I- Introduction :
L’eye is a sphere of’about 25 mm.
C’is a mobile organ contained in a cavity called the eyeball, which prevents him translational movement (before behind), but which allows it to rotate thanks to muscles allowing’direct the gaze in an infinite number of directions. C’do’we call the visual field, which can Reaching 200 °. The power of l’eye is equal to 59 Dioptries.
The cornea is a transparent membrane that allows us to see the’iris, colored diaphragm. The pupil is a diaphragm that allows light to pass, it can only measure 1 at 2 mm diameter to achieve intense light 8 mm in l’darkness.
l’eye is lined with 3 slips :
- sclera : c’is the white of the’eye, she is surrounded by’a very thin and transparent membrane, called conjunctiva,
- choroid : Full pigment layer which constitutes a darkroom ; it is highly vascularized,
- the retina : fabric very important and very fragile, c’is a sensory tissue transforming the light flow into nerve impulses.
Behind l’iris lies the lens. It is surrounded by the ciliary body to which it is held by the zonule of Zinn. The lens is transparent and can lose its transparency, With l’age among others.
Between the lens and the bottom of the’eye, we find the vitreous which is a transparent white gelatinous mass that maintains the shape of the’eye.
A l’before the’eye we delimit 2 zones :
- the anterior chamber between the cornea and the’iris. It is filled by the’aqueous humor.
- the posterior chamber between the’iris and lens.
The eyelids spread the tears in their blink.
Finally, the optic nerve functions as a transmission belt to the brain.
L’orbit : Situated in the upper part of the facial massif, true junction zones between the face and the skull bone, separated l’one of the’other through the nasal cavity, the two orbital cavities contain and protect the organs of the’vision device, especially the eyeballs and the extraocular muscles.
Quadrangular pyramid whose base is wide open in front and narrow top located behind, each orbit is made up of a set of’juxtaposed bones forming the’bone orbit, lined on its internal side d’a fibrous membrane : the periorbital. Numerous orifices dug in the bony walls put in communication the’orbit and neighboring regions and allow the passage of’arteries, veins, nerves for the eyeball or Annexes.
It orbits made :
- 4 walls or surfaces.
- 4 angles or edges.
- a base and a top.
- upper wall: orbital vault (frontal bone and small wing of the sphenoid).
- lower wall: floor (cheekbone "zygomatic", maxilla bone and bone Palatine).
- inner wall:(maxilla, us ethmoid a sphénoïde).
- outer wall: (cheekbone "zygomatic", and greater wing of sphenoid).
- Mountain peak: superior orbital fissure.
- Base: orbital rim.
II- eyelids :
The palpebral region is located in the’middle level of the face. It comes into close contact with the frontal region above (by l’eyebrow intermediary), the nasal region medially, Find jugale en bas et la région time latéralement. eyelids (four in number) have an important function as physiological, in protecting the eyeball (spreading the tear film on the cornea, shedding tears), qu’aesthetic where they participate, with eyebrows, to’expressiveness of the gaze.
- Have two sails mobile musculo-membranous that cover and protect the front of the eyeball.
- By their movements they spread on the surface of secreted tears eyeballs by the lacrimal glands.
- upper and lower eyelids separated by the palpebral fissure.
L’tear apparatus :
Formed by the lacrimal glands
- main gland located under the’orbit.
- glands located in the eyelids, the lacrimal
- start at the punctum by two channels : upper and lower.
that will give the common tear duct that ends in the lacrimal sac and nasolacrimal canal- nose that ends in the nasal cavity.
Les muscles oculomoteurs :
In each orbit, six oculomotor muscles allow mobilization of the bulb of the’eye in the different gaze directions : four rectus muscles – medial, superior, side and bottom – and two oblique muscles – upper and lower -. Each
muscle is surrounded by’a clean muscle fascia that s’unites forward to the bulb sheath of the’eye. Intermuscular fascias linking fascia adjacent muscles. L’set formed by the bulb of the’eye, the oculomotor muscles and their fasciae is suspended from the walls of the’orbit by a system of spans and aponeurotic ligaments.
The four rectus muscles and their fasciae limit posteriorly to the bulb of the’eye a conical space : the fasciomuscular cone, the apex of which is located at the’orbital apex.
The oculomotor muscles are striated muscles of’a particular type. Their arterial supply is important, but variable. They are innervated by cranial nerves three oculomotor : the oculomotor nerve, trochlear and abducens.
the conjunctiva :
The conjunctiva is a transparent mucous membrane, whose lid portion is continuous with the skin of the eyelids.
In the cul-de-sac, it is reflected and covers the eyeball up to’at limbus. On l’then called bulbar conjunctiva
the sclère :
the sclera, outermost tunic of the world, very solid and very resistant, formed of collagen and elastic fibers, around four fifths of the globe later.
On its surface s’insert the oculomotor muscles through their tendons and circulate the vasculonervous elements.
Its external diameter is 23 at 24 mm. It is perforated by the anterior ciliary arteries, four vortex veins and the two posterior ciliary arteries.
It protects against media intraocular trauma. His surgical interests is important : retinal detachment surgery or oculomotor surgery.
the cornea :
C’is the more rounded extension of the sclera. The sclera-corneal border s’call the limbus.
The cornea is highly innervated very sensitive. It is transparent and must remain so for good vision.
She is made of 5 different layers :
- corneal epithelium : squamous cell type renewing quickly.
- membrane de Bowman : transition layer 12 microns d’thickness.
- stroma : very thick (400 microns), it represents 90% of the’total corneal thickness. Its very specific connective tissue includes the usual elements of connective tissue. It contains l’water, organic substances, collagen…
- membrane de Descemet : 6 microns d’thickness.
- endothelium : 6 microns d’thickness, inner membrane, fragile, very fine. The quality and quantity of these cells vary with the’age.
