eye

• Eye


  Organ of sight, situated in the orbital cavity and supplied by the optic nerve, almost spherical in shape, about 2.5cm in diameter, the space between the eye and the orbital cavity is occupied by adipose tissue, the bony walls of the orbit and the fat help to protect the eye from injury, structurally the two eyes are separated but functionally they work in pair
• Layers of tissue in the walls of the eye


  • Outer fibrous layer: Sclera and Cornea
  • Middle vascular layer or uveal tract: Choroid, Ciliary body and Iris
  • Inner nervous tissue layer: Retina
• Sclera


  • Made-up of mainly of fibrous tissue, its external surface gives attachment to the extrinsic muscles of the eyeball, posteriorly in the region of the attachment of the optic nerve the sclera is perforated like a sieve - Lamina cribrosa, anteriorly the sclera becomes continuous with the cornea at the sclerocorneal junction, a circular canal is located in the sclera just behind the sclerocorneal junction - Sinus venosus sclerae / Canal of Schlemm, the external surface of the sclera is covered, in its anterior part, by the ocular conjunctiva, the rest of the sclera is in contact with a fascial sheath that surrounds the eyeball
• Cornea


  • More convex than the sclera, a transparent epithelial membrane, the junction of sclera and cornea is marked on the exterior of the eyeball by a groove - Sulcus sclerae, consists of five layers: Corneal epithelium, Anterior limiting membrane, Substantia propria, Posterior limiting lamina, Endothelium of anterior chamber
• Choroid


  • A network of blood vessels supported by connective tissue containing many pigmented cells that give it a dark brown color, it is the dark color of the choroid that darkens the interior of the eyeball, it also prevents the reflection of light within the eyeball, both these factors are necessary for formation of sharp images on the retina
• Ciliary Body


  • The anterior continuation of the choroid consisting of ciliary muscle and secretory epithelial cells, as many of the smooth muscle fibers are circular, the ciliary muscle acts like a sphincter, the lens is attached to the ciliary body by radiating suspensory ligaments, the ciliary body can be divided into a posterior part called the ciliary ring, and an anterior part made of the ciliary processes, the epithelial cells secrete aqueous humor into the anterior segment of the eye, the ciliary body is supplied with parasympathetic branches of oculomotor nerve
• Iris

  • The visible colored part of the eye and extends anteriorly from the ciliary body, lying behind the cornea and in front of the lens, it divides the anterior segment of the eye into anterior and posterior chambers which contain aqueous fluid, it is a circular body composed of pigment cells and two layers of smooth muscle fibers: Circular muscle fiber, Radiating muscle fiber, in its center there is an aperture - Pupil, the sphincter pupillae is a ring of circularly arranged muscles situated just around the pupil, its contraction narrows the pupil, the dilator pupillae is a smooth muscle which are arranged radially which dilates the pupil, the sphincter pupillae has the parasympathetic nerve supply while the dilator pupillae innervated by sympathetic nerves, the color of the iris is genetically determined and depends on the number of pigment cells present
• Iridocorneal Angle


  • The angle between the peripheral margins of the iris and of the cornea contains a spaces called iridocorneal angle, these spaces communicate with the anterior chamber, and laterally with the sinus venosus sclerae, aqueous humor passes into the posterior chamber of the eye, it passes through the pupil into the anterior chamber, from here it filters through the spaces of the iridocorneal angle to enter the sinus venosus sclerae through which it is drained into the veins of the region
• Lens


  • A highly elastic circular biconvex body, lying immediately behind the pupil, it consist of fibers enclosed within a capsule and it is suspended from the ciliary body by the suspensory ligaments, when the ciliary muscle contracts, it move forward, releasing its pull on the lens, increasing its thickness
• Suspensory Ligaments


  • Made up of fibers passing from the equator of the lens to the ciliary processes, changes in the tension on the suspensory ligaments, produced by contraction of the ciliary muscle, produce alterations in the convexity of the lens and enable the eye to focus at varying distance objects
• Retina


  • Innermost layer of the wall of the eye, an extremely delicate structure and is well adapted for stimulation by light rays, consists of two layers: Pigment cell layer (outer layer, one cell thick), Nerve cell layer (inner layer, several layers of nerve cells), the pigment layer contains photoreceptors which contain photosensitive pigments that convert the light stimulus into nerve impulse, photoreceptors are two: Rods and Cones, there are about 7 million cones in each retina, the rods are far more numerous, they number more than 100 million, rod cells contain a pigment called rhodopsin, it is made up of a protein opsin, and retinine which is a derived from vitamin A, cone cells contain a pigment called iodopsin, it is made up of photopsin and retinine, three types: Blue, Green and Red sensitive, the retina lines about three quarters of the eyeball and is thickened at the back, it thins anteriorly to end just behind the ciliary body, opposite the posterior pole of the eyeball the retina shows a central region about 6mm in diameter, in the center of this region an area about 2mm in diameter has a yellow color - Macula lutea / Yellow spot, in the center of the yellow spot is a little depression called fovea centralis consisting of only cones, towards the anterior part of the retina there are fewer cones than rods, about 0.5cm to the nasal side of the macula lutea all the nerve fibers of the retina converge to form the optic nerve, the small area of the retina where the optic nerve leaves the eye is optic disc / blind spot, it doesn't contain light sensitive cells
• Muscles of the Orbit

