Anatomy and PhysioIogyof the Human Eye

The human eye is a complex sense organ that allows us to see. n order to process
information, parts of the eye receive visual cues and other parts transmit that information to our
brains. While it might sound simple, the process is quite complex and can be disrupted by a
number of eye disorders.

Structure Of The Eye: The Orbit
The orbit helps protect the eye from injury. The orbit is comprised of the following bones:
O ethmoid
O frontal
O lacrimal
O maxillary
O palatine
O sphenoid
O zygomatic.


!ortions of these seven bones converge and form a pyramid-shaped socket that points
toward the back of the head. t is within this socket, which is typically called the eye socket,
where the actual eyeball rests
.
Surrounding the eye in the eye socket is a layer of fat. This layer cushions the eyeball
and promotes smooth movement of the eye within the orbit.

The EyebaII
The actual eyeball contains three layers:
O the outer Iayer, which is formed by the cornea and the sclera (see below)
O the middIe Iayer, which holds the primary blood supply for the eye and contains the iris and
the pupil (see below)
O the inner Iayer, which is comprised of the retina (see below).
The eyeball also contains three chambers of fluid:
O anterior chamber (between cornea and iris)
O 5osterior chamber (between the iris and lens)
O ;itreous chamber (between the lens and the retina).
The first two chambers are filled with aqueous humor, a watery fluid that provides
nourishment to the interior eye structures and keeps the eyeball inflated. The vitreous chamber
is filled with a thicker fluid called the vitreous humor.

O5tic Ner;e

n addition to numerous eye blood vessels, the eye also features the optic nerve. The
optic nerve runs from the back of the eyeball and through an opening in the orbit called the optic
foramen. From this opening, the nerve connects to the brain to transmit visual information.

n addition to the optic nerve, the eye contains other nerves. However, most of these
nerves carry non-visual information and convey messages about pain or help control motor
activity in the eye.








EyeIids

The eyelids are thin folds of skin and muscle that cover the eyes. Eyelids help protect
the eye from scratches, foreign objects and more and also work to lubricate the eyes.

While we are awake, eyelids carry secretions from the lacrimal (tear) glands across the
eye when we blink. When we sleep, the eyelids close, helping to keep moisture on the surface
of our eyes.

The eyelid actually has several layers, including:

O a fibrous layer to provide stability
O a layer of muscle that controls the opening and closing of the eyelid
O a layer of skin that contains glands and the eyelashes
O the conjunctiva, a mucous membrane that connects the eyeball to the eyelid and the eyeball
to the orbit.
Tear GIands

The lacrimal glands, or tear glands, help keep the eye moist. They are located under the
upper eyelids and extend inward from the outer corners of the eye. Each gland can feature as
many as 12 tear ducts. These ducts flow into a sac, which drops tears onto the eye via the
puncta lacrimal, a small opening at the inner corner of the eyelid. The secretions of other glands
within the eye help keep the tears from evaporating when they reach the eye's surface.

Aqueous Humour

The Aqueous Humour is located at the front of each eye in the human
body. .The aqueous humour is a watery fluid that fills the chamber called the "anterior
chamber of the eye" which is located immediately behind the cornea and in front of
the lens - click for diagram, and also the "posterior chamber of the eye" which is a very
narrow compartment located between the peripheral part of the iris, the suspensory
ligament of the lens, and theciliary processes.The aqueous humour is very slightly alkaline
salt solution that includes tiny quantities of sodium and chloride ions.
t is continually produced, mainly by the capillaries of the ciliary processes, and drains away
into Schlemm's canal, located at the junction of the cornea and the sclera.

horoid
The choroid is the layer of the eyeball located between the retina and the sclera .The
structure of the choroid itself consists mainly of a dense capillary plexus and of many
arterioles and venules transporting blood to and from this plexus.
t is a thin, highly vascular (i.e. it contains blood vessels)
membrane that is dark brown in colour and contains a
pigment that absorbs excess light and so prevents
blurred vision (due to too much light on the retina).
The choroid is loosely attached to the inner surface of
the sclera by the lamina fusa. The side of the choroid
closest to the centre of the eyeball is attached to the
retina. This transparent innermost layer of the choroid is
calledBruch's Membrane.

