Vision is a very important aspect to our daily lives, and it is something that some of us might take for granted. The eye is a complex organ that is composed of many different structures and tissues. It is my hope to spread some knowledge on the basic structure of the eye, the function, and some interesting pathologies.
Basic Anatomy
All of the information supplied for the anatomy section can be found in Vaughan & Asbury's General Ophthalmology - 17th Ed. (2008)
The Conjunctiva
This structure is a thin, transparent, mucous membrane that covers the posterior portion of the eyelids, and the anterior portion of the sclera. It is composed of 2-5 layers of stratified columnar epithelium.
The Sclera
The sclera is the fibrous, outer, protective coating of the eye. It is almost entirely composed of collagen fibers, and is a dense, white tissue.
The Cornea
This is a transparent tissue that is inserted into the sclera at the limbus. It is usually 550 um
thick in the center, and has a horizontal diameter of 11.75mm, and a vertical diameter of 10.6mm.
The cornea is made up of the following 5 or 6 layers:
- Epithelium- 5 or 6 layers
- Bowman's layer- acellular
- The Stroma- intertwining lamellae of collagen fibrils
- Descemets membrane- basal lamina for endothelium
- Endothelium- 1 layer of cells.
The Iris
This structure is an anterior extension of the ciliary body. It is a flat surface, with a round aperture in the center (the pupil).
It is a colored annular membrane that is suspended behind the cornea, and is found immediately
in front of the lens. The pigment in the iris comes from anterior extensions of neuroretina, and
retinal pigment epithelium.
The iris is the visible colored structure on the eye, that surrounds the pupil. It functions to
control the amount of light that enters the eye. Pupil size is determined by a balance between parasympathetic constriction, and sympathetic dilation.
The Ciliary Body
The ciliary body is found between the iris and the choroid. It consists of ciliary processes, and the ciliary muscle.
The Choroid
This is the posterior segment of the Uveal Tract, and is found between the retina and the sclera. It is made up of 3 layers of choroidal blood vessels, Large, Medium and Small. It forms the vascular layer, and supplies nutrition to the eye structures.
The Lens
This is a biconvex, avascular, colorless, and almost 100% transparent tissue. It is approximately 4mm thick, and 9mm in diameter.
It is suspended behind the iris by a suspensory ligament called the zonule.
With age, sub-epithelial lamellar fibers are continually produced, which makes the lens grow
larger, and less elastic as life goes on.
The nucleus and cortex of the lens are made up of long, concentric lamellae.
The lens is 65% water, and 35% Protein (which is the highest protein content of any tissue in the
body).
There are no pain fibers, blood vessels, or nerves located in the lens.
The Retina
The retina is a thin, semi-transparent, multi-layered sheet of neural tissue. It lines the inner aspect of 2/3 of the eyeball (also known as the globe).
Layers of the Retina
- Internal limiting membrane
- Nerve fiber layer (with ganglion cell axons passing to optic nerve)
- Ganglion cell layer
- Inner Plexiform Layer
- Inner nuclear layer of bipolar, amacrine, and horizontal cell bodies
- Outer plexiform layer
- Outer nuclear layer of photoreceptor cell nuclei
- External limiting membrane
- Photoreceptor layer of Rod and Cone inner and outer segments
- Retinal Pigment Epithelium
There are 6 extra ocular muscles that control the movement of the eye. There are 4 Rectus muscles that originate at a common ring tendon, and are named according the their insertion in the sclera; medial, lateral, inferior and superior. The movement made by these muscles respectively are : adduct, abduct, depress, and elevate the globe.
There are 2 oblique muscles that control the up and down movement
of the eye, the Superior Oblique muscle and the Inferior oblique muscle.
All of the extra ocular muscles are ensheathed by fascia.
The Optic Nerve
The optic nerve is composed of approximately 1 million ganglion cells from the retina. It emerges from the posterior surface of the globe, through the posterior scleral foramen, which is a short, circular opening in the sclera.
The image that is formed on the retina is transmitted to the brain via the Optic nerve, and the image is finally perceived by the brain.
Photoreceptors
The cornea and the lens of the eye will focus an inverted image onto the retina, which is the photoreceptor layer at the back of the eye. There are 2 types of photoreceptors : Rods and Cones.
Rods are more sensitive, and used for vision in dim light.
Cones are used in brighter light, for color vision, and for high acuity vision in animals with a fovea.
The Fovea is the central, high acuity region on the retina, where intervening cell layers and blood
vessels are pushed to the side. This area contains a lot of cones, and few rods.
(Hill et al 2008)
Embryology and Development
The eye is derived from 3 of the primitive embryonic layers, the surface ectoderm, the neural crest, the neural ectoderm and the mesoderm.
The surface ectoderm gives rise to the lens, the lacrimal gland, the epithelium of the cornea, conjunctiva, and epidermis of the eyelids.
The neural crest cells gives rise to the endothelium of the cornea, ciliary muscle, stroma of the iris, etc.
The neural ectoderm gives rise to the optic vesicle and optic cup, retina and retinal epithelium, sphincter muscles of the iris, the optic nerve fibers and ganglia.
Finally, the mesoderm contributes to extra ocular and lid muscles, and the orbital and ocular vascular epithelium.
Vaughan and Asbury (2008)
Growth
The Eyeball
At Birth, the eyeball is large in relation to the rest of the body. The ultimate size is reached at 7-8 years.
The Cornea
A newborn infant has a relatively large cornea, but reaches full size at 2 years.
