The Eye
Definition
Organ of the sense of sight
Situated in the orbital cavity
Structure
Spherical
Between the eye and the orbital cavity- fatty tissue;
The eyes are structurally separate; but are coordinated.
Two eyes - sense of depth and distance
Three layers
o Outer fibrous layer : sclera and cornea
o Middle vascular layer: choroid, ciliary body and iris
o The inner nervous tissue layer : retina.
o Inside the eyeball are the lens, aqueous fluid and vitreous body
Conjunctiva
o Transparent membrane lining the eyelids and the front of the eyeball
o Lower portion - lower conjunctival sac
o The medial and lateral angles - the medial canthus and the lateral canthus
Sclera
o Sclera - the tough outer wall of the eye - Firm fibrous membrane - white portion of the eye
o Outermost layer of the eye ball
o Continuous with the cornea anteriorlyo
o Gives attachment to the extraocular muscles of the eye
Cornea
o Continuation of the sclera anteriorly
o Transparent
o Convex
o Epithelial membrane
o Light rays pass through & reach the retina
o Convexity refracts the light rays (bends)
Choroid
o Choroid is the thin spongy layer between sclera and the retina
o Lining of the posterior five sixths of the inner surface of the sclera
o Rich in blood vessels
o Deep chocolate brown in color
Ciliary body
o Anterior continuation of the choroid
o Consists of smooth muscle fibres (ciliary muscle)
o And secretory cells - aqueous fluid into the anterior segment
o Suspensory ligament is attached to it
o The suspensory ligament attaches capsule of the lens
o Contraction or relaxation of the ciliary muscle changes the thickness of the lens - accommodation
o The ciliary body is supplied by parasympathetic branches of the oculomotor nerve.
Iris
o Visible coloured part of the eye - colour genetically determined
o Extends anteriorly from the ciliary body,
o Lies behind the cornea, in front of lens
o Anterior chamber in front of iris
o Posterior chamber Behind iris
o The chambers contain aqueous fluid secreted by the ciliray body
o Iris is composed of pigment cells and two layers of muscle fibres, one circular and one radiating
o Centre - hole or aperture - pupil. when iris contracts the pupil is constricted and when iris relaxes the iris is dilated -
o Parasympathetic supply - oculomotor
Lens
o Highly elastic
o Circular biconvex transparent body
o Behind pupil
o Suspended from the ciliary body by the suspensory ligament
o Enclosed within a transparent capsule
o Thickness controlled by the ciliary muscle
o Contraction of the muscle moves the lens forward thereby reducing the pull on the lens and increasing the thickness of the lens
Retina
o Innermost layer
o Stimulated by light rays
o Several layers of nerve cell bodies and their fibres
o Lying on a pigmented layer of epithelial cells
o The layer highly sensitive to light is the layer of rods and cones.
o macula lutea spot on the retina where finer detailed focus occurs.
The posterior 2/3 of the eye is occupied by the retina.
It contains millions of rod and cone cells which convert light energy into electrical signals sent to the brain via the optic nerve.
o Fovea centralis - only cone shaped cells : highest concentration of cone photoreceptors. Light rays are focused by the lens onto the fovea for straight ahead vision and fine detail.
o anteriorly there are fewer cones than rods
o contain photosensitive pigments which convert light rays into nerve impulses.
o The optic nerve and the central retinal blood vessels enter the back of the eye at the disc (also called the blind spot).
Optic nerve
o The nerve fibres originate in the retina
o All fibres converge to form the optic nerve
o Passes through the optic foramen of the sphenoid bone and meets the nerve from the opposite side at the optic chiasma
o Optic chiasma : In front of and above the pituitary gland
o Nerve fibres of the optic nerve from the nasal side of each retina cross over to the opposite side. The fibres from the temporal side do not cross but continue on the same side
Optic tracts
o These are the pathways of the optic nerves posterior to the optic chiasma
o Each tract consists of the nasal fibres of the opposite retina and the temporal fibres of the same side retina.
o Pass through the cerebrum to synapse with nerve cells of the lateral geniculate bodies of the thalamus. Thence the fibres pass as optic radiations to terminate int the visual area of the cerebral cortex in the occipital lobe of the cerebrum.
o Other neurones originating in the lateral geniculate bodies convey impulses from the eyes to the cerebellum where together with impulses from the smicircular canals, and from the skeletal muscles and joints, they contribute to the maintenance of the balance.
o Rhodopsin (visual purple) - degraded or bleached when exposed to light - regeneration
requires vitamin A.
o The time taken for the regeneration of rhodopsin is called dark-adaptation
o Other pigments present in cones respond to different wavelengths of visible ligt and are responsible for colour vision.
o 0.5 cm to the nasal side of the macula lutea all the nerve fibres of the retina converge to form the optic nerve which passes through the sphenoid bone to the cerebral cortex in the occipital lobe of the cerebrum.
o Optic disc or the blind spot - optic nerve leaves the eye.
