Ophthalmic anatomy for the veterinary technician (Proceedings)

Eyelids- cover and protect the globe, remove debris by blinking, spread the tear film, eyelid margins contain the meibomian glands

Cilia (eyelashes)

Vibrissae large sensory hairs

Third Eyelid – also protects the globe and helps to spread the tear film and remove debris

Lacrimal glands – two glands in the dog and cat; one in the lateral upper eyelid and one at the base of the third eyelid, secrete the aqueous portion of the tear film

Nasolacrimal ducts – originate at the puncta of the eyelids, then continues via the canaliculi to the duct proper; the duct ends at the nasal puncta

Conjunctiva – protective lining over globe, contains goblet cells that produce mucin as part of the tear film, also part of the immune system of the eye

Palpebral –lines the eyelids

Third eyelid – covers the third eyelid

Bulbar – lines the globe

Tear Film

Lipid – produced by the meibomian glands

Aqueous – produced by the lacrimal glands

Mucin – produced by the goblet cells

Cornea

Epithelium – outer layer of the cornea

Stroma – main portion of the cornea, consists of lamellae in specific orientation to ensure corneal clarity

Descemet's membrane – innermost layer of the cornea; does not retain fluorescein stain

Endothelium – single cell layer of the inner cornea, does not regenerate and functions as the "sump pump" for the cornea

Anterior Chamber – space between the cornea and iris, filled with aqueous humor

Aqueous Humor (AH) and Iridocorneal Angle (ICA)

The AH is produced by the ciliary body and is an ultrafiltrate of blood that travels through the vessels of the ciliary bodythe AH nourishes the cornea and carries waste away from the anterior segment

The AH exits through the ICA and through an alternative pathway called the uveoscleral pathway

The ICA is the meshwork filter at the junction of the cornea and iris; the anatomy of the ICA determines the predisposition of the eye to glaucoma

Uvea – contains most of the blood vessels in the eye

Iris the central opening forms the pupil and controls the amount of light that reaches the retina

The color of the iris is determined by the number of pigmented cells in the iris

Ciliary body – produces the aqueous humor and removes waste from the eye

Choroid – consists of blood vessels and support tissues for the retina

Main source of nutrition for adjacent retina consists of 4 layers:

1. Suprachoroidea

2. Large-vessel layer

3. Medium-size vessel and tapetum layer

4. Choriocapillaris

Lens

Lens capsule – thin, transparent envelope around the lens, necessary to place an artificial lens at cataract surgery

Zonules – fibers that support and control the lens play a role in accommodation disruption may lead to a lens luxation (loose lens) +/- glaucoma

Vitreous

This body takes up the majority of the eye

Role in light transmission and in supporting both the lens and the retina

The clear matrix is made up of 99% water and 1% other (collagen and hyaluronic acid)

Disruption may be associated with lens luxation +/- glaucoma

Retina

Transparent light-absorbing tissue in the back of the eye

Appears membrane-like when detached

Composed of 10 layers that translate light energy into nerve impulses that are then processed by the brain

Optic Nerve

Conduit for electrical impulses from the retina to reach the brain and visual cortex

The optic nerve is usually myelinated in dogs and non-myelinated in cats

Orbit

Bony cavity surrounding the globe with openings (foramina) for blood vessels and nerves to supply the eye

Protects the globe

Open vs. complete

Position of the orbit controls the visual field of the animal

Multiple bones make up the orbit; these bones vary by species

Dog: lacrimal, zygomatic, frontal, sphenoid, palatine, maxillary

Cat: lacrimal, zygomatic, frontal, sphenoid, palatine, maxillary

Vision

Dog: 20/70

Cat: 20/60

Red/green "color-blind"

Visual horizon is very different between breeds of dogs due to size and height of the head; different breeds of cats have fairly similar vision