eye overview

1. Anatomy and physiology the eye
DR. CHIKONDE

2. Introduction
► Vision is extremely important to human 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.
► In this session we will be dealing with:
1)The accessory structures of the eye.
2)the eyeball itself
3)Image Formation

3. Accessory Structures Of the eye
► What are the accessory structures of the eye?
► The accessory structures of the eye include
1)The eyelids
2)Eyelashes
3)Eyebrows
4)The lacrimal apparatus
5)Extrinsic eye muscles

4. Contn..

5. Contn…..
► EYELIDS – The upper and the lower eyelids, or palpebrae ,shade the
eyes during sleep, protect the eyes from excessive light and foreign
objects and spread lubricating secretions over the eyeballs.
► EYELASHES ANS EYEBROWS – The eyelashes, which protect from the
border of each eyelid, and the eyebrows, which arch transversely
above the upper eyelids, help protect the eyeballs from foreign
objects, perspiration, and the direct rays of the sun.

6. Contn…
► LACRIMAL APPARATUS – Each eye consists of
a)1 lacrimal gland
b) 2 lacrimal canaliculi
c)1 lacrimal sac
d)1 nasolacrimal duct
► The lacrimal glands are exocrine glands situated in the frontal bones on the
lateral aspect of each eye just behind the supraorbital margin.
► Each gland is approximately the size and shape of an almond and is composed
of secretory epithelial cells.
► The glands secrete tears composed of water mineral salts, antibodies and
lysozymes, a bacterial enzyme.

7. Contn…….
► EXTRINSIC EYE MUSCLES – Six extrinsic eye muscles move each eye:
1)The superior rectus 1)superior oblique
2)The inferior rectus 2)Inferior oblique
3)Lateral rectus
4)medial rectus
► They are supplied by cranial nerves III,IV and VI.
► The extrinsic eye muscles move the eyeball laterally,medially,superiorly
and inferiorly

9. Contn..

10. ANATOMY OF THE EYEBALL
► The adult eyeball measures about 2.5cm in diameter. Of its total
surface area only 1/6th is exposed, the remainder is recessed and
protected by the orbit, into which it fits. Anatomically, the wall of the
eyeball consist of three layers:
!) FIBROUS TUNIC- outermost protective layer -Sclera and the cornea
2) VASCULAR TUNIC-middle nutritive layer-the choroid, ciliary body
and iris
3) NERVOUS TUNIC-innermost most layer-the retina

11. Contn..

12. Contn…

13. FIBROUS TUNIC
► The fibrous tunic is the superficial coat of the eyeball. It consists
of the anterior cornea and the posterior sclera.
CORNEA- It is a transparent coat the covers the colored iris.
► Because it is curved ,the cornea helps to focus light onto the
retina
1)Consists of nonkeratinized stratified squamous epithelium in its
outer surface.
2)The middle coat of the cornea consists of collagen fibres and
fibroblasts.
3)The inner surface is simple squamous epithelium

14. Contn…..
► SCLERA- (scler=hard), the “white” of the eye is a layer of dense
connective tissue made up mostly of collagen fibres and
fibroblasts.
► The sclera covers the entire eyeball except the cornea .
► Function-It gives shape to the eyeball, makes it more rigid, and
protects its inner parts.
► At the junction of the sclera and the cornea is an opening known
as the scleral venous sinus (canal of Schlemm).A fluid called
aqueous humor drains into this sinus.

15. VASCULAR TUNIC
► The vascular tunic or uvea is the middle layer of the eyeball.
► It is composed of three parts:-the choroid, ciliary body and iris.
CHOROID-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.
► It contains melanocytes that produce the pigment melanin, which
causes this layer 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 the lens remains sharp
and clear.

16. Contn…
► CILIARY BODY-In the anterior portion of the vascular tunic, the choroid
becomes the ciliary body.
► The ciliary body appears dark brown in color because it contains
melanin-producing melanocytes.
► The ciliary body consists of ciliary processes and ciliary muscles.
► 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 ciliary muscles 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 far or near
vision.