Blood supply to the eye
o Internal carotid artery through its branches-
ophthalmic artery :-
ciliary arteries -
central retinal artery.
o Venous drainage is by a number of veins, central retinal vein cavernous sinus.
o Central retinal artery and vein are encased in the optic nerve. Entering the eye at the optic disc.
Interior of the eye ball
Anterior segment
the portion anterior to the lens
o the anterior segment is divided into the anterior and posterior chambers by the iris. between the cornea and the iris is the anterior chamber and between the iris and the lens behind is the posterior chamber
o contains the Aqueous fluid (humour)
o Secreted into the posterior chamber by ciliary glands
o It passes in front of the lens through the pupil into the anterior chamber and returns to the venous circulation through the canal of Schlemn (scleral venous sinus) in the angle between iris and cornea. - continuous production and drainage -
o Intraocular pressure remains 1.3 to 2.6 kPa (10 to 20 mm Hg) increase glaucoma.
o Aqueous -contains nutrients- to transparent structures cornea lens, lens capsule
Posterior segment
o the cavity behind the lens
contains vitreous body (humour)
o Soft coloureless, transparent, jelly-like substance
o Water salts mucoproteins
o Intraocular pressure maintained by the humours.
Extraocular muscles of the eye
o Six extrinsic muscles
o Attached to the eye ball at one end and to the walls of the orbital cavity at the other end.
o 4 straight muscles and 2 oblique muscles :
o Medial rectus - rotates inwards
o Lateral rectus - rotates outwards
o Superior rectus - rotates upwards
o Inferior rectus - rotates downwards
o Superior oblique - downwards and outwards
o Inferior oblique - upwards and outwards
o All these are Voluntary muscles - needed for convergence and accommodation
Nerve supply to the muscles of the eye
extrinsic muscles
- Superior rectus
- Inferior rectus
- Medial rectus
- Inferior oblique
Intrinsic muscles
- Iris
- Ciliary muscle
All of the above six muscles are supplied by oculomotor nerve
o Superior oblique - trochlear nerve
o &nblt;font face="Palatino Linotype" class="ws12">abducent nerve
Eye brows
o Arched
o Supra orbital
o Protect the eye from sweat, dust, and other foreign bodies
o Eye lids (palpebrae)
o Movable folds of tissue
o Upper and lower
o Free edges have hair - eye lashes
Layers of the eyelids are:
o Skin
o Areolar tissue
o Two muscles - the orbicularis oculi and levator palpebrae superioris
o Tarsal plate - thin sheet of dense conective tissue
o A lining of conjuctiva
Eyelid margins
o Along the edges sebaceous glands
o Ducts open into the hair follicles and on to the eyelid margins between hairs.
o Meibomian glands (tarsal glands) are modified sebaceous glands embedded in the tarsal plates - ducts open into the inside of the free margins of the eyelids - oily secretion - spread to delay evaporation of the tears
Lacrimal apparatus
o For each eye this consists of
o 1 lacrimal gland and its ducts
o 2 lacrimal canaliculi
o 1 lacrimal sac
o Nasolacrimal duct
The lacrimal glands
o In recesses in the frontal bones on the lateral aspect of each eye just behind the supraorbital margin
o Have secretory epithelial cells
o Tears are secreted - water mineral salts, antibodies and lysozyme, a bactericidal enzyme
o Several small ducts - pass over the front of the eye under the lids towards the medial canthus where it drains into two lacrimal canaliculi the opening of each is called the punctum
o The two canaliculi lie on lie one above the other separated by a small red body, the caruncle.
o The tears then drain into the lacrimal sac the upper end of the nasolacrimal duct
o Nasolacrimal duct opens into the nasal cavity opening at the level of the inferior concha.
Additional information
o The human eye works like a living camera. Like a camera, the eye processes light and takes mental snapshots of images, which are then developed in the brain. In order to create vision, all parts of the eye must work together as a team. The cornea, the clear, film-like part of the eye, is the leader. When light enters the dark pupil of the eye, the cornea bends it. The refracted light travels to the lens behind the pupil. The lens changes its shape to adjust the focus on both distant and near objects. This process is known as accommodation.
o The lens projects light from the outside world as an inverted image into the retina, which then acts like film in a camera and captures the image. The image is sent to the brain in the form of electric impulses to be developed.
