Structure of the Eye

The Outer Fibrous Layer:

Sclera:

The sclera or white of the eye forms the firm, fibrous outermost layer of the eye. It maintains the shape of the eye & gives attachment to the extraocular muscles. It is about 1mm thick. The sclera becomes thin (sieve- like membrane) at the site where the optic nerve pierces it. It is called Lmina cribrosa.

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.

• Intraocular - 1 mm
• Intraorbital - 25 mm
• Intraosseous - 4-10 mm
• Intracranial - 10 mm (Duke–Elder)