Eye Anatomy: Parts of the Eye and How We See

Contents of Eyeball

1. Aqueous humour
2. Lens
3. Vitreous body.

1. Aqueous humour

1. Aqueous humour Formation: It is secreted from the capillaries of the ciliary processes into the posterior chamber of the eyeball.

2. Aqueous humour Circulation: From the posterior chamber pupil- anterior chamber-

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3. Aqueous humour drains into space of the iridocomeal angle. (Here it is filtered by the lining membrane of the endothelial cells covering those spaces)- sinus venosus sclerae (canal of Schlemm) anterior ciliary veins- ophthalmic vein.

1. Accessory route: A portion of th aqueous humour may be absorbed as follows:
2. Via the blood vessels of the iris.
   Via the lymph spaces of the iris.
3. Fluid from the posterior chamber- zonular space- hyaloid fossa
4. Hyaloid canal- optic disc- perineural spaces around the nerve filaments of the optic nerve.

4. Aqueous humour Intraocular tension: Normal is 15-20 mm Hg.
If there is interference in the absorption of the aqueous humour into the sinus venosus sclerae; there will be an increase in the tension which results in the formation of glaucoma.

2. The lens

1. Definition: The lens is a transparent biconvex body, situated behind the iris and in front of the vitreous body .
2. Capsule: Lens is enclosed in a capsule which is a transparent elastic membrane.

3. Lens Features

1. The anterior surface is less convex than the posterior surface.

1. Lens central part is in contact with the pupillary membrane till it disappears.
2. After that, it forms the posterior boundary of the anterior chamber of the eyeball.
3. Lens periphery is separated from the iris by the aqueous humour of the posterior chamber.
4. Lens curvature is regulated by the ciliary muscles for focusing the near or distant objects.
5. These muscles are called muscles of accommodation.

2. Lens Posterior surface: It is more convex and occupies the hyaloid fossa on the anterior aspect of the vitreous body.

3. Lens Poles: The central points of the surfaces are called anterior and posterior poles.

4. Lens Axis: It is the line joining the two poles.

5. Lens Equator: The peripheral circumference of the lens is the equator.

6. Lens Dimensions:

1. Axis: 4 mm.
2. Transverse diameter: 9 to 10 mm.
3. Refractile index: 1.43.
4. Power of lens: 20 to 300

7. Lens Nutrition:

It is avascular. In foetal life, the posterior part of the lens capsule is supplied by hyaloid artery. It is a branch of the central artery of the retina.

8. Lens Structure

1. Capsule: The lens is composed of a series of more or less concentrically arranged fibres around a firm central part called the lens nucleus. So, the periphery is soft and the centre is hard.
2. Deep to the anterior part of the capsule, the lens is lined by epithelium.
3. It isformed by transparent, nucleated columnar cells.
4. These cells grow and fill the cavity of the lens vesicle.
5. These elongated cells are called lensfires.
6. These fibres run from periphery to the centre but never from pole to pole.

2. 1Applied:

1. In foetus, lens is soft but in old age, it becomes harder and flattened, thereby reducing the focusing power. This condition is called presbyopia.
2. In old age and also sometimes in early life, lens becomes opaque; this condition is called cataract.

3. The vitreous body

1. Vitreous Body Definition: It is a colourless, transparent jelly-like substance, situated behind the lens and on the innr surface of the retina.
   It occupies the posterior 4/Sth of the eyeball.

2. Vitreous Body Composition: It consists of

1. 90% water,
2. Salts,
3. Mucoprotein, and
4. Hyaluronic acid.

3. Hyaloid canal: It is traversed by a canal called hyaloid canal. In foetal life, it contains the hyaloid artery.

4. Hyaloid fossa: It is a fossa on the anterior surface of which lies the posterior surface of the lens.

5. Hyaloid membrane: Vitreous body is surrounded by its membrane. It is adherent to the retina in two places:

1. At optic disc: Immediately in front of the ora serrata, it is thickened by radial fibres called ciliary zonulae.
2. Ciliary processes.

6. Vitreous Body Nutrition: No blood vessels pierce it. Nutrition is carried by vessels of the retina and ciliary processes.

