Anatomy of Eye

Eyeball ModelThe Almighty has equipped the miraculous human body with the gift of five senses with which we see, touch, feel, hear, smell, taste the things and the world around us. Vision, being one of them, is a very important gift that completes our existence on earth. Without vision and the gift of sight, life would be so much difficult and the function of this gift of life is possible only with the help of two beautiful eyes that we have made with such precision so that we can appreciate the beautiful and not so beautiful around world around us.

Therefore, we came up with this post in which a brief detail about the Anatomy of eye will be discussed which will help us to understand the functions of human eye and its various pathologies more clearly.

The eyeball

The eyeball is of shape of  an oblate spheroid which is kept distended by fluid present inside it, hence, it can be called a cystic structure (cystic means cyst like that is sac containing fluid). An oblate spheroid means a symmetrical ellipse which has a greater equatorial diameter than the vertical diameter and that vertical diameter will divide the equatorial diameter into two equal parts. The equator thus lies between the two poles which are situated on the maximum width of the eyeball. These poles are called the anterior pole and the posterior pole. Anterior means in front and posterior means behind.

Dimensions of an adult eyeball

Eyeball StructuresAntero-posterior diameter: 24 mm

Horizontal diameter: 23.5 mm

Vertical diameter: 23 mm

Circumference: 75 mm

Volume: 6.5 ml

Weight: 7 grams

Coverings of the eyeball

The eyeball has basically three coverings:

Cornea Transparent1. The outer covering: It is formed by the sclera and the cornea. They form the fibrous covering of the eye in order to protect the eye from external environment. Therefore this covering can be also be called an elastic but dense membrane which supports the eyeball. The sclera forms the five-sixth of this fibrous covering and cornea forms the anterior most one-sixth of this fibrous layer. The sclera is also called the white of the eye and is thus opaque. The cornea is a transparent membrane which covers the coloured iris and black pupil. Cornea is an avascular structure that means it does not have its blood supply. The The membrane which covers the white or the sclera of the eye is called conjunctiva, mainly the bulbar conjunctiva. Conjunctiva also covers the back of the eyelid and therefore joins the eyelids to the eyeball hence called conjunctiva that is to join. The conjunctiva which covers the back of the lids is called the palpebral conjunctiva. The junction between the cornea and the conjunctiva is called the limbus.

2. The middle covering: It is formed by the uveal tract that is a triad of iris, ciliary body and the choroid. This middle covering is also known as the vascular covering of the eyeball as it is mainly related to the nutrition of the various structures of the eyeball as it is highly vascular that is supplied by blood vessels. The iris is the part of the eye which has different colours in different individuals, races etc. The reason we call some people black eyed, brown eyed, green eyed, hazel eyed or blue eyed is due to the color of the iris. The iris lies in front( anterior most) amongst the other two parts of the uveal tract and can be compared to a circular disc with hole or an aperture in the centre which is known as the pupil. The pupil is normally black in colour and light enters through it. The pupil decreases in size or increases in size according to the intensity and brightness of the light with the help of contraction and relaxation of muscles of iris.In fact it is the iris which contracts and relaxes the make the hole or the pupil to narrow or widen in size.

The iris has free margin anteriorly and posteriorly is attached to ciliary body. The ciliary body is a triangle like structure with base forwards. It is mainly related to production and flow of aqueous humour (a fluid present in the anterior chamber of the eye). The ciliary body ends at orra serrata from which the innermost covering of the eye-the retina begins.

The ciliary body continues behind as the choroid which is in contact with the sclera from within and is highly vascular. There is a space between the sclera and the cornea which is called the Epichoroidal or the Suprachoroidal space. Choroid is also lined from its inner side (the side towards the inner retina) by a thin membrane or sheath or covering called Bruch’s membrane or the lamina vitrea.

3. The inner layer: The retina forms the innermost layer of the eyeball. This layer is also called the nervous layer of the eyeball as it is richly supplied by nerve fibres and is mainly formed by the different kind of nerve cells, their fibres and their synapses, this layer is thus responsible for vision. The retina is further made up of ten layers which can be seen under the microscope if a histological section is made out of it.

Chambers of the eyeball

Eyeball AnatomyThe eyeball is mainly divided into two segments i.e Anterior and Posterior segment.

1. Anterior segment: It includes all the structures in front of the lens or it is segment anterior to the lens. The lens is placed behind the iris, the ciliary body is lying on both its lateral sides giving out the zonules which hold the lens to its normal position. In front of the lens or anterior to it is the pupil. Therefore, the anterior segment can be divided into further two chambers:

  • Anterior chamber
  • Posterior chamber

a. The anterior chamber: It is the part of the anterior segment up to the iris. Therefore, its boundaries would be:

  • In front-Back of cornea.
  • In back-front of iris and part of ciliary body and part of the lens visible through the pupil.

In normal adults, the depth of the anterior chamber is 2.5mm.

It is slightly shallower in hypermetropes and deeper in myopes.

The anterior chamber contains the aqueous humour. It is approximately 0.25 ml in adults.

The peripheral part of the anterior chamber which is narrowest (that is at the sides of the anterior chamber) forms the ‘angle of the anterior chamber’. It has in front -the junction of the cornea and the sclera and behind-the root of the iris. The angle of the anterior chamber form an important drainage pathway for the aqueous humour and thus is one of the main sites involved in Glaucoma.

b. Posterior chamber: Its boundaries are:

  • In front: The back portion of the iris and some portion of the ciliary body and the pupil.
  • In back: The lens with its zonules.

It contains approximately 0.06 ml of aqueous humour.

2. Posterior segment: It is the segment of the eyeball behind the lens. It is bounded at the back by the choroid and the retina. It contains a gel like material or fluid which fills this space. It is called ‘the vitreous humour’.

Appendages of the eye

Eyeball AnatomyThe eyeball is attached to the various extraocular muscles which help in the movement of the eyeball. The eyeball is placed inside the bony orbit of the skull. It is protected by the eyelids which act as curtains closing and opening that is blinking and attached with help of the conjunctiva. It is also protected by the tears whose production and drainage is via lacrimal apparatus that would be discussed in detail in the separate post. Thus the following structures are the appendages of the eyeball which help it to thrive, support and mainly protect from the external environment:

1. Extraocular muscles

2. Lacrimal apparatus

3. Eyelids and eyebrows

4. Conjunctiva

Clinical anatomy of the eye

External Eyelid Structures

Blood supply of the eye

All the arteries of the eye in man are branches of Ophthalmic artery which is a branch of internal carotid artery.

Venous supply: Most of the blood is drained to the cavernous sinus via the ophthalmic veins.

The superior ophthalmic vein communicates with the angular vein at the root of the nose.

The inferior ophthalmic vein anastomoses with the pterygoid plexus.

Nerve supply

Sensory supply: Optic nerve, ophthalmic division of trigeminal nerve, some of the lacrimal and nasociliary branches.

Motor supply: Oculomotor nerve, trochlear nerve, abducens nerve.

Parasympathetic supply: Edinger Westphal nucleus which synapses with short ciliary nerves in the ciliary ganglion and accompanies the third cranial nerve that is the oculomotor nerve.

Sympathetic supply: T1 fibres which synapse in superior ciliary ganglion.

Reference

1. A.K Khurana, MD “Comprehensive Ophthalmology” (New Age International Publisher, Inc 2012) Page no. 3-4.

2. Parsons, Disease of the Eye (Elsevier, 21 Edition), Page no. 5-16.