  • Include the muscles of eyelids and those that move the eyeballs, the eyeball is moved by six extrinsic muscles attached to one end to the eyeball and at the other to the walls of the orbital cavity, the muscles are: Straight / rectus muscles (four), Oblique muscles (two)
• Blood and Nerve Supply to the Eye

  • The eye is supplied with arterial blood by the ciliary arteries and the central retinal artery, venous drainage is by several veins, including the central retinal vein which eventually empty into a deep venous sinus, the central rental artery and vein are encased in the optic nerve, which enters the eye at the optic disc, the oculomotor nerve supply the intrinsic eye muscles of the iris and ciliary body
• Interior of the Eye

  • The anterior segment of the eye is incompletely divided into anterior and posterior chambers by the iris, both chambers contain a clear aqueous fluid secreted into the posterior chamber, there is a continuous production and drainage, but the intraocular pressure remains constant between 1.3 and 2.6 kPa (10 - 20 mmHg), behind the lens and filling the posterior segment of the eyeball is the vitreous body / vitreous humor, this is a soft, colorless, transparent, jelly like substance composed of 99% water, some salts and mucoprotein, the eyes keeps its shape because of the intraocular pressure exerted by the vitreous body and the aqueous fluid, it remains constant throughout the life
• Optic Nerves


  • The fibers of the optic nerve originates in the retina, and they converge to form the optic nerve about 0.5cm to the nasal side of the macula lutea, the nerve pierces the choroid and sclera to pass backwards and medially through the orbital cavity, it then passes through the optic foramen of the sphenoid bone, backwards and medially to meet the nerve from the other eye at the optic chiasma
• Optic Chiasma


  • Situated immediately in front of and above the pituitary gland, which is in the hypophyseal fossa of the sphenoid bone, in this point, the nerve fibers of the nasal side of each retina cross over to the opposite side, the nerves from temporal side don't cross but continue backwards on the same side, this crossing over provides both cerebral hemisphere with sensory input from each eye
• Optic Tracts


  • The pathways of the optic nerves, posterior to optic chiasma, each tract consists of the nasal fibers from the retina of one eye and the temporal fibers from the retina of the other, the optic tracts passes backwards to synapse with nerve cells of the lateral geniculated bodies of the thalamus, from there the nerve fibers proceed backwards and medially as the optic radiation to terminate in the visual area of the cerebral cortex in the occipital lobe
• Eyebrows


  • Two arched ridges of the supraorbital margins of the frontal bone, numerous hairs project obliquely from the surface of the skin, they protects the eyeball from sweat, dust and other foreign bodies
• Eyelids (Palpebrae)


  • Two movable folds of tissue situated above and below the front of each eye, on their free edges there are short curved hairs - Eyelashes, the layers of tissue forming eyelids: Thin covering of skin, Thin sheet of subcutaneous connective tissue, Two muscles: Orbicularis oculi and Levator palpebrae superioris, Thin sheet of dense connective tissue: Tarsal plate, Lining of conjunctiva
• Conjunctiva


  • A fine transparent membrane that lines the eyelids and the front of the eyeball, where it lines the eyelids, it consists of highly vascular columnar epithelium, corneal conjunctiva consists of avascular stratified epithelium, when the eyelids are closed the conjunctiva becomes a closed sac, the space between the upper and lower eyelids - Palpable fissure, the medial and lateral ends of the fissure - Angle / Canthus, the lateral canthus is intact with sclera, at the medial canthus, the upper and lower lids are separated by a triangular interval - Lacus lacrimalis, when the eyelids are closed a capillary space separates the posterior surface of the lids from the cornea and the anterior part of the sclera, this space is filled with conjunctival sac which is filled with a thin film of lacrimal fluid
• Eyelid Margins


  • Along the edges of the lids are numerous sebaceous glands, some with duct opening into the hair follicles of the eyelashes and some on to the eyelid margins between the hairs, tarsal glands are modified sebaceous glands embedded in the tarsal plate with ducts that open on to the inside of the free margin of the eyelids, they secrete an oil material spread over conjunctiva by blinking which delays evaporation of tears
• Lacrimal Apparatus


  • For each eye lacrimal apparatus consists of: Lacrimal glands and ducts, Lacrimal canaliculi, Lacrimal sac, Nasolacrimal duct, the lacrimal glands are exocrine glands situated in recesses in the frontal bones on the lateral aspects of each eye just behind the supraorbital margin, each gland is approximately the size and shape of an almond, it is composed of secretory epithelial cells, the glands secreted tears composed of water, mineral salts, antibodies and lysozyme a bactericidal enzyme, the tears leave the lacrimal gland by several small ducts and pass over the front of the eye under the lids towards the medial canthus where they drain into the two lacrimal canaliculi, the opening of each lacrimal canaliculi is lacrimal punctum, the two canaliculi lie one above the other separated by a small red body - Lacrimal caruncle, lateral to the caruncle there is a fold of conjunctiva - Plica semilunaris, each eyelid margin has a slight elevation - Lacrimal papillae, the tears then drains into the lacrimal sac which is the upper expanded end of the nasolacrimal duct, this is a membranous canal approximately 2cm long, extending from the lower part of the lacrimal sac to the nasal cavity, opening at the level of inferior concha