The eye ball

The eye ball

Sense of sight appreciated through retina of the eyeball is one of the five special senses. Its importance is obvious in the varied ways of natural protection. Bony orbit, projecting nose and various coats protect the precious retina. Each and every component of its three coats is assisting the retina to focus the light properly. Lots of advance have been made in correcting the defects of the eye. Eyes can be donated at the time of death, and a ''will'' can be prepared accordingly.Eye sees the out side world. The inside of the eyeball can be seen through the ophthalmologist where by health of small vessels can be visualized in normal subjects, in diabetics and in hypertensive individuals. About 75% of afferent reach the brain through the eyes. Adequate rest to eye muscles is important.Could a good place for rest be the classroom where palpable part of orbicular is oculist closes the eyes gently? The eyeball is the organ of sight. The camera closely resembles the eyeball in its structure. It is almost spherical in shape and has a diameter of about 2.5 cm. It is made up of three concentric coats. The outer of fibrous coat also called the uveal tract consists of choroid, the cilia body and the iris. The inner or nervous coat is the retina. Light entering the eyeball passes through several refracting media. From before backwards these are the cornea, the aqueous humor, the lens and the vitreous body.             

Scleroses

The sclera (skleros=hard) is opens and forms the posterior five-sixths of the eyeball. It is composed of dense fibrous tissue which is firm and maintains the shape of the eyeball. It is thickest behind, near the entrance of the optic nerve, and thinnest about 6 mm behind the sclerocorneal junction where the recti muscles are inserted. However,it is weakest at the entrance of the optic nerve. Here the sclera shows numerous perforations for passage of fibres of the optic nerve. Because of its sieve-like appearance, this region is called the lamina cribrosa (crib=sieve).The outer surface of the sclera is white and smooth, it is covered by tenon's capsule. Its anterior part is covered by conjunctiva through which it can be seen as the white of the eye. The inner surface is brown and grooved for the ciliary nerves and vessels.It is separated from the choroid by the perichoroidal space which contains a delicate cellular tissue, termed the suprachoroidal lamina or lamina fusca of the sclera. The sclera is continuous anteriorly with the cornea at the sclerocorneal junction or limbus. The deep part of the limbus contains a circular canal known as the sinus venosus sclerae or the canal of schlemm. The aqueous humour drains into the anterior scleral or ciliary veins through this sinus.   

Iris

This is the anterior part of the uveal tract. It forms a circular curtain with an opening in the centre, called the pupil. By adjusting the size of the pupil, it controls the amount of light entering the eye, and thus behaves like an adjustable diaphragm. It is placed vertically between the cornea and the lens, thus divides the anterior segment of the eye into anterior and posterior chambers, both containing aqueous humour. Its peripheral margin is attached to the middle of the anterior surface of the ciliary body and is separated from the cornea by the iridocorneal angle or angle of the anterior chamber. The central free margin forming the boundary of the pupil rests against the lens.The anterior surface of the iris is covered by a single layer of mesothelium,and the pigmented cells which are continuous with those of the ciliary body. The main bulk of the iris is formed by stroma made up of blood vessels and loose connective tissue in which there are pigment cells. The long posterior and the anterior ciliary arteries join to form the major arterial ciecle at the periphery of the iris.From this circle vessels converge towards the free margin of the iris and join together to from the minor arterial circle of the iris.The colour of the iris is determined by the number of pigment cells in its connective tissue.If the pigment cells are absent, the iris is blue in colour due to the diffusion of light in front of the black posterior surface. The iris contains a well-developed ring of muscle called the sphincter pupillae which lies near the margin of the pupil. Its nerve supply (parasympathetic) is similar to that of the ciliary muscle. The dilator pupillae is an ill-defined sheet of radial muscle fibres placed near the posterior surface of the iris. It is supplied by sympathetic nerves.

