The organs of sight are a pair of eyes. The study of structure, functions and diseases of the eye is called ophthalmology. (G. ophthalmos = eye. logos = discourse).
The eyes are situated in deep protective bony cavities, called the orbits, or eye sockets, of the skull. Between the eye and the bony wall of the orbit are connective tissue, adipose tissue, ligaments, muscles and glandular tissue. The ligaments suspend the eye in such a way that muscles can move it to let one see up, down and on sides.
Anatomy or Structure of the Eye
The structure of eye is a hollow, spherical organ, about 2.5 cm. in diameter and about 6 to 8 gm. in weight. It has two parts: wall and contents.
A. Wall of the Eye.
Wall of the eye is composed Ot three concentric coats : outer fibrous coat or corneoscleral coat, middle vascular coat or uvea and inner nervous coat or retina. Of these, the fibrous coat alone is complete. Others are incomplete on the front side.
1. Fibrous Coat. This coat is thick and tough. It protects the eyeball, maintains its form, and provides firm surface for the insertion of eye muscles. It has two distinct but unequal regions : sclera and cornea.
(i) Sclera. It forms the posterior five-sixths of the fibrous coat. It is opaque, bluish-white sheet of fibroelastic tissue. It contains collagen fibres. It is largely hidden in the orbit. A small part of it is visible from in front, and is commonly called “white” of the eye.
(ii) Cornea. It forms the anterior one-sixth of the fibrous coat. It is transparent, circular and fully visible from infront. It is composed of a peculiar variety of connective tissue covered externally by stratified non- keratinized squamous epithelium and internally by simple squamous epithelium. It lacks blood vessels. The cornea is kept moist by tears and mucus from conjunctival glands. It is nourished by lymph from adjacent area. The cornea is thicker than the sclera and is slightly bulged forward. Its curved surface refracts (bends) light rays toward the retina.
Conjunctiva. The cornea and exposed part of sclera are covered externally by a thin, transparent membrane, the conjunctiva. The latter is composed of a stratified epithelium and is continuous with the epidermis that lines the eyelids. The conjunctiva, thus, has 2 parts : ocular conjunctiva that covers the front of the eye and palpebral conjunctiva which lines the eyelids. In sore or “pink” eyes, the conjunctiva gets inflammed, causing conjunctivitis.
2. Vascular Coat. The vascular coat shows three regions : choroid, iris and ciliary body.
(i) Choroid. It forms the greater part of the vascular coat, and lies in contact with the sclera. It is composed of a soft connective tissue containing abundant pigment cells and blood vessels. It is dark- brown in colour. It darkens the cavity of the eyeball to prevent internal reflection of light that might blur the image. Blood vessels of the choroid nourish the retina
(ii) Iris. At the junction of the sclera and the cornea, the vascular coat sharply bends into the cavity of the eyeball to form a thin, coloured partition. This partition is called iris. It is perforated at the middle by an aperture called pupil. The iris contains two sets of smooth muscles : sphincters and dilators, of ectodermal origin. These muscles regulate the amount of light entering the eyeball by varying the size of the pupil. The sphincter muscles are arranged in rings. Their contraction makes the pupil smaller in bright light so that less light enters the eye. The dilator muscles are arranged in a radial manner. Their contraction widens the pupil in dim light to let in more light. Iris, by regulating the size of the pupil, allows light to pass only through the centre of the lens, which is optically the most effective part. The parasympathetic fibres constrict the pupil and the sympathetic fibres dilate it. The amount and location of the pigmen melanin in the iris imparts characteristic colour to the eye (various shades of greenish-blue, gray and brown).
Albino Eye. Albinos lack pigment in the skin, hair and iris; and the pink colour of their iris is due to reflection of light from the blood vessels of iris.
(iii) Ciliary Body. Behind the peripheral margin of the iris, the vascular coat is thickened to form the ciliary body. The inner surface of the ciliary body is thrown into radiating folds, the ciliary processes, which project into the eyeball. The ciliary body contains smooth ciliary muscles of two types : circular and meridional .
The retina is a very delicate coat and lines the whole of the vacular coat. Like the latter, it also shows three parts: optic, ciliary and iridial.
(i) Optic Part. It lies in contact with the choroid. It is thick and composed of four layers of cells. Beginning from the choroid side, it has a layer of pigmented cells, a layer of receptor cells, a layer absorbing property of choroid in reducing the scattering of light in the eye. The receptor cells synapse with the bipolar cells, which in turn synapse with the ganglion cells. The axons of the ganglion cells form the optic nerve. A thin limiting membrane covers the inner surface of the retina.
A small area of the optical part of the retina lying exactly opposite the centre of the cornea is called the macula lutea, or yellow spot. It has a yellow pigment (xanthophyll)’. The macula lutea has at its middle a shallow depression, the fovea centralis. The fovea has cone cells only, and is the place of most distinct vision. Away from the fovea, the rod and cone cells occur in equal numbers, and at the periphery of the retina, the rods are more numerous than the cones. This is why we see better in dimlight by looking out of the corner of the eye. The functional retina terminates anteriorly along an irregular border, the ora serrata. The optic nerve leaves the eyeball by piercing the eye coats at the back. The point on the retina from where the optic nerve starts is called the blind spot, or optic disc, as it lacks the receptor cells and is insensitive to light.
(ii) Ciliary Part. It is thin, being composed only of a layer of pigmented cells. It lies in contact with the ciliary body.
(iii) Iridial Part. It is also thin and is likewise composed only of a layer of pigmented cells. It lies in contact with the iris.
The ciliary body and the iris are, thus, composed partly of the vascular coat and partly of the retina.
Lens. A solid but elastic body, the lens, lies just behind the iris. It consists of a transparent, laminated fibrous tissue and is enclosed in a thin, transparent, elastic membrane, the lens capsule. It is biconvex, but more convex behind than in front. It is held in place by a firm, elastic, ring-like frame, the suspensory ligament, which extends from the equatorial edge of the lens to the ciliary processes. The lens provides fine focus of light on the retina.
B. Contents of Eyeball.
The lens and suspensory ligament divide the cavity of the eyeball into two chambers: the anterior small aqueous chamber and the posterior large vitreous chamber.
The aqueous chamber itself consists of two chambers : large anterior in front of the iris and small posterior between the iris and the lens. Both the parts of aqueous chamber are filled with a clear, watery fluid, the aqueous humour, secreted by the ciliary processes of the ciliary body into the posterior chamber whence it passes into the auterior chamber. The aqueous humour is continuously secreted and drained into the vènous system via the canal of Schlemm.
The vitreous chamber is full of a thick, transparent, jelly-like substance, the vitreous humour, or vitreous body. The latter is apparently secreted by the retina during development of the eye.
The humours maintain the form of the eyeball by keeping it taut (inflated) with their hydrostatic pressure. They also support the lens and help in focussing light rays too. Aqueous and vitreous humours both contribute to intraocular pressure Obstruction in the flow of aqueous humour into the blood causes glaucoma, a very painful rise in intraocular pressure, which may damage the retina and cause blindness. Vitreous humour is not replaced.