Nasolacrimal system – introduction

15 Nasolacrimal system – introduction


The nasolacrimal system is made up of a secretory component (the pre-ocular tear film) and an excretory component (nasolacrimal drainage system). Secretion of the tear film is from various glands – the lacrimal and third eyelid glands, meibomian glands and goblet cells – each contributing a different constituent as detailed below. The drainage system consists of the nasolacrimal punctae, canaliculi and duct.



THE PRE-OCULAR TEAR FILM


An adequate supply of tears is necessary to maintain ocular integrity and normal function. The pre-ocular tear film covers the corneal and conjunctival surfaces and its close proximity to these underlying structures means that it is important in both health and disease. The pre-ocular tear film has a pH of 6.8–8.0 (mean 7.5). This is important when considering topical medication – drops with a different pH are more likely to sting on application for example. Functions of the pre-ocular tear film include:


• maintaining an optically uniform corneal surface

• removing foreign material and debris from the conjunctiva and cornea

• permitting the passage of oxygen and nutrients

• possessing antibacterial properties.

Traditionally the pre-ocular tear film is considered to be made up of three layers, although the structure and interactions of these layers are probably much more complicated than this! However, for simplification, we will stay with this classification, which divides the tear film into outer, middle and inner layers as follows:



Outer oily layer


This thin layer is produced by the meibomian glands. The glands are present in the palpebral conjunctiva with their openings visible as a line of grey dots on the eyelid margin. This layer helps to decrease evaporation, improves the stability of the whole of the tear film and provides a protective oily barrier at the lid margins.



Middle aqueous layer


This is produced from lacrimal and nictitans glands and constitutes the bulk of the tear film. The lacrimal gland, located dorsolateral to the globe within the periorbita of the lacrimal bone, provides the majority of the aqueous tear production, with the nictitans gland producing approximately a quarter to one-third of the total amount. This aqueous layer allows oxygen and nutrient diffusion, facilitates smooth movement of the eyeball and contributes to corneal protection.

Related posts:

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  4. Limbal melanoma

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Nasolacrimal system – introduction

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