The eye is a complex and sensor organ that is gatheringof visual information.The eye is made up ofthree main parts: eyeball (globe), orbit (eye socket), accessory (adnexal)structures.· AccessorystructureThe eye’s accessorystructures contain the eyelids, conjunctiva, caruncle and lacrimal glands.· Orbit (eyesocket)The orbit (eye socket) is made up of bone. It containsthe eyeball and the connective tissues that protect the eye. The muscles attachto the eyeball make it easily move in different directions.
· The eyeballThe eyeball (globe) is rich in bloodvessels. Human eyes are roughly spherical, ?lled with a transparent gel-likesubstance called the vitreous humor. Itsupports the internal structures and maintains the shape of the eye.
The eyeball consists ofthree concentric layers (tunics), whose names re?ect their basic functions: a?brous tunic, consisting of the sclera and the cornea; a vascular pigmentedtunic, comprising the choroid, ciliary body, and iris; and a nervous tunic, theretina, see Figure(3). ü The Fibrous TunicThe ?brous tunic is the outermost layerof eye tissue. It is a tough and inelastic corneoscleral envelope.
The corneais dense fibrous connective tissue. It is transparentto allow passing the light. The sclera istough and white connective tissue.
Function: protection and support.ü Vascular tunic The vascular tunic is the middle layer, also known asuvea. It consists of the choroid, the ciliary body and the iris, which isperforated by the pupil. The iris is thin, colored part of the eye.
It islocated in front of the eye between the lens and the cornea.Aqueous humorsecreted by the ciliary body, ?ows into the posterior chamber, through thepupil and out of globe through drainage apparatus at the angle of the anteriorchamber. The choroid contains blood vessels that supply oxygen and nutrient andremove the waste products of the retinal cells. Function: nutritive.
ü The nervous tunic The nervoustunic is the inner layer which includes the retina and lens. The retinaconsists of receptors and neurons and concerned with the initial processing of visual information. Function:visualization.1.1.1 CorneoscleralEnvelopeThe cornea and sclera together form the outerfibrous tunic of the eye and withstand both internal and external forces tomaintain the shape of the eyeball. Although both of these structures consistmainly of collagen fibrils, their optical properties are different.1.
1.1.1 CorneaThe cornea must be transparent, refract light,contain the IOP and provide a protective interface with the environment. Eachof these functions is provided by a highly specialized substructuralorganization. The cornea does not contain blood vessels,it receives nutrients from tears and the aqueous humor in the anterior chamber.The cornea acts as the eye’s outermost lens. Itacts as a window that focuses and controls the amount of light to the eye.
Thecornea contributes between 65-75 % of the eye’s total focusing power.When light strikes the cornea, it refracts theincoming light onto the lens. The lens further refocuses that light onto theretina. The retina is a layer of light-sensingcells lining in the back of the eye that starts the translation of light intovision. The cornea also acts as a filter, screening out someof the most damaging ultraviolet (UV) wavelengths in sunlight.
Without thisprotection, the lens and the retina would be highly sensitive to injury from UV radiation.The corneal tissue is arranged in ?ve layers. Theyare: epithelium, Bowman’s membrane, the stroma, Descemet’s membrane and theendothelium, see Figure (4).
1. Corneal epitheliumThe epithelium is the cornea’s outermostregion. It is ?lled with tiny nerve endings that make the cornea highlysensitive to pain.
The epithelium functions primarily to: (1) block the passageof foreign material, such as water, dust, and bacteria, into other layers ofthe cornea and the eye. (2) provide a smooth surface that absorbs oxygen andcell nutrients from tears then distribute these nutrients to the rest of thecornea’s layer. The basement membrane is the part of the epithelium where theepithelial cells anchor and organize.2. Bowman’s layerBowman’s membrane lies directly belowthe basement membrane of the epithelium. It is a transparent sheet of tissue,is composed of strong layered of collagen ?brils.
Bowman’s membrane protectsthe cornea from injury. Once injured, it resiliently regenerates, leaving ascar when the injury is deeper. If these scars are large and located in thecenter, some vision loss can occur.3. Corneal stroma Thestroma lies under Bowman’s layer.
