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 sli
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
extra-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
