Eye in Cross Section : Anatomy : The Eyes Have It

Click on a label to display the definition. Tap on the image or pinch out and pinch in to resize the image

Conjunctiva:

mucous membrane covering anterior eye
bulbar conjunctiva covers sclera
palpebral conjunctiva covers inner surface of eyelids
keeps cornea moist and fends off infection
its blood vessels become dilated in inflammation and trauma to conjunctiva and adjacent structures

Schlemm's canal:

Extraocular muscle:

one of six muscles that move eye
there are four rectus muscles (medial, lateral, superior, inferior) and two oblique muscles (superior and inferior)
when disease weakens or shortens these muscles, eyes may go out of alignment and move poorly

Vitreous cavity:

filled with clear gel called "vitreous humor"
vitreous humor (usually just called "vitreous") made up of hyaluronic acid, collagen fibers, and dilute salt water solution
collagen is thickest in peripheral portion called "vitreous cortex"
vitreous cortex has firm attachments to anterior retina and border of optic disc
with aging, inflammation, high myopia, or trauma, vitreous degenerates into lakes of water that place stress on its attachments to retina, and...
vitreous detaches itself from optic disc, leaving floater, and...
peripheral vitreous cortex detaches from retina, sometimes tearing hole in retina
if fluid seeps under retinal hole, retina may detach
retinal detachment must be emergently repaired surgically to preserve vision

Lens:

protein-rich, bean-shaped structure suspended by zonular ligaments
responsible for 1/3 of refracting power of eye (the cornea accounts for 2/3)
opacification (cataract) occurs with advancing age, inflammation, or trauma
can be removed surgically without harming eye and replaced with clear artificial implant ("intraocular lens’)

Macula:

retinal region containing high density of cones and ganglion cells specialized in visual acuity and color vision
affected in many retina disorders, most commonly in age-related macular degeneration

Optic nerve:

contains retinal ganglion cell axons travelling to optic chiasm and on to lateral geniculate body
affected by many diseases that may also affect brain

Fovea:

central depression within macula in which retina consists only of cones
foveal pit comes about because other retinal tissue is pushed aside so light can travel directly to cones

Retinal artery & vein:

vessels that travel in innermost retinal layer
arteries nourish only retinal ganglion cells and their axons (deeper layers get blood from choroid)

Retina:

inner layer of posterior wall of eye (see Retina in Cross Section)
contains receptors that convert light energy into signals that brain can interpret

Choroid:

Sclera:

Ciliary body:

has two components: secretory epithelium and muscle
secretory epithelium manufactures aqueous fluid
muscle controls accommodation, change in refracting power of lens
when ciliary muscle contracts, zonules loosen, lens assumes more rounded shape, which gives it more refracting power and allows eye to focus on objects viewed at reading distance
accommodation is gradually lost with aging ("presbyopia"); lens stiffens and cannot react to loosening of zonules

Zonules:

fibers that extend from ciliary muscle to lens like hammock
when ciliary muscle relaxes, zonules are tightly stretched and lens is kept in elongated shape
when ciliary muscle contracts, zonules loosen and lens is allowed to take more rounded shape
as lens becomes more round, it has greater refracting power, so that eye can focus on objects viewed at reading distance (see "accommodation")

Posterior Chamber:

Iris:

diaphragm that controls light entering eye by constricting or enlarging
constriction governed by circumferential sphincter muscle controlled by parasympathetic nervous system
dilation governed by radial dilator muscle controlled by sympathetic nervous system
lined posteriorly by pigment epithelium
anterior stroma made of collagen, muscle and pigment cells
color of iris governed by amount of pigment in stroma

Pupil:

Anterior chamber:

filled with aqueous fluid produced by ciliary body
bounded by cornea in front and iris and lens in back
with iris inflammation, cells and protein leak into aqueous fluid to make it turbid ("flare")
with infection of iris or adjacent structures, white blood cells settle in bottom of anterior chamber to form "hypopyon"
With eye trauma, tearing of iris vessels causes blood to settle in bottom of anterior chamber to form "hyphema"

Cornea:

transparent anterior refracting surface of eye
accounts for 2/3 of refracting power of eye (lens accounts for 1/3)
three main layers: outer epithelium, collagenous stroma, and inner endothelium
if corneal epithelium is abraded or infected, it will regenerate without harming vision
if stroma is damaged, scar forms and opacifies cornea so as to reduce vision
endothelium keeps aqueous fluid from entering cornea so as to preserve its transparency
if endothelium is damaged, cornea swells and loses transparency