The eye is composed of three layers - the fibrous tunic, vascular tunic, and retina. The fibrous tunic includes the sclera and transparent cornea. The vascular tunic contains the choroid, ciliary body, and iris. The retina lines the inside of the eye and contains light-sensitive rod and cone cells. Light passes through the cornea and lens and strikes the retina, where it is converted to nerve signals sent to the brain via the optic nerve. The iris controls the size of the pupil to regulate light entry. Various structures work together to focus light and maintain the eye's shape and pressure.

1. Structure of Eye

2. Vision
 Vision, the act of seeing, is extremely important to survival.
 More than half the sensory receptors in the human body are located in the eyes, and a
large part of the cerebral cortex is devoted to processing visual information.
Surface view of eye
 Eye is a sense organ sensitive to light (visible
portion of electromagnetic spectrum 380-760 nm)
 The eyes are held in protective bony sockets of
the skull called orbits
 Human eye is about 24mm in diameter and
weighs 6-8 g
 The eye is composed of accessary structures and
photoreceptors

3. Accessory structures of the eye include the eyelids, eyelashes, eyebrows, lacrimal
apparatus, and extrinsic eye muscles

4. Anterior view of the lacrimal apparatus

5. Anatomy of the Eyeball
 The adult eye ball measures about 2.5 cm (1 inch) in diameter.
 Of its total surface area, only the anterior one-sixth is exposed; the remainder is recessed
and protected by the orbit, into which it fits.
 Anatomically, the wall of the eyeball consists of three layers:
1. Fibrous tunic (anterior cornea and posterior sclera)
2. Vascular tunic (choroid, ciliary body, and iris)
3. Retina (contains photoreceptor cells-rods and cones and cells bodies and neurons
supplying optic nerve)

6. 1. Fibrous Tunic
 The fibrous tunic is the superficial layer of the eyeball and consists of the anterior cornea
and posterior sclera
 The cornea is a transparent coat that covers the coloured iris, it helps focus light onto the
retina; its outer surface consists of non keratinized stratified squamous epithelium, the
middle coat consists of collagen fibers and fibroblasts, and the inner surface is simple
squamous epithelium.
 Since the central part of the cornea receives oxygen from the outside air, contact lenses
that are worn for long periods of time must be permeable to oxygen
 The sclera, the “white” of the eye, is a layer of dense connective tissue made up mostly of
collagen fibers and fibroblasts; the sclera covers the entire eyeball except the cornea; it
gives shape to the eyeball, makes it more rigid, protects its inner parts, and serves as a site
of attachment for the extrinsic eye muscles.
 At the junction of the sclera and cornea is an opening known as the scleral venous sinus
(canal of Schlemm); a fluid called aqueous humor, drains into this sinus

7. Anatomy of the eyeball: The wall of the eyeball consists of three layers: the fibrous tunic, the
vascular tunic, and the retina.

8. 2. Vascular Tunic
 The vascular tunic or uvea is the middle layer of the eyeball; it is composed of three
parts: choroid, ciliary body, and iris
 The highly vascularized choroid, which is the posterior portion of the vascular tunic,
lines most of the internal surface of the sclera; its numerous blood vessels provide
nutrients to the posterior surface of the retina. The choroid also contains melanocytes
that produce the pigment melanin, which causes this layer to appear dark brown in color.
 Melanin in the choroid absorbs stray light rays, which prevents reflection and scattering
of light within the eyeball. As a result, the image cast on the retina by the cornea and lens
remains sharp and clear.
 Albinos lack melanin in all parts of the body, including the eye. They often need to wear
sunglasses, even indoors, because even moderately bright light is perceived as bright
glare due to light scattering.

9.  In the anterior portion of the vascular tunic, the choroid becomes the ciliary body; it
extends from the oraserrata, the jagged anterior margin of the retina, to a point just
posterior to the junction of the sclera and cornea.
 Like the choroid, the ciliary body appears dark brown in color because it contains
melanin-producing melanocytes.
 In addition, the ciliary body consists of ciliary processes and ciliary muscle.
 The ciliary processes are protrusions or folds on the internal surface of the ciliary body.
 They contain blood capillaries that secrete aqueous humor.
 Extending from the ciliary process are zonular fibers (suspensory ligaments) that attach
to the lens.
 The fibers consist of thin, hollow fibrils that resemble elastic connective tissue fibers.
The ciliary muscle is a circular band of smooth muscle.
 Contraction or relaxation of the ciliary muscle changes the tightness of the zonular
fibers, which alters the shape of the lens, adapting it for near or far vision.

