge at the top of the eye socket and the other incorporated into the third eyelid. Each produces approximately half the aqueous tear volume. Excess tears are collected at the inner corner of the eye and drained by the nasolacrimal duct into the nasal cavity near the front of the nose.

The tear film actually comprises three layers. The outer layer is a lipid or oily layer produced by meibomian glands in the eyelids. This layer acts as a barrier and prevents tears from evaporating and spilling over the eyelids. The middle or aqueous layer consists of the watery tears produced by the lacrimal glands. The inner layer is composed of mucus secreted by the conjunctiva. Mucus is a wetting agent and holds the aqueous layer against the surface of the eye.

Diseases of the tearing mechanism produce either a dry eye or a watery eye.