Its nutrition is provided by the tears that bring oxygen essentially, some by & rsquo; aqueous humor and blood vessels at the limbus.
the choroid :
The choroid is a highly vascular layer that provides
nutrition of the’iris and retina. It is located between the sclera and the retina.
It contains many colored pigments and thus forms a screen. She maintains the’inside the’eye in darkroom.
C’is a circular diaphragm automatically adjusting according to the amount of light received.
When the diameter is small, there is less’aberrations : the rays which are in excess are eliminated by the diaphragm and the’image that forms on the retina is sharp.
The night, it n’there is not much light, the pupil dilates, l’image that forms on the retina n’is sharper : c’is nocturnal myopia.
L’iris is responsible for the color of the’eye. The color of the’eye depends on the’thickness of l’range formed by the pigment lamellae and its melanin concentration. Plus, l’fan is thick and contains melanin, more I’eye is dark.
The nutrition of the’iris is provided by the’aqueous humor in which she bathes, and some small arterioles.
The muscles which are responsible for the variation in diameter of the’irises are :
- dilator : contract l’iris, c’that is to say dilates the pupil,
- sphincter : reduces the diameter of the pupil.
The pupils :
Its diameter is in normal light 3 at 6 mm. L’increase in pupil diameter s’call : mydriase, and the decrease in this diameter s’call : myosis.
the Lens : c’is a transparent biconvex lens. It is vascularized.
Can vary its courburte, d’where variation of its power. C’is’accommodation.
The lens bomb, it increases its power.
With l’age, there is loss of’elasticity of the lens. C’is presbyopia.
Yes the s’opacifie, there cataract.
The lens is surrounded by a capsule. This capsule have set the fibers of the zonule of Zinn. his power is 16 dioptries.
The metabolism is provided by the’aqueous humor.
L’aqueous humor :
It is produced by the ciliary process. It goes from the posterior chamber into the anterior chamber through the pupil. In the anterior chamber, she is
removed at the trabecular meshwork (in L’iridocorneal angle) or it goes into Schlemm's canal.
The trabecular meshwork is a kind of filter. If the trabecular meshwork becomes blocked (debris of’iris, excess protein), we then increase the pressure d’where glaucoma.
L’aqueous humor is composed mainly of’water, but also vitamin C, de glucose, d’Lactic acid, protein. It is renewed 2-3 hours.
Its role is particularly nourishing (corneal endothelium and iris), repairer, control of intraocular pressure, as well as maintaining the shape of the’eye.
The vitreous body :
C’is transparent connective tissue. It is surrounded by a membrane called hyaloid membrane.
C’is a filling material. It represents 4/5 of the volume of’eye, and is the first constituent of the’eye.
Its role is to maintain the rigidity of the eye-ball, and maintain the retina in place well pressed against the back of the eyeball.
Its structure makes it intervene in the maintenance of the intraocular pressure and allows it to’absorb the pressures to which it is subjected without altering the function of the’eye. It consists of 95% d’water.
The retina :
C’is a sensitive and fragile tissue. C’is the innermost membrane.
She as thick 1/10 at 4/10 mm. It is highly vascularized : important network of veins and arteries.
- ACR : central retinal artery : which is a branch of the’ophthalmic artery emerges at the center of the papilla then divides into two branches an ascending and l’other descendant each divides in a dichotomous fashion.
- The veins follow the course of the arteries to form the VCR : central retinal vein.
The retina is a hypersensitive plate. It is traversed by numerous small vessels. She is
of hundreds of millions of nerve cells : the rods and cones. The role of these
cells is crucial. They allow to see the details, the lights, colors, shapes and movements.
The light entering the’eye must pass through the retina to reach the sensitive layer of cones and rods.
The rods and cones are the photoreceptor cells. It is these cells that capture
l’nerve impulses and transmit it to the brain to decode it and form an image.
It was much sticks (130 millions) that cones (6-7 millions). The diameter of the cone is much smaller than that of the rods. Plus on s’away from the central part, more cones are scarce and their diameter increases.
The macula and the fovea :
In the central elliptical area is the maximum cones. This area allows a clear vision. This measure area 3 mm in the major axis and 2 mm in the minor axis. it s’call the macula. The macula or yellow spot, appears located at the posterior pole of the center as a fine excavation.
The fovea is a region of the retina located in the macula, near the’optical axis of l’eye. This region is of utmost importance for vision. C’is she who gives the most precise vision, in daylight, c’that is to say during the day. When we set an object, we turn our eyes so as to align the’image on that part of the retina.
The fovea is the central part of the macula. It measures 1300 at 1500 microns. It contains 400 000 cones.
In an even more central vision we find the fovea. It measures 300 at 400 microns in diameter and contains 25 000 cones.
The cones need more light than the rods to be excited. The cones respond more daylight than at night lighting. The rods need much less light to react, they provide night vision.
It exists 3 kinds of cones that react to lengths of’different wave : bleu, vert, rouge. The cones are therefore responsible for color vision.
The sticks are not involved in color vision. Night work only sticks, c’is for this reason that at night all cats are gray !
The optic nerve :
Transmits information to the brain
All the optical fibers from the visual cells converge toward a specific point on the retina : papilla. This point will therefore not contain visual cells but only the nerve fibers.
The papilla is therefore a point of the’eye that does not see. On l’also calls the blind spot. At this point also opens the venous and arterial network of the retina.
The optical fibers are joined all there to form a cable called the optic nerve. It measures 4 mm diameter and 5 cm de long.
There is an optical eye by nerve, therefore 2 optical nerves in all. These 2 nerves cross in an area called the optic chiasm. At this place s’crosses only part of the fibers : fibers from the nasal retina.
Dr Hidouci's course – Faculty of Constantine