Eye is the organ of sight situated in the orbital cavity. It is almost spherical in shape and is about 2.5 cm in diameter. The volume of an eyeball is approximately 7cc. The space between the eye and the orbital cavity is occupied by fatty tissue. The bony wall of the orbit and the fat helps to protect the eye from injury.
The eye is a delicate organ which is protected by several structures such as eyebrows, eyelids, eyelashes and extraocular muscles.
Eyebrows:
Eyebrows are two arched ridges of the supraorbital margins of the frontal bone. Numerous hair projects obliquely from the surface of the skin. They protect the eyeball from sweat, dust and other foreign bodies.
Eyelids and Eyelashes:
The eyelids are two movable folds of tissue situated above and below the front of each eye. There is short curved hair, the eyelashes situated on their free edges.
Lacrimal Apparatus:
Lacrimal apparatus consists of,
Extraocular Muscles of The Eye:
Arterial Supply:
The eye is supplied by the short (about 20 in number) and long ciliary (2 in number) arteries and the central retinal artery. These are branches of the ophthalmic artery, one of the branches of the internal carotid artery.
Venous Drainage:
Venous drainage is done by the short ciliary veins, anterior ciliary veins, 4 vortex veins and the central retinal vein. These eventually empty into the cavernous sinus.
Nerve Supply to The Eye:
Both anterior and posterior chambers contain a clear aqueous humor fluid secreted into the posterior chamber by the ciliary epithelium. It passes infront of the lens, through the pupil into the anterior chamber and returns to the venous circulation through the
Lens:
Lens is a transparent, circular, biconvex structure lying immediately behind the pupil. It is suspended from the ciliary body by the suspensory ligament or zonule of Zinn. It is enclosed within a transparent capsule.
Vitreous:
The Outer Fibrous Layer:
Sclera:
Cornea:
Cornea forms the anterior 1/6 of the eye. The transparent, ellipsoid, anterior part of the eyeball is known as the cornea. It is the main refracting surface of the eye. The dioptric power is +43 to +45 D.
Limbus:
The junction of cornea & sclera is known as the limbus. There is a minute arcade of blood vessels about 1mm broad present at the limbus.
The Middle Vascular Layer:
Iris:
Iris is a colored free circular diaphragm with an aperture in the centre the pupil. It divides the anterior segment of the eye into anterior & posterior chambers which contain aqueous humor secreted by the ciliary body. It consists of endothelium, stroma, pigment cells and two groups of plain muscle fibres, one circular (sphincter pupillae) and the other radiating (dilator pupillae).
Ciliary body:
Ciliary body is triangular in shape with base forwards. The iris is attached to the middle of the base. It consists of non-striated muscle fibres (ciliary muscles), stroma and secretory epithelial cells. It consists of two main parts namely pars plicata & pars plana.
Choroid:
Choroid is a dark brown highly vascular layer situated between the sclera and retina. It extends from the ora serrata upto the aperture of the optic nerve in the sclera.
The Inner Nervous Tissue Layer:
Retina:
Retina is composed of ten layers of nerve cells and nerve fibres lying on a pigmented epithelial layer. It lines about ¾ of the eyeball. Macula lutea is a yellow area of the retina situated in the posterior part with a central depression called fovea centralis. It is the most sensitive part of the retina.
Optic disc:
Optic disc is a circular, pink colored disc of 1.5 mm diameter. It has only nerve fibre layer so it does not excite any visual response. It is know as the blind spot.
The Optic Nerve:
The optic nerve extends from the lamina cribrosa up to the optic chiasma. The total length of the optic nerve is 5cm. It has four parts namely.