The Lens
The lens is more spherical at birth, producing a greater refractive power, to help compensate for a short anteroposterior diameter of the eye. Lens grows throughout life, making it flatter.
The Iris
At birth, there is little to no pigment on the anterior surface of iris, so the posterior pigment layer shows through the translucent layer, giving most infant e
yes a bluish color. As pigment begins to appear in the anterior surface, the iris assumes its definitive color. If lots of pigment develops, the eye will be brown. If not a lot go pigment
develops, the eyes will be blue, green or hazel.
Vaughan and Asbury (2008)
Function
As most of us know, the main function of the eye is to provide vision. We depend on our eyes to see colors, shapes, objects etc. A quick, easy, summary of how everything proceeds.
We look at an image. The light reflects off that image and goes in through our pupil. The light is focused onto the retina by the lens. This stimulates Rods and Cones in the eye, and the image will be passed to the brain via the Optic Nerve.
Common Terms/Concepts
Below are some important terms that are related with the eye.
Emmetropia: When there is an absence of refractive error (IE: you do not need glasses)
Ametropia: When there IS a refractive error (So you do need glasses)
Accommodation: When the eye changes refractive power to focus on objects that are close up.
Presbyopia: With aging, eyes will lose accommodation power. Symptoms such as not being able to read fine print will arise around age 42-46. This will happen to everyone. It can be fixed with simple reading glasses, or progressive lenses.
Myopia: This is also known as nearsightedness. It occurs when the image of a distant object is focused in front of the retina. This can be treated using a concave(minus) lens, which moves the image back to the retina.
A highly myopic prescription can lead to degenerative retinal changes, such as retinal detachment.
Hyperopia: AKA Farsightedness. When the image is focused behind the retina, and it is hard to see things close up.
Astigmatism: The eye produces an image with multiple focal points. This is when your eye is shaped more like a football, than a round globe. It can be correct
ed for with eyeglasses, or contact lenses.
Vaughan and Asbury (2008)
Vaughan and Asbury(2008)
Pathologies
There are many pathologies that are associated with the eye. I will concern myself with a few of them that are very common, and may affect us in some way.
Blindness
This is defined as lacking visual perception due to physiological or neurological factors (Wikipedia 2009). OR
A loss of vision sufficient to prevent one from being self supporting in an occupation, and making one dependent on others. (Vaughan and Asbury, 2008).
Some causes of blindness are as follows:
- Cataracts
- Glaucoma
- Leprosy
- Herpes Simplex Keratitis
- Retinal Detatchment
- Inherited retinal degenerative disorders
According to WHO, there are ~ 50 million blind people in the world today, and 135 million who are suffering from significant visual disability.
Cataracts
A cataract is defined as the clouding of the eyes natural lens.( Lee and Bailey, 2009) The lens is made up of water and protein. Sometimes, the protein clumps together, forming a cloud in a small area of the lens. This cloud grows with time, covering more of the lens, and making it
harder to see.
Types of Cataracts (http://www.ehponline.org/docs/2005/113-3/eye.jpg)
- Subscapular Cataract: this type of cataract forms at the back of the lens, and is commonly found in people with diabetes, high farsightedness, or one who is taking a high dosage of steroids.
- Nuclear Cataract: Forms in the nucleus, the center of the lens. Forms due to natural aging causes.
- Cortical Cataract: forms in the lens cortex, and gradually extends inwards. Is commonly found in diabetics.
Vision may be blurred, light from a lamp of the sun may seem to bright, or glaring, oncoming headlights may seem brighter than before, or you might not be able to see colors as clearly.
Causes
It is not entirely sure what causes cataracts, but studies have proven that the following are associated with cataract development:
- UV exposure
- Other types of radiation
- High Salt consumption
- Cigarette Smoke
- Heavy Alcohol consumption
Glaucoma
Glaucoma is a leading cause of blindness in our world today. It results from changes in the optic disc, and visual field defects (Agarwal et al, 2009). It is characterized by loss of retinal ganglion cells, and axons over many years. It is caused by increased pressure in the eye. There are a few types of glaucoma ;
Open Angle Glaucoma
This is the most common type. People with this disease are still able to see, read, drive etc, because vision loss isn't obvious until it is too late. Vision loss is permanent, therefore it is important to be checked regularly by an optometrist who can check the intraocular pressures of your eyes.
Closed Angle Glaucoma
This type is less common, and must be treated by laser surgery. It starts by having a slow, dull ache over one eye, change in vision, and seeing blurry halos around lights. If this happens, it is an emergency and should see a vision care provider immediately.
Treatment : Drops may be given to lower the IOP of the eye. For Open Angle Glaucoma, it is a life long disease and you may have to continuously use drops to keep it under control. For a Closed Angle Glaucoma attack, IOPs must be decreased before permanent vision loss occurs, therefore an Opthamologist may have to make a small slit in the iris with a laser, to release some pressure. (CNIB)
For more information regarding the pathologies above, as well as some more eye pathologies, please visit the CNIB website.
References
Agarwal R, Gupta SK, Agarwal P, Saxena R, Agrawal SS. Current concepts in the pathophysiology of glaucoma. Indian J Ophthalmol 2009;57:257-66
Hill, R.W, Wyse, G.A., Anderson, M. Animal Physiology. Second Edition. 2008
Lee, j., and Bailey, G. Cataracts. 2009. As hosted at http://www.allaboutvision.com/conditions/cataracts.htm
www.cnib.ca
Vaughan & Asbury's General Ophthalmology - 17th Ed. (2008)
Note: All of the black and white pictures found in the anatomy section were from Vaughan and Asbury (2008)
