Ocular Anatomy

A Vision Teacher ‘s Guide

The Human Eye

Eyelid

• In addition to tear spreading, the eyelid is primarily responsible for corneal nutrition

• Also provides protection to cornea

• Lashes offer additional protection

Lacrimal System/Tear Film

• Lacrimal system is responsible for tear production and drainage

• Made up of 3 layers • Created primarily by lacrimal

apparatus and meibomian glands• Lubricate the eyeball, provides

oxygen/nutrition for cornea, has antibacterial properties and helps wash away debris

• Also have unique composition which keeps surface of cornea slick

Lacrimal System

Cornea

• Made up of 5 layers• Specialized Transparent Tissue

– No blood vessels• Primarily responsible for

refracting light – Does more of the job than the lens

• More nerve endings than anywhere else in the body– Protection to the eye

• The only part of the eye that is transplanted from one person to another

Aqueous humor

• Fills space between cornea and iris • Continuously produced by ciliary body• Flows into chamber through the pupil• Drains from eye through trabecular

meshwork to canal of schlemm• Nourishes the cornea and lens• Gives front of eyeball form and shape

– Anterior chamber is area between the cornea and the iris: filled with aqueous

– Posterior chamber is the area behind the iris and in front of the lens: filled with aqueous

Limbus

• Juncture between the cornea and the sclera

• Nourishes peripheral cornea…assists in corneal wound healing

• Pathway for aqueous outflow (contains trabecular meshwork and canal of schlemm)

Conjunctiva

• Thin translucent mucous membrane starts at the limbus and covers the sclera and inner surface of the eyelid

• Has some responsibility of tear production

• Subject to infection…problems from contact lens use

• Can be degraded by environmental conditions heat, wind, dust, etc.

Sclera

• Whites of the eye• Made up of 3 layers• Tough, fibrous tissue: site of

extra-ocular muscle attachment

• Opaque...allows no light to enter

• Subject to inflammation

Iris

• The colored part of the eye…unique to every individual like a fingerprint– Color is dependent on the amount of

pigment

• A diaphragm, the iris has tiny muscles that control the light levels in the eye

• Has 2 layers• Pupil is located in the center of the iris

– pupil = hole: it is not an eye structure per se

Lens

• Transparent, biconvex structure, held in place by ciliary zonules

• Composed of 6 layers• Refracts light • Nutrition comes from aqueous humor…

insoluble deposits of proteins build up over time = cataracts– A clouding of the lens and capsule

• Live long enough and you WILL have some degree of cataract

• cataracts also caused by other agents

Ciliary body

• Connects the choroid with the iris• Has three parts including:

– The ciliary muscle is ring shaped muscle that controls the shape of the lens (accommodation)

– The ciliary process is the attachment site for the zonules and produces the aqueous in the pars plicata

– The ciliary ring is attached to the choroid and is composed of the pars plana. The pars plana has no known function in the post-fetal eye thus this is a safe area through which surgical instruments may be inserted

Zonules

• Attach the lens to the ciliary body

• May become broken or stretched causing the lens to move out-of-place

Vitreous

• A thick, transparent gel like substance that fills the center of the eyeball, giving it form and shape

• A canal runs through the vitreous from optic disk to the lens. It is a developmental leftover from the hyaloid artery. Usually regresses but may persist and result in floaters

• May see reference to hyaloid membrane. This transparent tissue surrounds the vitreous and separates it from the retina

• Central retinal veins and arteries extend in bundles, exit and enter respectively through the optic nerve

Choroid

• A brown vascular sheet lying between the sclera and the retina

• This is the blood supply for the retina

Retina

• Most internal layer of eye, facing the vitreous

• Converts light energy into electrical energy which is then sent to the brain via the optic nerve

• Actually an extension of brain tissue • Composed of 10 layers…contains

photoreceptors: cones, near center (responsible for seeing detail and color) and rods, in periphery (responsible for seeing in low light and seeing movement)

• Point of sharpest vision is in the fovea; located in the center of the macula

Ora Serrata

• A serrated juncture between the retina and ciliary body marking the transition between non-sensitive tissue and the retinal portion with many layers and specialized photoreceptor cells

Intra-ocular muscles

• Purpose is to move eyes • Maintain binocularity• 6 muscles

– medial rectus (MR)—moves the eye toward the nose– lateral rectus (LR)—moves the eye away from the

nose– superior rectus (SR)—primarily moves the eye

upward and secondarily rotates the top of the eye toward the nose

– inferior rectus (IR)—primarily moves the eye downward and secondarily rotates the top of the eye away from the nose

– superior oblique (SO)—primarily rotates the top of the eye toward the nose and secondarily moves the eye downward

– inferior oblique (IO)—primarily rotates the top of the eye away from the nose and secondarily moves the eye upward

Optic Nerve

• Purpose is for energy transmission to brain

• Subject to underdevelopment, damage, inflammation

• Contains over 1 million nerve fibers…once severed cannot be reconnected=no “eye transplant”

• Upon examination only the head can be seen by doctor. Should appear as yellowish pink, flat and with distinct margins

• The cup to disk ratio is evaluation as a measure of health…increase in size of cup may indicate elevated pressure

Optic Nerve Pathways/Visual Cortex • Message is carried down the optic

nerve through pathways to occipital cortex; here vision becomes sight

• At the optic chiasm, the nasal nerve fibers cross; temporal nerve fibers go straight back to cortex; this arrangement impacts on visual fields

• Results in visual field losses can be predicted based on where damage is located on the optic nerve

• When damage is located anterior of the optic chiasm; it is likely there will be a cortical component to the field loss