Eye
Anatomy of the Eye
External Anatomy and Accessory Structures
Lacrimal Apparatus
Parts:
1. Lacrimal Gland: are situated superior to the lateral aspect of
each eye
2. Lacrimal Canals: passageway for tears to enter the
nasolacrimal
duct
3. Nasolacrimal Duct: empties excess tears into the nasal cavity
Lysozyme: an antibacterial enzyme that protects the eyes from bacteria
Eyelids ( Palpebrae): protective in nature
Parts:
Medial Canthus: contains the caruncle which produces oily
secretions
Lateral Canthus
Conjunctiva: lines the internal surface of the eyelids and anterior surface
of the eyeball
A mucous membrane
Secretes mucous that lubricates the eye
Conjunctivitis: inflammation of the conjunctiva
Characterized by redness of the eye
Eyelashes: project from the border of the eyelid
Ciliary Glands: modified sweat glands
Help lubricate the eye
Tarsal Glands: secrete oily substance
Sty: inflammation of a ciliary or tarsal gland
Internal Anatomy of the Eye
3 Tunics:
Fibrous Tunic: outermost
Dense avascular connective tissue
Sclera: makes up most of the fibrous tunic
Seen as the white of the eye
Cornea: is in the front of the eye
Is transparent
Area where light first enters the eye
Vascular Tunic (uvea): the middle tunic
Choroid: blood rich nutrient layer
Contains the pigment that prevents light from scattering
Ciliary Body: anterior portion of the choroid
Made up mostly of ciliary muscles which control lens
shape
Ciliary Processes: group of capillaries that secrete aqueous humor
Iris: most anterior part of the vascular tunic
Incomplete, with a rounded opening
Compose of circularly and radially arranged smooth muscle
fibers
Acts as a reflex to control the amount of light going into the
eye
Pupil: the opening in the iris
Area where light passes through
Sensory Tunic: inner most tunic
Retina: bilayered, delicate layer in the posterior of the eye
Pigmented Epithelial Layer: covers the ciliary body and posterior
side of the iris
Neural Layer: transparent inner layer
Goes only as far as the ciliary body
Contains photoreceptors
Rods and Cones: photoreceptors of the eye
Found over the entire neural retina except where the optic
nerve leaves the eye
Optic Disc: area where the optic nerve leaves the eye
Also called the blind spot
Macula Lutea: lateral to the blind spot and posterior to the lens
Is an area of high cone density
Fovea Centralis: a very small indentation in the eye
Contains the most cones and is the area of most accurate
vision
Lens: structure that focuses light on the retina
Flexible, see through structure
Suspensory Ligament: holds the lens vertically
Attaches the lens to the ciliary body
Cataracts: condition that occurs when the lens becomes hard and opaque
Causes hazy or obstructed vision
Divisions of the Lens
Anterior Segment: anterior to the lens
Aqueous Humor: clear, watery fluid
Helps maintain intraocular pressure
Provides nutrients for the lens and cornea
Scleral Venous Sinus (Canal of Schlemm): the area where
the aqueous humor is reabsorbed
Posterior Segment: behind the lens
Vitrous Humor (body): gel-like substance
Provides the major internal reinforcement of the
posterior part of the eye
Helps keep the retina pressed against the wall of the
eye
Is formed only before birth
Glaucoma: caused by compression of the retina and optic nerve
Occurs when drainage of aqueous humor is impaired which causes
an increase in intraocular pressure
Can cause pain and/or blindness
Microscopic Anatomy of the Retina
3 Major Neuronal Populations:
1. Photoreceptors
2. Bipolar Cells
3. Ganglion Cells
Rods: specialized receptors for dim light
Visual interpretation is in gray tones
Most numerous in the outer edges of the retina
Cones: color receptors
Allow high levels of visual acuity
Function only under high light intensity
Optic Nerve: ganglion cell axons that leave the retina in a tight bundle
Visual Pathways to the Brain
Optic Chiasma: area where the optic nerves from each eye meet and cross
over to the opposite side
Visual Acuity, and Astigmatism
Emmetropic Eye: the normal eye
Is able to accommodate perfectly and have perfect visual acuity
Myopia: nearsightedness
Can see close objects with no problem, but have trouble with
distant ones
Hyperopia: farsightedness
No problem seeing distant objects, but trouble with close ones
Astigmatism: irregularities in the curvature of the lens and/or cornea
Characterized by blurred vision
Near-Point Accommodation
Presbyopia: occurs in the elderly
Occurs when lens elasticity decreases
Near point accommodation: test to measure lens elasticity
Is closer in children than in adults
Visual Acuity: sharpness of vision
Tested with the Snellen eye chart
Based on the fact that the emmetropic eye can see letters of a certain size
clearly at specific distances
Color Blindness: tested using Ishihara’s color plates
Testing for deficiencies in cones