THE EYEBALL
Each eyeball is a cystic structure kept distended by the pressure inside it. Although, generally referred to as a globe, the eyeball is not a sphere but an ablate spheroid.
The central point 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.
Dimensions of an adult eyeball
Anteroposterior diameter - 24 mm
Horizontal diameter - 23.5 mm
Vertical diameter- 23 mm
Circumference - 75 mm
Volume- 6.5 ml
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 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 which from anterior to posterior 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 divided 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, viz., iris, cornea and two aqueous humour-filled spaces : anterior and posterior chambers.
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 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 aqueous humour.
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.
OPTOMETRY-SHARP VISION
Optometrist