Basic eye anatomy

Summary

A basic understanding of the many parts of the eye (including cornea, pupil, lens, sclera, conjunctiva, etc.) is required before delving into further detail.

Introduction

The eye has many distinct parts. In this article, we will cover the major parts of the eye to provide you with a very basic understanding of eye anatomy. This basic anatomy is essential in understanding the pathophysiology of conditions that affect the eye.

The sclera

This is the white part of the eye, visible to the naked eye. It is formed of opaque tissue and is attached to extraocular muscles which control the movement of the eye.

The conjunctiva

A thin layer of transparent connective tissue covers the front part of the sclera and folds back on itself into the inside part of the eyelids. This layer of connective tissue is known as the conjunctiva.

The cornea

The dome-shaped transparent part of the eye is called the cornea. The cornea provides most of the refractive power of the eye through which light beams are focused.

The tears which line the surface of the cornea provide a medium for oxygen exchange, since the cornea itself is avascular.

The cornea has five layers:

• Epithelium

• Bowman membrane

• Stroma

• Descemet membrane

• Endothelium

Each of these five layers have separate functions, however, we will not be covering those in this basic anatomy article.

The iris

Through the cornea, we can see the coloured part of the eye, called the iris. The iris forms an aperture through which light beams can travel into the eye, known as the pupil.

The iris also contains muscles that control the size of the pupil. The iris will constrict the pupil upon exposure to bright light and will dilate in exposure to darkness.

The lens

Once light beams pass through the pupil, they pass through a transparent globular structure inside the eye, known as the lens.

The lens can change shape to focus on objects at different distances. This is known as accommodation.

The lens is attached to the ciliary muscle through zonular fibres. The ciliary muscle contracts and relaxes to control the shape of the eye. It also produces a liquid called aqueous humour.

The choroid

This is a vascular layer between the sclera and the retina. Along with the ciliary body and iris, these three structures make up the uveal tract or the uvea.

The vitreous

Behind the lens is a gel-like substance called the vitreous. The vitreous functions to maintain the shape of the eye. It is mainly composed of water and so allows light to pass through to the back of the eye on the retina.

The retina

The inside layer of the eye where light is detected is called the retina. This is where light beams are focused onto. The part of the retina that handles central vision is called the macula and medial to the macula is the optic disc, where the optic nerve enters the back of the eye.