Transcript Heart and circulation.

Eye- Structure and Refraction
Prof. K. Sivapalan
Structure of the Eye.
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Sclera.
Cornea.
Conjunctiva.
Choroid.
Retina
Iris and pupil.
Ciliary body, muscles and
lens.
• Anterior and posterior
chambers.
• Vitreous
• Optic disc, macula.
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Lacrimal gland and tearing.
• Lacrimal gland secretes
tear which flows through
lacrimal duct of the upper
eye lid.
• It moistens the cornea
and washes it- removing
dust.
• Lower eye lids collect the
tears which go through
naso-lacrimal duct.
• Increased by
parasympathetic.
• Xerophthalmia- damage
to cornea.
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Nutrition of Eye.
• Retina gets nutrients from
retinal artery that enters
eye with optic nerve.
• Fovea has no vessols.
Nurished by diffusion
from choroid.
• Other structures get the
nutrients from vessels in
the choroid.
• There are no vessels in
the anterior chamber,
lens, posterior chamber
and vitreous.
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Vitreous and Aqueous Humor.
• Vitreous is clear
gelatinous material
through which nutritious
material diffuse.
• The space between the
lens and the cornea is
divided into anterior and
posterior chambers by
the iris and filled with
aqueous humor.
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Aqueous humor.
• Ciliary body has ciliary processes in the posterior
chamber where aqueous humor is secreted by active
transport of sodium and other nutrients.
• The fluid passes through the pupil into anterior chamber
and into the angle between the cornea and the iris.
• It is absorbed through the trabeculae into the Chanal of
Schlemm and into veins.
• It is formed at a rate of 2.5 μL / min. and is responsible for
the intra-ocular pressure of 15 [10-20] mm Hg and
nourishes the cornea and lens.
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Abnormalities of Intra-ocular
Pressure.
• Dehydration reduces intra-ocular pressure.
• It can result in abnormal curvatures in cornea.
• Obstruction to absorption results in increase in
pressure [Glaucoma]
• Increased pressure compresses retina, optic
nerve and retinal vessels.
• The optic nerve is affected at the optic disc
because it is the weak point in the sclera.
• It results in loss of vision in peripheral fieldconical vision at early stages. This can
progress to total blindness.
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Principles of Optics.
• Light rays are refracted when they pass from one
medium to another.
• Parallel rays [more than 6 meters] striking on biconvex
lens are refracted to principal focus.
• The principal focus is in the principal axis.
• Rays from closer objects are diverging and focus on a
point further than principal focus.
• Biconcave lens causes the rays to diverge.
• Larger the curvature greater the refractive power.
• Refractive power is the reciprocal of focal length in
meters.
• If the focal length is 25 cm, refractive power is 1/0.25= 4
diopters. Refractive power of human eye = 60 diopters
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Refraction in Eye.
• When light rays from a distant object falls on the cornea,
it is refracted:
• On entry into cornea,
• On entry into aqueous humor,
• On entry into lens, and
• On entry into vitreous humor.
• Finally an image is formed on the retina. The image is up
side down.
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Accommodation.
• Light rays from an object closer
than 6 meters will be diverging and
the image will be formed behind the
retina.
• In a camera, the lens is moved
forwards to get the image on the
film.
• In the eye, the lens curvature is
increased to increase the refractive
power and the image is formed on
the retina.
• The reflex mechanism is
accommodation.
• This can increase the refractive
power by about 12 diopters.
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Ciliary body and lens.
• Suspensory ligaments from the ciliary body keep lens
in place, under tension when the ciliary muscles are
at rest.
• The lens is pulled into a flattened shape by the
tension because the lens substance is malleable and
the lens capsule is elastic.
• Contraction of circular muscles reduce tension by
sphincter like action and the meridional muscles
reduce tension because they are attached to corneoscleral junction and ciliary body.
• Reduction in tension results in increase in the
curvature and refractive power of the lens.
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Near Point.
• Light rays from very near to eye cannot be
focused on the retina.
• The nearest point from which an object can be
focused on the retina is the ‘near point’.
• Near point receds through out the life, 9 cm at
10 years, 10.5 at 20 years and 83 cm at 60
years.
• This is known as presbyopia and can be
corrected by convex lens.
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Refractive errors.
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Astigmatism.
• When light rays go through lens with surfaces
as part of sphere, the rays converge at the
focal point.
• If light goes through cylindrical lens, the rays
converge in a line parallel to the lens.
• When the surface of the cornea happens to be
a part of distorted sphere, rays in different axis
focus at different points resulting in blurred
image.
• This can be identified by chart with radiating
lines and corrected by cylindrical lens.
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Iris and pupil.
• Iris is a pigmented, opaque membrane attached
to ciliary body.
• Pupil is the opening at the centre if iris through
which light rays reach the lens.
• The diameter of the pupil can be altered from 8
mm to pinpoint by the circular and radial muscles.
• The size of the pupil determines the amount of
light reaching the retina.
• Smaller diameter increases the depth of focus
and reduces spherical aberration of the periphery
of the lens.
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