17. Contn….
► 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.(black
,blue, green)
► Principle function of the iris is to regulate the amount of light
entering the eyeball through the pupil ,the hole in the centre of the
iris.

18. Contn….
► When bright light stimulates the eye, parasympathetic fibers of the
oculomotor(III) nerve stimulate the circular muscles (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 (dilator pupillae) of the iris to contract. causing an
increase in the pupils size(dilation).

19. RETINA
► The third and inner coat of the eyeball, the retina, lines the posterior
,three quarters of the eyeball and is the beginning of the visual
pathway.
► 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 .
► 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.
► Three distinct layers of retinal neurons- the photoreceptor layer, the
bipolar cell layer and the ganglion cell layer.

20. Contn…

21. Contn…
► Photoreceptor cells are specialized cells that begin the process
by which light rays are ultimately converted to nerve impulses.
► The two types of photoreceptors are :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 color vision .
► Brighter light stimulate cones ,which produce color 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.

22. Rods and Cones

23. Contn....
► OPTIC DISC-It is within the retina. It is the site where the
optic nerve exits the eyeball and is also called the blind
spot. It is called the blind spot because there are no rods
and cones and due to the absence of these ,we cannot see
images of the objects.
► MACULA LUTEA-It is also called the yellow spot. It is small
flat spot in the exact center of the posterior portion of
the retina.
► Fovea centralis-It is a small depression in the center of
the macula lutea ,containing only cones. It is the area of
highest visual acuity or resolution(sharpness of vision)

25. Contn….
LENS-
► Behind the pupil and iris, within the cavity of the
eyeball,is the lens.
► Proteins called crystallines are arranged like the layers of
an onion in the cells of the lens.
► The lens helps to focus images on the retina to facilitate
clear vision.

26. Interior Of The Eyeball
► The lens divide the interior of the eyeball into two
cavities, the anterior cavity and vitreous chamber.
► Anterior cavity –the space anterior to the lens,consists of
two chambers.
► The anterior chamber lies between cornea and iris.
► The posterior chamber lies behind the iris and filled with
aqueous humor.
► Vitreous chamber large posterior cavity of the eyeball It
lies between the lens and the retina.
► Within the vitreous chamber is the vitreous body ,a
transparent jelly like substance that hold the retina.

28. Intraocular Pressure
► The pressure in the eye is called the intraocular pressure.
► It is produced mainly by the aqueous humor and partly by
the vitreous humor .
► Normally the pressure is about 16 mm of Hg(millimeters of
mercury).
► The intraocular pressure maintains the shape of the
eyeball and prevents it from collapsing.

29. Intraocular pressure

30. IMAGE FORMATION
► There are three processes for an image formation on
the retina.
(a)The refraction
(b)Accommodation
(c) Constriction
► Refraction of the light rays-
► As the light rays enter the eye,they are reflected at
anterior and posterior surfaces of the cornea.
► Both surfaces of the lens of the eye further refract the
light rays so they come into exact focus of the retina.

31. Contn…
► Images formed on the retina are inverted .
► 75%of total refraction of light occurs at the cornea.The
lens provide the remaining 25%of focusing power.

32. Accommodation and the near point of vision.
► The lens of the eye is convex on both its anterior and posterior
surfaces, so that the lens will refract incoming light rays towards
each other , so that they eventually intersect.
► Its focusing power increases as its curvature becomes greater.
► When the eye is focusing on a close object the lens become more
curved , causing greater refraction of the light rays. This
increase in curvature of the lens for near vision called
accommodation.
► Near point of vision is the maximum distance from the eye that
an object can be clearly focused with maximum
accommodation. The distance is about 10cm in young adult.

33. Contn…

34. Constriction of the pupil
► Constriction of the pupil is the narrowing of the diameter of the
hole through which light enters the eye due to the contraction of
the circular muscles of the iris.

35. References…
► Gerard J Tortora andBryan Derrickson Principles of
anatomy and physiology,13th edition ,page no 642-656.