7. Suspensory ligament of the lens: The ciliary zonule splits into two layers enclosing the lens:

1. One layer is thin; it lines the hyaloid fossa.
2. The other layer is thick and it is called the suspensory ligament.

1. Its periphery is attached to the ciliary process and ora serrata.
2. Centrally, it is attached to the lens slightly in front of its margin.

8. Vitreous Body Function: In rest position, it keeps the anterior surface of the lens flattened under tension.

When the ciliary muscles contract, this ligament is relaxed and causes the anterior surface to become convex thereby accommodating to focus near objects.

Vitreous Body Applied anatomy :

1. The zonular fibres constituting the suspensory ligament of lens are very tough structures.
2. Hence, cataract extraction in immature state is not usually done because of the tough zonule. In immatue state, it is removed by zonulysis.

Compartments of eyeball

1. Chambers of the eyeball: The eyeball is divided into two compartments by the lens and suspensory ligament

1. Anterior compartment
2. Posterior compartment

1. Anterior compartment: It is further divided by the iris into

1. Anterior chamber
2. Posterior chamber

1. Anterior chamber: It is 3 mm deep centrally.

1. Anterior chamber Boundary

1. Anteriorly: Posterior surface of the cornea.
2. Posteriorly: Anterior surface of the iris and central part of the lens opposite the pupil.

1. At periphery: It is limited by the angle between the cornea and iris, called the iridocomeal angle.
   This angle is traversed by fibres called pectinte ligament.
   The spaces though this ligament drai ino canal of Schlemm. Obliteration of this angle prevents the absorption of the aqueous humour, thereby increasing the interocular tension, i.e. glaucoma.

2. Anterior chamber Contents: Aqueous humour coming from posterior chamber though the pupil.

2. Posterior chambers

1. Posterior chambers Boundary:

1. Anteriorly: Posterior surface of the iris.
2. Posteriorly: Anterior surface of the lens and suspensory ligament.
   Triangular in cross-section, i.e. pupil and lens coming in contact form the apex, ciliary processes form the base.

2. Posterior chambers Communication: With the anterior chamber through the pupil.

2. Posterior compartment: It contains the vitreous body.

1. Posterior compartment Definition: It is a colourless, transparent jelly-like substance, situated behind the lens and on the inner surface of the retina.
It occupies the posterior 4/Sth of the eyeball.

2. Posterior compartment Composition: It consists of

1. 90% water,
2. Salts,
3. Mucoprotein, and
4. Hyaluronic acid.

3. Hyaloid canal: It is traversed by a canal called hyaloid canal. In foetal life, it contains the hyaloid artery.

4. Hyaloid fossa: It is a fossa on the anterior surface of the vitreous body. The lens is logged in the fossa. It is also called lenticular fossa or patellar fossa.

5. Hyaloid membrane:

1. Vitreous body is surrounded by its membrane. It is adherent to the retina in two places:
2. At optic disc: Immediately in front of the ora serrata, it is thickened by radial fibres called ciliary zonulae, Ciliary processes.

7. Nutrition: No blood vessels pierce it. Nutrition is carried by vessels of the retina and ciliary processes.

8. Suspensory ligament of the lens: The ciliary zonule splits into two layers enclosing the lens:

1. One layer is thin; it lines the hyaloid fossa.
2. The other layer is thick, and it is called the suspensory ligament.

1. Its periphery is attached to the ciliary process and ora serrata.
2. Centrally, it is attached to the lens slightly in front of its margin.

9. Posterior compartment Function:

1. In resting position, it keeps the anterior surface of the lens flattened under tension.
2. When the ciliary muscles contract, this ligament is relaxed and causes the anterior surface to become convex thereby accommodating to focus

Posterior compartment Applied anatomy :

1. The zonular fibres constituting the suspensory ligament of lens are very tough structures.
2. Hence, cataract extraction in immature state is not usually done because of the tough zonule.
3. In immature state, it is removed by zonulysis.
4. Zonulysis causes melting of zonular fibres. The lens floats up and is removed.