p Essentials of Anatomy Seeley Stephens Tate SPECIAL SENSES PART III VISION AND THE EYE

 Eyebrows:  Perspiration, sunlight  Eyelids & lashes  Foreign objects (blink reflex)  Lubricate eyes with blinking (20/min)  Conjunctiva  Membrane inner eyelid/ covers anterior eye  Lubrication  Small amt tears  mucus ACCESSORY STRUCTURES OF THE EYE

LACRIMAL APPARATUS

 Rectus muscles  Superior, inferior  Medial, lateral  Oblique muscles  Superior, inferior EXTRINSIC EYE MUSCLES

 3 Tunics (layers)  Fibrous  Vascular  Nervous ANATOMY OF THE EYE

 Sclera: firm white outer connective tissue  maintain shape, attachment of muscle, protect  Cornea: transparent, anterior eye  allow light to enter, refracts light FIBROUS TUNIC

 most blood vessels  CHOROID: Thin vascular network and melanin- containing cells, appears black  CILIARY BODY: smooth muscles,  LENS: flexible transparent disc  IRIS: colored part, contractile muscles, anterior to lens, regulates diameter of the pupil  PUPIL: opening, light passes through VASCULAR TUNIC

 Pigmented Retina: w/choroid keeps light from reflecting into the eye  Sensory Retina: contains rods/cones, interneurons  Rods: 20 x more common, sense light  Cones: need more light, sense color  6-7 million  Long (Red), Short (blue), Medium (green) varieties NERVOUS TUNIC

 Rod cells: rhodopsin (opsin and retinal) pigment  Light changes retinal’s shape  Night blindness: vitamin A deficiency  Cone cells: blue, red, green opsin NERVOUS TUNIC

 Rods/cones synapse with bipolar sensory cells  then ganglion (cell bodies) which converge to form optic nerve NERVOUS TUNIC

 Fovea Centralis: only cone cells, light most focused  Macula lutea: yellow spot contains fovea centralis  Optic disc: blood vessels enter NERVOUS TUNIC  Optic disc:  blood vessels enter  optic nerve exits  “blind spot” of the eye  no photoreceptors Macula lutea Fovea Centralis

 Anterior and Posterior chambers:  aqueous humor-watery fluid of eye, produced by ciliary body  Maintain pressure, refracts light  Vitreous chamber:  vitreous humor – jellylike substance helps maintain pressure, hold lens and retina in place CHAMBERS OF THE EYE

FUNCTIONS OF THE EYE

 Cornea: Convex, light is bent so the rays converge  Lens: also convex, fine focus adjustments by changing shape  Ciliary body maintains pressure on lens, relatively flat = long distance  Object closer than 20 feet, ciliary muscles contract, ciliary body closer to lens, lens flexes more spherical  Accommodation: process of ciliary body moving lens to focus on objects closer than 20 feet ACCOMMODATION

EYE TESTS & DISORDERS

 Snellen Eye Chart  At 20 feet you can read a letter that most humans can read at 20 feet  20/200 means you can read at 20 feet a letter most can read at 200 feet  LogMAR chart is considered more reliable font, spacing, number of letters STATION 1: VISUAL ACUITY

STATION 2: ASTIGMATISM  Misshapen cornea or lens  Blurry, fuzzy vision at all distances  Snellen Astigmatism Test  If lines appear wavy or different colors (thinner), you may have astigmatism  Clear vision, same thickness same spacing

 Myopoia: Near Sighted (trouble with distant)  Eye focuses light in front of the retina  Long eyeball, strong lens MYOPIA & HYPEROPIA  Hyperopia:  Far Sighted  trouble close objects  5-10% of Americans  Flat lens, short eyeball

 Decrease in ability of eye to accommodate near vision  Part of aging process  Lens is no longer flexible PRESBYOPIA

 Optic Disc – no photoreceptors BLIND SPOT

 August Seebeck used 300 colored papers (1837). Participants matched papers to a sample color.  Holmgren (1877) used colored wool. Colors that are difficult to distinguish can indicate color vision difficulties.  Ishichara color plates are commonly seen for color vision deficiency testing. STATION 4: COLOR VISION

 Can be complete or only decrease in perception  Most common cause is faulty cone development  More frequent in males (8% males with northern european ancestry, only 1% of females)  X-linked genetic trait COLOR DEFICIENCY (COLOR BLINDNESS)

 Conjunctivitis: inflammation (usually bacterial, aka pinkeye)  Neonatal ophthalamia: severe form, high risk of blindness to infant if mother is infected  Chlamydial conjunctivitis  Gonorrheal EYE DISORDERS  Trachoma: leading infectious cause of blindness; scarring of cornea

 A clouding in the lens  Proteins that make up the lens clump together  Most are related to aging  Smoking, diabetes, radiation also suspected causes CATARACT (b) The same scene viewed with cataracts. (a) Normal vision

 Cataract: (22 million U.S.)  Clouding of lens due to age, infection, trauma  Glaucoma: (2 million)  Pressure build up from aqueous humor  Diabetes meliitus:  A major cause of blindness in U.S.  Damage to optic nerve, cataracts, retinal detach CAUSES OF BLINDNESS