Topic 3
The nasolacrimal drainage system serves as a conduit for tear flow from the external eye to the nasal cavity. It consists of the puncta, canaliculi, lacrimal sac, and nasolacrimal duct
Puncta are openings 0.3 mm in diameter located on the medial aspect of the upper and lower eyelid margins
-Avascular therefore pale compared to surrounding
-Epiphora is an overflow of tears onto the face
Due to lack of drainage
caniculi
Initially vertical (2mm) and then horizontal (8mm)
They then converge and dilate forming ampulla
Lined by non-keratinised stratified squamous cells
And surrounded by elastic tissue thus can dilate 2-3 x
-The lacrimal sac is located in the lacrimal fossa
(Anteriorly frontal process of maxilla, post. lacrimal bone) ;
divided into fundus and body as illustrated above.
-nasolacrimal duct = 12-mm superior intraosseous portion and a 5-mm inferior membranous portion
intraosseous →running through the nasolacrimal canal part of maxillary bone
inferior part → within nasal mucosa and eventually opens into the inferior meatus
-Lacrimal gland (almond shaped) in lacrimal fossa of frontal bone Consisting of 2 parts : Orbital and palpebral When eye is inverted palpebral can be seen Both parts have several ducts (orbital less ) opening up in the eyelid They secrete the aqueous layer of the tear film (90%) Containing nutrients + o2, electrolytes, enzymes and antibodies therefore -maintain pH, provide nutrients to cornea and washes away Metabolites and provides protection against microorganisms -Smoothes irregularities of the corneal surface, enhancing optical quality
The tear film exist of three layers
- 1.innermost mucus layers
- It is the thinnest and consist of Mucin which is secreted by conjunctival goblet cells and glands of Manz which make the corneal hydrophilic
- 2.Aquous layer
- Largest part and intermediate layer and is secreted by main and accessory lacrimal glands which is mainly water and contains electrolytes and nutrients as well as antibacterial properties
- 3. Lipid layer
- Outermost layer and is formed by the glands of the eyelid and it prevents the overflow of tears and prevents rapid evaporation and lubricates the eyelids from the inside
Functions of the tear film
- Keeps the conductive and cornea moist and provides oxygen to the corneal epithelium which is the metabolically very active
- Clears debris and noxious substances
- Prevents infection due to antibacterial properties
- Improves movement of lids over globe as it is a lubricant
Secretion of tears
- It’s a continuous process by the lacrimal glands