The structure of the choroid itself consists mainly of a dense capillary plexus and of many
arterioles and venules transporting blood to and from this plexus
ornea
The cornea is the transparent circular part of the front of the human eyeball. t has
an important optical function as it refracts light entering the eye through the pupil and onto
the lens (which then focuses the light onto the retina).The degree of curvature of the cornea
varies between individuals and also throughout the life of an individual. t is more prominent
in youth than later in life, when it can become flatter in shape.
The cornea has a complex structure that specialist texts describe in terms of the following
layers (from the outside inwards):
1. Several strata of epithelial cells, continuous with those of the conjunctiva;
2. A thick central fibrous structure called the substantia propria ;
3. A homogeneous elastic lamina ;
4. A single layer of endothelial cells forming part of the lining membrane of the anterior
chamber of the eyeball.
The cornea a non-vascular structure (which means that it does not contain any blood
vessels) as the capillaries that supply it with nutrients terminate in loops at its
circumference. t is supplied by many nerves derived from the ciliary nerves. These enter
the laminated tissue of the cornea. t is therefore extremely sensitive.

o;ea
The fo;ea is a small depression forming a shallow pit in the retina at the back of
each eye in the human body. Because it contains a large number of the light-sensitive
photo-detector cells called cones, the fovea is the area of greatest acuity of vision.
This means that when an eye is directed at an object, the part of the image of that object
formed on the retina that falls onto the fovea is the part of the image that will be perceived in
the greatest detail. The fovea is slightly yellow in appearance and so was first called the
"0ow Spot" or "Macua Lut0a" of Smmerring (the scientist who first discovered and
documentedit).
The existence of such an area is only known to occur in humans, the quadrumana (a
group of primates comprising apes and monkeys), and some saurian reptiles. The subject
of visual (also known as "physiological") optics is a key component of many courses within
the fields of both biology and physics. t is also an essential consideration in the design of
displays and control units used in many applications from televisions and mobile telephones
to advanced aircraft. n the context of engineering "visual optics" is one of several medical
and psychological topics in the important area of "Human Factors".
HyaIoid Membrane
The hyaIoid membrane is a transparent membrane that encloses the vitreous
humour, separating it from the retina. n front of the ora serrata (the area in which the retina
terminates as a jagged margin towards the front of the eyeball as it approaches the cilliary
body) the hyaloid membrane is thickened by radial fibres and is called the Zonule of Zinn or
(another name for the same thing, thezonula ciliaris). The subject of visual (also known as
"physiological") optics is a key component of many courses within the fields of both biology
and physics. t is also an essential consideration in the design of displays and control units
used in many applications from televisions and mobile telephones to advanced aircraft. n
the context of engineering "visual optics" is one of several medical and psychological topics
in the important area of "Human Factors".
For further detail about components of the human eye mentioned on these pages,
we recommend Gray's Anatomy as one of the most informative yet inexpensive sources of
detailed information.
Iris
The Iris is the colored part of the human eye.
That is, the anterior surface of the iris has different colors in different individuals and is also
marked by lines that converge toward the pupil (as studied in iridology). However, the
posterior (back) surface of this iris has a deep purple tint due to two layers of pigmented
columnar epithelium. This pigmented epithelium is usually referred to as the "pars iridica
retinae" but is sometimes called simply "uvea" due to the similarity of its color to that of a
ripe purple grape. The Iris is a thin circular contractile curtain located in the aqueous
humour - in front of the lens but behind the cornea. t contains a circular aperture (or "hole" -
in non-medical terms!) called the pupil and located just to the nasal side of the centre of the
iris.
A simple description of the iris is that it is a colored diaphragm of variable size
whose function is to adjust the size of the pupil to regulate the amount of light admitted into
the eye. t does this via the 5u5iIIary refIex (which is also known as the "light reflex"). That
is, when bright light reaches the retina, nerves of the parasympathetic nervous system are
stimulated, a ring of muscle around the margin of the iris contracts, the size of the pupil is
reduced, and hence less light is able to enter the eye. Conversely, in dim lighting conditions
the pupil opens due to stimulation of the sympathetic nervous system that contracts of
radiating muscles, hence increases the size of the pupil.
The iris is composed of a series of layers, including: (1.) Flattened endothelial cells
on a hyaline basement-membrane; (2.) Stroma - consisting of fibres and cells; (3.) Muscular
Fibre - consisting of circular and radiating fibres; (4.) !igment - the location of pigment cells
differing in different irides; (5.) Arteries of the iris, and (6.) Nerves of the Choroid and ris.