10.  The iris, the colored portion of the eyeball, is shaped like a flattened donut.
 It is suspended between the cornea and the lens and is attached at its outer margin to the
ciliary processes.
 It consists of melanocytes and circular and radial smooth muscle fibers.
 The amount of melanin in the iris determines the eye color.
 The eyes appear brown to black when the iris contains a large amount of melanin, blue
when its melanin concentration is very low, and green when its melanin concentration is
moderate.
 A principal function of the iris is to regulate the amount of light entering the eyeball through
the pupil, the hole in the center of the iris.
 The pupil appears black because, as you look through the lens, you see the heavily
pigmented back of the eye (choroid and retina).
 However, if bright light is directed into the pupil, the reflected light is red because of the
blood vessels on the surface of the retina.
 It is for this reason that a person’s eyes appear red in a photograph (red eye) when the flash
is directed into the pupil.

11. Responses of the pupil to light of varying brightness: Contraction of the circular muscles
causes constriction of the pupil; contraction of the radial muscles causes dilation of the pupil
 Autonomic reflexes regulate pupil diameter in response to light levels; when bright light
stimulates the eye, parasympathetic fibers of the oculomotor (III) nerve stimulate the
circular muscles or sphincter pupillae of the iris to contract, causing a decrease in the
size of the pupil (constriction).
 In dim light, sympathetic neurons stimulate the radial muscles or dilator pupillae of the
iris to contract, causing an increase in the pupil’s size (dilation).

12. 3. Retina
 The third and inner layer of the eyeball, the retina, lines the posterior three-quarters of the
eyeball and is the beginning of the visual pathway
 This layer’s anatomy can be viewed with an ophthalmoscope, an instrument that shines
light into the eye and allows an observer to peer through the pupil, providing a magnified
image of the retina and its blood vessels as well as the optic (II) nerve
 A normal retina, as seen through an ophthalmoscope; Blood vessels in the retina can be
viewed directly and examined for pathological changes.
 The optic disc is the site where the optic nerve exits the eyeball. The fovea centralis is
the area of highest visual acuity

13.  The surface of the retina is the only place in the body where blood vessels can be viewed
directly and examined for pathological changes, such as those that occur with
hypertension, diabetes mellitus, cataracts, and age-related macular disease.
 Several landmarks are visible through an ophthalmoscope.
 The optic disc is the site where the optic (II) nerve exits the eyeball.
 Bundled together with the optic nerve are the central retinal artery, a branch of the
ophthalmic artery, and the central retinal vein.
 Branches of the central retinal artery fan out to nourish the anterior surface of the retina;
the central retinal vein drains blood from the retina through the optic disc.
 Also visible are the macula lutea and fovea centralis.
 The retina consists of a pigmented layer and a neural layer.
 The pigmented layer is a sheet of melanin-containing epithelial cells located between the
choroid and the neural part of the retina.
 The melanin in the pigmented layer of the retina, as in the choroid, also helps to absorb
stray light rays.

14.  The neural (sensory) layer of the retina is a multi-layered outgrowth of the brain that
processes visual data extensively before sending nerve impulses into axons that form
the optic nerve.
 Three distinct layers of retinal neurons- the photoreceptor layer, the bipolar cell layer,
and the ganglion cell layer are separated by two zones, the outer and inner synaptic
layers, where synaptic contacts are made.
 The light passes through the ganglion and bipolar cell layers and both synaptic layers
before it reaches the photoreceptor layer.
 Two other types of cells present in the bipolar cell layer of the retina are called
horizontal cells and amacrine cells.
 These cells form laterally directed neural circuits that modify the signals being
transmitted along the pathway from photoreceptors to bipolar cells to ganglion cells.

15. Microscopic structure of the
retina. The downward blue arrow
at left indicates the direction of
the signals passing through
the neural layer of the retina.
Eventually, nerve impulses arise
in ganglion cells and propagate
along their axons, which make up
the optic (II) nerve.
In the retina, visual signals pass
from photoreceptors to bipolar
cells to ganglion cells.

