THE EYES AND VISION Vision is the dominant

THE EYES AND VISION • Vision is the dominant sense in humans • 70% of sensory receptors in humans are in the eyes • 40% of the cerebral cortex is involved in processing visual information • The eye (or eyeball) is the visual organ – It is globe shaped with diameter 2. 5 cm (1 inch) – It is slightly flattened from above downwards – Its anterior part is small and forms 1/6 of the eye ball while it posterior part is large and forms 5/6 of the eye ball. – Lies in bony orbit

Accessory structures of the eye The followings are the accessory parts of the eye: • Eyebrows • Eyelids or palpebrae – Upper & lower separated by palpebral fissure – Corners: medial & lateral canthi – Eyelashes 3

EYELIDS - It protects the eye ball - It opens and closes voluntarily and reflexly - The margins of the eyelids have sensitive hairs called cilia. - Each of the cilia has a follicle surrounded by a nerve plexus - Blinking of eyelids occur when sensory nerve are activated. - Prevent dust particles from reaching eyeball. - The opening between the two eyelids is known as palpebral fissure. 4

• Eyelid tarsal plates give structure – Where orbicularis oculi muscles attach (close eyes) • Levator palpebrae superioris muscle – Lifts upper lid voluntarily (inserts on tarsal plate) 5

• Tarsal glands – modified sebaceous (oil) glands in tarsal plates • Conjunctiva - transparent mucus membrane of stratified columnar epithelium which covers the exposed part of the eye. – Palpebral conjunctiva – Bulbar conjunctiva • Covers white of eye but not the cornea (transparent tissue over the iris and pupil) 6

• Conjunctiva • It covers the anterior surface and also reflected into the inner surface of eyelids. – Palpebral conjunctiva: The part of conjunctiva covering the eyelids – Bulbar conjunctiva : The part of conjunctiva covering the eye ball. . The surface of conjunctiva is lubricated by a thin film of tears secreted by lacrimal glands 7

Lacrimal apparatus • Responsible for tears – The fluid has mucus, antibodies and lysozyme • Lacrimal gland in orbit superolateral to eye • Tears pass out through puncta into canaliculi into sac into nasolacrimal duct • Empty into nasal cavity (sniffles) 8

LACRIMAL GLAND • It is situated in the shelter of bone forming the upper and outer border of wall of the eye socket • Tears flow from lacrimal gland over the surface of conjunctiva and drains into nose through lacrimal duct, lacrimal sac and nasolacrimal duct. • Conjunctiva is kept moist and is protected from infection due to its continuous washing and lubrication by tears • Tears also contains lysozyme that kills bacteria. • Secretion of tears is controlled by the parasympathetic fiber of facial nerve.

LACRIMAL GLAND • At birth the nasolacrimal duct may not be fully developed causing a watery eye. • Acquired obstruction of the nasolacrimal duct is a common cause of water eye in adult which may lead to acute infection of the sac.

Extraocular (extrinsic) eye muscles: 6 in # “EOMs intact” means they all work right • Four are rectus muscles (straight) – Originate from common tendinous or anular ring, at posterior point of orbit • Two are oblique: superior and inferior 11

Extraocular (extrinsic) eye muscles Cranial nerve innervations: • Lateral rectus: VI (Abducens n. ) – abducts eye outward • Medial, superior, inferior rectus & inf oblique: III (Oculomotor n. ) – able to look up and in if all work • Superior oblique: IV (Trochlear n. ) – moves eye down and out 12

Innervation 13

• Double vision: diplopia (what the patient experiences) – Eyes do not look at the same point in the visual field • Misalignment: strabismus (what is observed when shine a light: not reflected in the same place on both eyes) – can be a cause of diplopia – – Cross eyed Gaze & movements not conjugate (together) Medial or lateral, fixed or not Many causes • Weakness or paralysis of extrinsic muscle of eye – Surgical correction necessary • Oculomotor nerve problem, other problems • Lazy eye: amblyopia – Cover/uncover test at 5 yo – If don’t patch good eye by 6, brain ignores lazy eye and visual pathway degenerates: eye functionally blind NOTE: some neurological development and connections have a window of time - need stimuli to develop, or ability lost 14

3 Layers form the external wall of the eye 1. (outer) Fibrous: dense connective tissue – – Sclera – white of the eye Cornea • • • 2. 100 s of sheets of collagen fibers between sheets of epithelium and endothelium Clear because regular alignment Role in light bending Avascular but does have pain receptors Regenerates (middle) Vascular: uvea – – – 3. Choroid – posterior, pigmented Ciliary body Iris (colored part: see next slide) (inner) Sensory – Retina and optic nerve 15

1. (outer layer) Fibrous: dense connective tissue – – 2. Sclera – white of the eye Cornea (middle) Vascular: uvea – – Choroid – posterior, pigmented Ciliary body • • • – 3. Muscles – control lens shape Processes – secrete aqueous humor Zonule (attaches lens) Iris (inner layer) Sensory – Retina and optic nerve 16

Layers of external wall of eye continued 1. (outer) Fibrous: dense connective tissue – – 2. Sclera – white of the eye Cornea (middle) Vascular: uvea – – Choroid – posterior, pigmented Ciliary body – Iris Pigmented put incomplete: pupil lets in light Sphincter of pupil: circularly arranged smooth muscle parasympathetic control for bright light and/or close vision Dilator of pupil: radiating smooth muscle – sympathetic control for dim light and/or distance vision (inner) Sensory 3. – Retina 17

Layers of external wall of eye continued 1. (outer) Fibrous: dense connective tissue – Sclera – white of the eye – Cornea 2. (middle) Vascular: uvea – Choroid – posterior, pigmented – Ciliary body – Iris 3. (inner) Sensory – Retina -------will cover after the chambers 18

some pictures… 19

Chambers and fluids (see previous pics) • Vitreous humor in posterior segment – Jellylike – Forms in embryo and lasts life-time • Anterior segment filled with aqueous humor – liquid, replaced continuously – Anterior chamber between cornea and iris – Posterior chamber between iris and lens – Glaucoma when problem with drainage resulting in increased intraocular pressure 20

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