ANATOMY OF THE EYE-audio

This chapter gives only a brief account of the anatomy of the eyeball and its related structures. The detailed anatomy of different structures is described in the relevant chapters.

THE EYEBALL

Each eyeball ( Fig. 1.1) is a cystic structure kept distended by the pressure inside it. Although, generally referred to as a globe, the eyeball is not on the maximal convexities of the anterior and posterior curvatures of the eyeball is called the anterior and posterior pole, respectively. The equator of the eyeball lies at the mid plane between the two poles (Fig. 1.2).

DIMENSIONS OF AN ADULT EYEBALL

Anteroposterior diameter 24 mm
Horizontal diameter 23.5 mm
Vertical diameter 23 mm
Circumference 75 mm
Volume 6.5 mm
Weight 7 gm
COATS OF THE EYEBALL

The eyeball comprises three coats: outer (fibrous coat) , middle (vascular coat) and inner (nervous coat).

1.Fibrous coat . It is a dense strong wall which protects the intraocular contents. Anterior 1/6th of this fibrous coat is transparent and is called cornea. Posterior 5/6th opaque part is called sclera. Cornea is set into the sclera like a watch glass. Junction of the cornea and sclera is called limbus. Conjunctiva is firmly attached at the limbus.

2.Vascular coat (uveal tissue). It supplies nutrition to the various structures of the eyeball. It consists of three parts, from anterior to posterior, which are: iris, ciliary body and choroid.

3.Nervous coat(retina). It is concerned with visual functions.

SEGMENTS AND CHAMBERS OF THE EYEBALL

The eyeball can be devided into two segments: anterior and posterior.

1.Anterior segment. It includes crystalline lens ( which is suspended from the ciliary body by zonules), and structures anterior to it, vit., iris, cornea and two aqueous humour-filled spaces: anterior and posterior chambers.
Fig. 1.1 Gross anatomy of the eyeball

a. Anterior chamber. It is bounded anteriorly by the back of cornea, and posteriorly by the iris and part of ciliary body. The anterior chamber is about 2.5 mm deep in the centre in normal adults. It is slightly shallower in hypermetropes and deeper in myopes, but is almost equal in the two eyes of the same individual. It contains about 0.25 ml of the same individual. It contains about 0.25 ml of the aqueous humour.
Fig. 1.2 Poles and equators of the eyeball.

b. Posterior chamber. It is a triangular space containing 0.06 ml of aqueous humour. It is bounded anteriorly by the posterior surface of iris and part of ciliary body, posteriorly by the crystalline lens and its zonules, and laterally by the ciliary body.

2. Posterior segment. It includes the structures posterior to lens, viz., vitreous humour (a gel like material which fills the space behind the lens) , retina, choroid and optic disc.

VISUAL PATHWAY

Each eyeball acts as a camera; it perceives the images and relays the sensations to the brain (occipital cortex) via visual pathway which comprises optic nerves, optic chiasma, optic tracts, geniculate bodies and optic radiations ( Fig. 1.3).
Fig. 1.3. Gross anatomy of the visual pathway.

ORBIT, EXTRAOCULAR MUSCLES, AND APPENDAGES OF THE EYE

Each eyeball is suspended by extraocular muscles and fascial sheaths in a quadrilateral pyramid-shaped bony cavity called orbit (Fig. 1.4). Each eyeball is located in the anterior orbit, nearer to the roof and lateral wall than to the floor and medial wall. Each eye is protected anteriorly by two shutters called the eyelids. The anterior part of the sclera and posterior surface of lids are lined by a thin membrane called conjunctiva are to be kept moist by tears which are produced by lacrimal gland and drained by the lacrimal passages. These structures (eyelids, eyebrows, conjunctiva and lacrimal apparatus) are collectively called ‘the appendages of the eye’.

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