Anatomy of the Eyeball

Cavities of the Eyeball separated by the lens anterior posterior

Anterior Cavity between lens and cornea filled with aqueous humor (watery fluid) Divided into: - anterior chamber (anterior to iris) - posterior chamber (posterior to iris)

Posterior Cavity between lens and retina filled with vitreous body (jelly-like substance) also called vitreous chamber

Lens elastic, transparent, biconcave structure separates anterior and posterior cavities of eyeball suspended from ciliary body by suspensory ligaments tension on suspensory ligaments controls shape of lens

Layers of Eyeball (tunics) fibrous - outer - sclera - cornea vascular - middle - choroid - ciliary body - iris retina - inner nervous

Layers of Eyeball iris sclera cornea pupil choroid ciliary body retina

Sclera - outer fibrous “white of the eye” outermost protects eye thick, tough connective tissue capsule that maintains shape of eye serves as point of attachment for extrinsic muscles makes up 5/6 of sclera

Cornea - outer fibrous anterior 1/6 of fibrous tunic; continuous with sclera bulges forward, forming convex surface - refracts light rays as they enters eye transparent - allows light rays to pass lacks blood vessels receives nutrition from lymph has five layers

Cornea - outer fibrous (cont.) has touch and pain receptors injury causes scarring most exposed part of eye great ability to repair itself only tissue that can be transplanted from person to person without rejection

Scleral Venous Sinus also called canal of Schlemm junction of sclera and cornea drains aqueous humor from eyeball

Choroid - middle vascular layer vascular layer; blood rich contains dark pigment produced by melanocytes - absorbs pigment and prevents scatter of light after it passes through retina anterior portion becomes ciliary body and iris

Ciliary Body - middle vascular layer thickest part of vascular tunic forms internal ring in anterior part of eyeball within are projections or folds called ciliary muscles - secrete aqueous humor into anterior cavity lens is attached via suspensory ligaments

Iris - middle vascular layer extends out from ciliary body anterior to lens thin diaphragm of connective tissue seen from outside as colored portion of eye has rounded opening called pupil regulates amount of light entering posterior cavity of eyeball through pupil

Pupils bright light or close up - pupils constrict dim light or distance - pupils dilate

suspensory ligaments anterior chamber lens cornea posterior chamber pupil iris

Retina - inner nervous layer light sensitive is where light rays form an image image travels via optic nerve to cerebral cortex if image is not focused correctly, corrective glasses or lenses are required contains photoreceptors - rods and cones

Photoreceptors rods - 20 million - recognize gray tones and dim light cones - 6 million - recognize primary colors together they interpret intermediary colors in moonlight only rods are functioning; therefore we cannot see colors

Fovea Centralis depressed area in center of macula lutea - yellowish spot just lateral to optic axis of eyeball has highest concentration of cones in retina produces sharpest vision and best color perception

Optic Disc also called blind spot medial to optic axis fibers from ganglion cells exit eyeball to form optic nerve no photoreceptors; light striking this area produces no image

Color Blindness inability to distinguish colors caused by a lack or deficiency in one of the three cone photopigments most common type is red-green color blindness inherited condition affecting males more often than females - sex-linked

Intraocular Pressure caused when scleral venous sinus is obstructed and reabsorption of aqueous humor cannot keep up with its secretion pressure in chambers pushes lens back and puts pressure on vitreous body which in turn presses on retina which obstructs blood supply

Intraocular Pressure (cont.) retinal cells die and optic nerve may atrophy causing blindness (glaucoma) symptoms usually go unnoticed until damage is irreversible disease can be detected by use of tonometer used to measure intraocular pressure

Glaucoma group of eye diseases characterized by an increase in intraocular pressure pressure causes pathological changes in optic disk and visual field defects

Normal Flow of Intraocular Fluid

Abnormal Flow of Intraocular Fluid (most common type) egress is partially blocked causing increased accumulation of fluid causing increase pressure and eventual blindness

Acute Closed-Angle Abnormal Flow of Intraocular Fluid egress is totally blocked causing permanent blindness suddenly

Accessory Structures eyelids lacrimal apparatus extrinsic muscles cranial nerves

Eyelid Composed of: - skin covers outer surface - conjunctiva covers inner surface of eyelid and anterior surface of eyeball (except cornea)

Eyelid (cont.) Composed of: - tarsal glands modified sebaceous gland (oil) open at edge of each eyelid also called Meibomian glands - muscles orbicularis oculi - surrounds eye levator palpebrae - in upper eyelid

Lacrimal Apparatus lacrimal gland superior and inferior canaliculi lacrimal sac nasolacrimal duct

Lacrimal Gland located in upper portion of each orbit secretes constant flow of tears - wash anterior surface of eyeball - maintain moist and clean environment for cornea and conjunctiva - contain antibacterial enzyme lysozyme that helps prevent eye infections

Superior and Inferior Canaliculi collect tears after they have washed over eyeball Lacrimal Sac collects tears from canaliculi Nasolacrimal Duct connects lacrimal sac to nasal cavity where tears are swallowed

Extrinsic Muscles arise from bones of the orbit inserted into broad tendons on sclera Six extrinsic eyeball muscles: lateral rectus medial rectus superior rectus inferior oblique inferior rectus superior oblique

superior oblique superior rectus trochlea inferior oblique lateral rectus (cut) medial rectus inferior rectus

Primary Actions of the Eye Muscles abduction adduction elevation depression

Abduction contraction of lateral rectus moves pupil away from nose Adduction contraction of medial rectus moves pupil towards nose

Elevation contraction of superior rectus or inferior oblique muscles moves pupil upward Depression contraction of inferior rectus or superior oblique muscles moves the pupil downward

Cranial Nerves of Eyeball Innervation oculomotor (III) - branches innervate superior rectus, medial rectus, inferior oblique, and inferior rectus trochlear (IV) - innervates superior oblique abducens (VI) - innervates the lateral rectus