The secretory system produces a sufficient quantity and quality of tear film to allow the ocular surface to function normally. A smooth refractive surface, ocular comfort and the health of the cornea and conjunctiva, all depend on a stable tear film.
The secretory system consists of two types of glands:
- The main lacrimal gland lies in the upper, outer part of the orbit of the eye and is about the size and shape of an almond.
This gland is responsible for the secretion of ‘reflex’ (aqueous) tears only. In response to parasympathetic stimulation, the main lacrimal gland produces aqueous tears when nerve endings on the ocular surface are stimulated (e.g. wind), when bright light stimulates the retina or when the emotional centre in the brain is stimulated (crying).
Certain condition, such as the wear of contact lenses, can decrease the sensitivity of nerve endings on the surface of the eye, leading to decreased reflex aqueous secretion. This reduction in reflex tearing is enough to lead to dry eye sensations
- The numerous accessory lacrimal glands are scattered around the conjunctival sac and maintain the ‘basal’ (unstimulated) tear secretion that is necessary to keep the eye moist. These glands are regulated by the autonomous system and continuously produce the majority of the three-layered precorneal tear film
The goblet cells, the tarsal crypts of Henlé and limbal glands of Manz contribute to the inner mucus layer
The accessory lacrimal glands of Wolfring and Krause are responsible for the basal secretion of the middle aqueous layer
The outer lipid layer is mainly produced by the Meibomian glands, but also by the glands of Zeis and the glands of Moll
Anatomy of the glands of the eye
Mechanism of secretion
During reflex tearing, the aqueous component is secreted by the main lacrimal gland into the conjunctival sac via numerous excretory ducts (an average of 12) that end in the superior fornix of the eye.
For basal secretion, the scattered accessory lacrimal glands produce their components directly into the conjunctival sac.
|The conjunctival sac is formed by the inner surface of the eyelid, which forms a pocket that extends upwards and downwards. The conjunctival sac has a maximum volume of approximately 30 μl.|
All of the components of the tear fluid accumulate in the superior and inferior fornices of the eye (which are the grooves between the lid margin and the eyeball, found mostly in the lateral part of the eye) and overlap the cornea for about 1 mm as a straight line. Most of the tears travel in the fornices and conjunctival sac. In these areas, the aqueous fluid is sealed between the mucus layer (produced by the goblet cells of the conjunctiva) and the lipid layer (secreted by the Meibomian glands).