Lens
The Iens is an important part of the eye. This Iens is a transparent structure
enclosed in a thin transparent capsule. t is located behind the pupil of the eye and encircled
by the ciliary processes - that slightly overlap its edges. The lens of the eye helps to refract
light travelling through the eye (which first refracted by the cornea). The lens focuses light
into an image on the retina. t is able to do this because the shape of the lens is changed
according to the distance from the eye of the object(s) the person is looking at.
This adjustment of shape of the lens is called accommodation and is achieved by the
contraction and relaxation of the ciliary muscle.
The Structure of the Lens
The ca5suIe of the Iens is a transparent, brittle, yet highly elastic membrane.
This capsule is thicker in front of the lens than behind it. The Iens itself is a transparent,
biconvex body of approx. 9-10 mm diameter and approx. 4 mm from front to back.
The basic structure of the lens is composed of concentric layers.
O5tic Pa5iIIa
The O5tic Pa5iIIa is also known as the Optic Disc.This, the O5tic Pa5iIIa (or Optic
Disc)
is the location on the retina of the eye at which the optic nerve leaves the eye
- transmitting signals from the eye to the brain.
The subject of visual (also known as "physiological") optics is a key component of
many courses within the fields of both biology and physics. t is also an essential
consideration in the design of displays and control units used in many applications from
televisions and mobile telephones to advanced aircraft. n the context of engineering "visual
optics" is one of several medical and psychological topics in the important area of "Human
Factors".
Pu5iI
The Pu5iI is located in the centre of each eye in the human body.
t generally appears to be the dark "centre" of the eye, but can be more accurately
described as the circular aperture in the centre of the iris through which light passes into the
eye. The size of the pupil (and therefore the amount of light that is admitted into the eye) is
regulated by the 5u5iIIary refIex (also known as the "light reflex"). That is, when bright light
reaches the retina, nerves of the parasympathetic nervous system are stimulated, a ring of
muscle around the margin of the iris contracts, the size of the pupil is reduced, hence less
light is able to enter the eye. Conversely, in dim lighting conditions the pupil opens due to
stimulation of the sympathetic nervous system that contracts of radiating muscles, hence
increases the size of the pupil.
Note that although some animals' eyes
are basically structured in a similar
way to human eyes, they may appear
to be very different.
E.g. Differently shaped pupils of cats
compared with people.

The subject of visual (also known as "physiological") optics is a key component of

many courses within the fields of both biology and physics. t is also an essential
consideration in the design of displays and control units used in many applications from
televisions and mobile telephones to advanced aircraft. n the context of engineering "visual
optics" is one of several medical and psychological topics in the important area of "Human
Factors".
#etina
The retina is located at the back of the human eye.The retina may be described as
the "screen" on which an image is formed by light that has passed into the eye via
the cornea, aqueous humour, pupil, lens, then the hyaloid and finally the vitreous
humour before reaching the retina.
The function of the retina is not just to be the screen onto which an image may be
formed (necessary but not sufficient), but also to collect the information contained in that
image and transmit it to the brain in a suitable form for use by the body.
The retinal "screen" is therefore a light-sensitive structure lining the interior of the eye. t
contains photosensitive cells (called rods and cones) and their associated nerve fibers that
convert the light they detect into nerve impulses that are then sent onto the brain along the
optic nerve.
ScIera
The scIera is the tough white sheath that forms the outer-layer of the ball.
t is also referred to by other terms, including the sclerotic and the sclerotic coat (both
having exactly the same meaning as the scIera).n all cases these names are due to the
the extreme density and hardness of the sclera (sclerotic layer). t is a firm fibrous
membrane that maintains the shape of the eye as an approximately globe shape. t is much
thicker towards the back/posterior aspect of the eye than towards the front/anterior of the
eye.The white sclera continues around the eye; most of which is not visible while the
eyeball is located in its socket within the face/skull. The main area of the eye that is not
covered by the area is the front part of the eye that is protected by the
transparent cornea instead.
The Structure of the ScIera
The sclera is composed of white fibrous tissue intermixed with fine elastic fibers and
corpuscles of flattened connective-tissue. These fibers are grouped together in bundles.
Blood supply to the sclera is via small (but not very numerous) interlinking capillaries.
The nerves connected to the sclera are from the ciliary nerves.
kCCLSS CI VISICN
ight waves from an object (such as a tree) enter the eye
first through the cornea, which is the clear dome at the front of the
eye. The light then progresses through the pupil, the circular
opening in the center of the colored iris.
Fluctuations in incoming light change the size of the eye's pupil. When the light entering
the eye is bright enough, the pupil will constrict (get smaller), due to the pupillary light response.
nitially, the light waves are bent or converged first by the cornea, and then further by
thecrystalline lens (located immediately behind the iris and the pupil), to a nodal point (N)
located immediately behind the back surface of the lens. At that point, the image becomes
reversed (turned backwards) and inverted (turned upside-down).
The light continues through the vitreous humor, the clear gel that makes up about 80%
of the eye's volume, and then, ideally, back to a clear focus on the retina, behind the vitreous.
The small central area of the retina is the macula, which provides the best vision of any location
in the retina. f the eye is considered to be a type of camera (albeit, an extremely complex one),
the retina is equivalent to the film inside of the camera, registering the tiny photons of light
interacting with it.
Within the layers of the retina, light impulses are changed into electrical signals. Then
they are sent through the optic nerve, along the visual pathway, to the occipital cortex at the
posterior (back) of the brain. Here, the electrical signals are interpreted or "seen by the brain
as a visual image.
Actually, then, we do not "see with our eyes but, rather, with our brains. Our eyes
merely are the beginnings of the visual process.