Tears are necessary for the proper functioning of the protective apparatus of the eye. The so-called tear film covering the cornea and conjunctiva consists of three layers: a lipid, aqueous and mucus layer. The lipid layer is produced by the Meibomian palpebral glands. Watery component is secreted by the lacrimal gland located in top-temporal part of the orbit and by lacrimal glands additionally located in the conjunctiva (Krause and Wolfring glands). In contrast, the mucous layer is produced by goblet cells contained in the conjunctiva, crypts of Henle and glands of Manz.
Proper tear film covers the eye in such a way that the mucous layer is in direct contact with the surface of the eye, aqueous layer is next and there is lipid layer from the outer side. The mucous layer provides good adhesion to the surface of the eye, the aqueous layer hydrates, and the lipid layer prevents evaporation of tears.
Tear drainage system - the so-called lacrimal pathway consists of the lacrimal puncta located in the medial part of both eyes, lacrimal canaliculi, the lacrimal sac and nasolacrimal duct draining fluid into the nasal cavity. For this reason, when we cry we start to "sniff", as there appear tears in the nasal cavity drained away from the eye.
Both the shortage of tears - the so-called dry eye, and their excess indicate the state of disorder.
Excessive tearing - may result from excessive tear secretion, e.g. in the case of diseases causing eye irritation, like conjunctivitis. The second group of causes results from the impairment of the outflow of tears, which can be caused by:
- incorrect position of the lacrimal puncta (eg. due to unwinding of the lower eyelid – “ectropion”)
- obstruction on any part of the tear drainage paths (from the puncta to the nasolacrimal duct)
- the failure of the so called lacrimal pump (suction mechanism of tears sucked into tear drainage system), which can occur eg. at the lower eyelid laxity