16.  Photoreceptors are specialized cells that begin the process by which light rays are
ultimately converted to nerve impulses.
 There are two types of photoreceptors: rods and cones.
 Each retina has about 6 million cones and 120 million rods.
 Rods allow us to see in dim light, such as moonlight. Because rods do not provide colour
vision, in dim light we can see only black, white, and all shades of gray in between.
 Brighter lights stimulate cones, which produce colour vision. Three types of cones are
present in the retina:
1. Blue cones, which are sensitive to blue light
2. Green cones, which are sensitive to green light
3. Red cones, which are sensitive to red light
 Colour vision results from the stimulation of various combinations of these three types
of cones. Most of our experiences are mediated by the cone system, the loss of which
produces legal blindness. A person who loses rod vision mainly has difficulty seeing in
dim light and thus should not venture out at night/dim light.

17.  From photoreceptors, information flows through the outer synaptic layer to bipolar cells and
then from bipolar cells through the inner synaptic layer to ganglion cells.
 The axons of ganglion cells extend posteriorly to the optic disc and exit the eyeball as the
optic (II) nerve.
 The optic disc is also called the blind spot, because it contains no rods or cones, we cannot
see images that strike the blind spot.
 The macula lutea is in the exact center of the posterior portion of the retina, at the visual
axis of the eye
 The fovea centralis, a small depression in the center of the macula lutea, contains only
cones.
 In addition, the layers of bipolar and ganglion cells, which scatter light to some extent, do
not cover the cones here; these layers are displaced to the periphery of the fovea centralis;
as a result, the fovea centralis is the area of highest visual acuity or resolution
 A main reason that you move your head and eyes while looking at something is to place
images of interest on your fovea centralis

18.  Rods are absent from the fovea centralis and are more plentiful toward the periphery of
the retina, because rod vision is more sensitive than cone vision, you can see a faint
object (such as a dim star) better if you gaze slightly to one side rather than looking
directly at it.

19. Lens
 Behind the pupil and iris, within the cavity of the eyeball, is the lens
 Within the cells of the lens, proteins called crystallins, arranged like the layers of an
onion, make up the refractive media of the lens, which normally is perfectly transparent
and lacks blood vessels.
 It is enclosed by a clear connective tissue capsule and held in position by encircling
zonular fibers, which attach to the ciliary processes.
 The lens helps focus images on the retina to facilitate clear vision

20. Interior of the Eyeball
 The lens divides the interior of the eyeball into two cavities: the anterior cavity and
vitreous chamber.
 The anterior cavity-the space anterior to the lens consists of two chambers; the anterior
chamber lies between the cornea and the iris and the posterior chamber lies behind the iris
and in front of the zonular fibers and lens
 Both chambers of the anterior cavity are filled with aqueous humor, a transparent watery
fluid that nourishes the lens and cornea.
 Aqueous humor continually filters out of blood capillaries in the ciliary processes of the
ciliary body and enters the posterior chamber; it then flows forward between the iris and
the lens, through the pupil, and into the anterior chamber.
 From the anterior chamber, aqueous humor drains into the scleral venous sinus (canal of
Schlemm) and then into the blood. Normally, aqueous humor is completely replaced about
every 90 minutes.
 The larger posterior cavity of the eyeball is the vitreous chamber, which lies between the
lens and the retina.

21. Section of eye through the anterior portion of the eyeball at the junction of the cornea and
sclera. Arrows indicate the flow of aqueous humor

22.  Within the vitreous chamber is the vitreous body, a transparent jellylike substance that
holds the retina flush against the choroid, giving the retina an even surface for the
reception of clear images. It occupies about four-fifths of the eyeball.
 Unlike the aqueous humor, the vitreous body does not undergo constant replacement.
 It is formed during embryonic life and consists of mostly water plus collagen fibers and
hyaluronic acid. The vitreous body also contains phagocytic cells that remove debris,
keeping this part of the eye clear for unobstructed vision. Occasionally, collections of
debris may cast a shadow on the retina and create the appearance of specks that dart in and
out of the field of vision.
 The pressure in the eye, called intraocular pressure, is produced mainly by the aqueous
humor and partly by the vitreous body; normally it is about 16 mmHg).
 The intraocular pressure maintains the shape of the eyeball and prevents it from
collapsing. Puncture wounds to the eyeball may cause the loss of aqueous humor and the
vitreous body. This in turn causes a decrease in intraocular pressure, a detached retina, and
in some cases blindness.