Inner coat/retina

This is the thin, delicate inner layer of the eyeball. It is continuous posteriorly with the optic nerve. The outer surface of the retina (formed by pigment cells) is attached to the choroid, while the inner surface is in contact with the hyaloid membrane (of the vitreous) Opposite the entrance of the optic nerve (inferomedial to the posterior pole) there is a circular area known as the optic disc. It is 1.5 mm in diameter. The retina diminishes in thickness from behind forwards and is divided into optic,ciliary and iridial parts. The optic part of the retina contains nervous tissue and is sensitive to light. It extends from the optic disc to the posterior end of the ciliary body. The anterior margin of the optic part of the retina forms a wavy line called the ora serrata. Beyond the ora serrata, the retina is continued forwards as a thin, non-nervous insensitive  layer that covers the ciliary body and iris, forming the ciliary and iridial parts are made up of two layers of epithelial cells. The depressed area of the optic disc is called the physiological cup. It contains no rods or cones and is therefore insensitive to light,i.e it is the physiological blind spot. At the posterior pole of the eye 3 mm lateral to the
optic disc, there is another depression of similar size, called the macula lutea. It is avascular and yellow in colour. The centre of the macula is further depressed to from the fovea centralis. This is the thinnest part of the retina. It contains cones only, and is the site  of maximum acuity of vision. The rods and cones are the light receptors of the eye. The rods contain a pigment called visual purple. They can respond to dim light (scotopic vision). The periphery of the retina contains only rods, but the fovea has none at all. The cones respond only to bright light ( photopic vision ) and are sensitive to colour. The fovea centralis has only cones. Their number diminishes towards the periphery of the retina. The retina is supplied by the central artery. This is an end artery. In the optic disc, it divides into an upper and a lower branch, each giving off nasal and temporal branches. The artery supplies the deeper layers of the retina up to the bipolar cells. The rods and cones are supplied by diffusion from the capillaries of the choroid. The retina veins run with the arteries.

Aqueous humour             
This is a clear fluid which fills the space between the cornea is front and the lens anterior segment. This space is divided by the iris into anterior and posterior chambers which freely communicate with each other through the pupil. The aqueous humour is secreted into the posterior chamber from the capillaries in the ciliary processes. It passes into the anterior chamber through the pupil. From the anterior chamber, it is drained into the anterior ciliary veins through the space of the iridocorneal angle or angle of anterior chamber and the canal of schlem. Interference with the drainage of the aqueous humour into the canal of schlemm results in an
increase of intraocular pressure ( glaucoma ). This produces cupping of the optic disc and pressure atrophy of the retina causing blindness. The intraocular pressure is due chiefly  to the aqueous humour which maintains the constancy of the optical dimensions of the eyeball. The aqueous is rich in ascorbic acid, glucose and amino acids, and nourishes the avascular tissues of the cornea and lens.

Lens            
The lens is a transparent biconvex structure which is placed between the anterior and posterior segments of the eye. It is circular in outline and has a diameter of 1 cm. The central points of the anterior and posterior surfaces are called the anterior and posterior poles. The line connecting the poles constitutes the axis of the lens, while the marginal circumference is termed the equator. The chief advantage of the lense is that can very its dioptric power. It contributes about 15 dioptres to the total of 58 dioptric power of the eye ( A dioptre is the inverse of the focal length in meters.A lens having a focal length of half meter has a power of two dioptres ).The posterior surface of the lens is more convex than the anterior. The anterior surface is kept flattened by the tension of the suspensory ligament. When the ligament is relaxed by contraction of the ciliary muscle, the anterior surface becomes more convex due to elasticity of the lens substance. The lens is enclosed in a transparent, structureless elastic capsule which is thickest anteriorly near the circumference. Deep to capsule, the anterior surface of the lens is covered by a capsular epithelium. At the centre of the anterior surface, the epithelium is made up of a single layer of cubical cells, but at the periphery, the cells elongate to produce the fibres of  the lens substance. The centre (nucleus) of the lens is firm, where as the periphery is soft and is made up of more recently formed fibres.