Each eye is represented in the form of a spherical eye ball which is moved in the eye orbit with the help of six eye muscles (extraocular muscles) namely superior oblique, inferior oblique, superior rectus, inferior rectus, external rectus and internal rectus.
These six muscles are governed by the cranial nerves III (oculomotor), IV (trochlear) and VI (abducens). Eye movement disturbances can cause images to fail to focus on corresponding portions of the retina, thus resulting in double vision (diplopia). Or, as in the case of paralysis, one eye may not fix on the object at all, resulting in monocular, rather than binocular, vision. Eyeball measures about 2.5 cm. in diameter and is hollow.
Its wall is formed of three layers or coats-the outermost is called fibrous coat, the middle one as vascular coat and the inner one as nervous coat.
The outer coat of the eyeball is thick and tough layer made of dense, avascular connective tissue. It provides form and shape to the eyeball. Fibrous coat consists of two parts, the sclera and cornea. Sclera constitutes about five-sixth of the outer coat. It is white and opaque, and popularly called white of the eye.
Most part of sclera is concealed in the orbit. It protects and maintains shape of eye ball. Cornea is the anterior transparent part of sclera and constitutes about 1/6th of the fibrous coat.
It is non-vascular (due to which its transplantation is successful) and convex anteriorly. It is actually a highly organized group of cells and proteins. The cornea is transparent because the collagen fibres in this region are more regularly arranged and do not reflect light. It receives its nutrients from the tears and aqueous humour that fills the chamber behind it. The cornea functions to aid the bending of light and in part, contributes to the formation of a clear image. If the cornea is not shaped properly or if its transparency is lost, the image will not be focused on the retina and blurred vision will result. The cornea also serves as a filter, screening out some of most damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, the lens and the retina would be highly susceptible to injury from UV radiations.
The cornea is covered by a thin and transparent membrane called conjunctiva composed of stratified epithelium and continued over the inner surface of the lids. It does not cover the part of the cornea over the iris.
The middle coat of the eyeball is differentiated into three regions namely choroid, ciliary body and iris. Choroid is delicate, highly vascular and pigmented part which lies in contact with the sclera. It provides dark colour due to presence of black pigment (melanin) cells to the interior of the eyeball. It prevents reflection of light by absorbing it within the eye. The blood vessels of choroid nourish the retina.
It is the part of vascular coat immediately behind the peripheral margin of the iris.
Ciliary body is thicker and less vascular than choroid. Its inner surface is folded to form ciliary processes. Present within the ciliary body are ciliary muscles. These ciliary muscles are circular sheet of smooth muscle fibres that form bundles of circular and radial muscles which alter the shape of the lens during accommodation. Hence, accommodation is an adjustment for distant and close vision during which contraction of ciliary muscles releases tension in the suspensor ligaments and allows the lens to elastically recoil and bulge out on both of its sides. This increases the convexity of the lens and increases the level of refraction of light passing through it. Iris – It is the anterior part of vascular coat which lies behind the cornea.
Iris is the circular, muscular diaphragm containing the pigment which gives the eye its colour. It controls the amount of light entering the eye. The central perforation of iris is called pupil whose size is regulated by iridium muscles [i.
e. radial muscle (whose contraction dilates pupil) and circular muscles (whose contraction constricts pupil)]. Both of these muscles are under the control of the autonomic nervous system. Present behind the iris is a biconvex and transparent body, the lens. It is formed of laminated fibrous tissue and is capable of undergoing change in its curvature. The lens is covered by lens capsule and suspended to the body by suspensory ligament.
The ability of the lens to produce a sharp image on the retina is partially, a function of its elasticity (elasticity is due to presence of elastin protein present in the epithelial cells forming lens). When focusing on close objects the lens must be more spherical (convex) than when focusing on distant objects. As an individual gets older, the lens gradually loses this ability to accommodate or adjusts to near vision. The lens is progressively less able to bend light, probably due to protein denaturation within the cells of the lens.
The innermost coat consists of a delicate non-vascular light sensitive coat, called retina. Retina is the innermost coat of the eyeball.
It is formed of the three portions, the optic part which lies along choroid, the ciliary part in contact with ciliary body and irridial part which lies in contact with the posterior side of iris. The circular margin of optic region where it continuous upon ciliary processes is naturally crenated (crenated means having a bivalved fluted fan shaped margin as in certain leaves) and hence called ora serrata The optic part of the retina is differentiated into two parts-pigmented and nervous. The pigmented part is made of cuboidal cells with dark brown granules and fringe like protoplasmic processes. It continues beyond ora serrata. The inner nervous part is transparent and is made of three layers which from outside to inside include the layer of receptor cells, layer of bipolar ganglion cells and layer of ganglion cells.
The receptor cells include rods and cones.