It is the thickest layer (about 500µm),dominating the mechanical response of the cornea to injury and accounting about90 % of the cornea’s thickness. It consists primarily of water 78%, collagen16%, and non-collagenous proteins 7%. Itdoes not contain any blood vessels. Collagen gives the cornea its elasticity,strength, and form. The unique shape and arrangementof collagen are essential in producing the cornea’s light-conductingtransparency. The Microscopic Organization of CollagenThe corneal stroma depends mostly on thedegree of its collagen ?brils (spatial order) which are narrow in diameter andclosely packed in a regular array. Di?erent types of the collagen present inthe human cornea (I, III, VI, XII).
By scanning electron microscopy, it seems that the average diameter of collagen ?brils ishighly uniform (about 31nm), remaining constant across the cornea before risingat the limbus. There is a significant increase inspacing from the central cornea (about 57nm) to the peripheral cornea (about62nm), followed larger increase at the limbusitself. The proteoglycan matrix (also known as ground substance) is a gel-likesubstance, consisting mainly of water 14.
The collagen in stroma also plays animportant role on the macroscopic level, where it confers shape and strength.The stromal ?brils are organized into three hundred to ?ve hundred ?at bundles,or lamellae, which run uninterrupted from limbus to limbus like thin belts upto 0.2mm broad and about 1 ? 2cm thick. Fibrils within a given lamella runapproximately parallel but tend to makelarge angles with those in adjacent lamellae, Figure (5). 4. Descemet’s membrane Descemet’s membrane is locatedbeneath the stroma, it is a thin but strong sheet of tissue.
It is composed of collagen fibers (differentfrom those of the stroma) and is made by the endothelial cells that lie belowit. Descemet’s membrane plays an important role incorneal hydration and in the maintenance of the endothelium after wounding andsurgery, regenerating readily after injury. Considering its thickness (? 10µm) and unique composition, it maybe speculated whetherDescemet’s layer has a specialized function,besides the function as a basement membrane that could be in mechanicalsupport, ?ltration or liquid barrier 15. 5. CornealendotheliumTheendothelium is the thin and the innermost layer of the cornea. Endothelialcells are essential in keeping the cornea clear. Normally, fluid leaks slowlyfrom inside the eye into the stroma.
The excess fluid pumps out of the stromaby the endothelium. Without this pumping,the stroma would swell with water. A perfect balance in a healthy eye ismaintained between the fluid moving into the cornea and fluid being pumped outof the cornea. Once endothelium cells are destroyed by disease or trauma, theydo not recover.
Too much damage to endothelial cells can lead to corneal edema(swelling caused by excess ?uid) and blindness, with corneal transplantationthe only available therapy. 1.1.
1.2 LimbusThe limbus is the areathat the cornea meets the sclera and conjunctiva. Functionsof limbus are nourishingthe peripheral cornea, providing an outflowfor the aqueous humor, assisting in corneal epithelial regeneration.In anatomically it is di?cult todetermine exactly where the cornea ends and the sclera begins. Transitionalzone is approximately 1.
5?2mm. By using synchrotronX-ray di?raction confirmed the presence of circumferentialannulus of collagen ?brils located in the limbus of the human eye 16. Sincethe preferred orientation of the collagen ?brils is circular at the limbus, itis the weakest region in the corneoscleral wrapper by IOP pressure. From aconsideration of the mechanics of the system,it seems probable that the purpose of this annulus is to help maintain thecorrect curvature of the cornea; studieson bovine tissue, proposed microstructural models of possible integrationarrangements between the central cornea and the limbus 17 support thisassumption.1.1.
1.3 Sclera The sclera gives the eye most of its white color. It isrelatively avascular and consists almost entirely of collagen. The scleraprotects the intraocular contents from injury and the function of the collagenin the sclera is obviously structural. The strength and resilience of thesclera are imparted by the close interlacing of the collagen fibers whichaccount for 80 % of the dry weight. Its mechanicalstrength serves to contain the IOP and at the same time prevents deformationsof the globe by resisting the stresses induced by contractions of theextraocular muscles. The thickness of sclera is not uniform, being thinner in femalesthan in males.
There is also increase in scleral thickness, together withopacity in relation to age 18.The sclera is pierced by the optic nerve, forming a thin netlike lamina.This structure provides support and anchorage for the optic nerve ?bres passingthrough it and also reinforces the globeat its weakest point. The bands of collagen bundles of the sclera are mostlyparallel to the surface, but they cross each other in all directions and maydivide and reunite, see Figure (6). Within each bundle,the collagen ?brils are parallel and show wide variation in diameter andspacing (ranging from 25–230 nm), which is distinctly di?erent from that of thecornea and account for the opacity of the sclera 19.