Diagram source: www.medicenet.com	Image source: www.intranet.tdmu.edu.ua

Introduction

The eye serves a very important function- the sense of light. Human beings have two slightly protruding spherical eyes located in the socket. The eyeball has approximately 24 mm in diameter.

Layers of The Eye

The eye is made up of three coats. The outermost layers are sclera and cornea. The middle layer, known as vascular tunic consists of choroid, cilliary body and iris. The innermost is retina.

Sclera is the part of the eye commonly known as the ‘white’. It forms the supporting wall of the eye ball and is continuous anteriorly with clear cornea. It also acts as a tough protection from injury and provide attachment for extra ocular muscle to move the eye. It looks bluish on infant due to its thin layer and become yellowish on elderly because of pigmentation.

Choroid is the middle vascularised layer between the retina and the sclera. It provides nutrition to the retina.

Ciliary body connects the choroid to the iris. It contains the ciliary muscle and ciliary epithelium. The contraction and relaxation of ciliary muscle will change the shape of the lens when the eyes focus on something. The ciliary epithelium produces aqueous humour into the anterior chamber.

Iris is the coloured part of the eye, located between cornea and lens. In the middle of the iris is an opening, called pupil. Sphincter and dilator papillae muscle responsible for controlling the diameter and size of the pupil and thus amount of light reaching the retina. The colour of the iris gives the eye its colour. Blue eyes have less pigment cells compared to brown eyes. Iris also divides the anterior segment into anterior chamber and posterior chamber that contain aqueous humour.

Chambers of the Eyeball	Optical Components of The Eye
Anterior chamber: The space between cornea and iris Posterior chamber: The space between iris and lens Vitreous chamber: The space between lens and retina	Diagram source: www.animations.physics.unsw.edu.au

The optical components are transparent part that admit, bend and focus the light onto retinal cells to form image. These components are cornea, aqueous humour, crystalline lens and vitreous body.

Cornea is a clear and transparent membrane that consists of five layers; epithelium, Bowman membrane, stroma, Descemet membrane and endothelium. It does not contain blood vessel and pigments but full of nerve that makes the cornea very sensitive. Tears in front of cornea is important to maintain the structure. The convex shape of the anterior cornea responsible to focus the light onto retina.

Iris divides the anterior segment into anterior and posterior chamber. The two chambers contain of aqueous humour produced by ciliary body. The aqueous humour flows through the narrow cleft between the front of the lens and the back of iris, to escape through the pupil into the anterior chamber and to drain out of the eye via Trabecular Meshwork. From here, it drains into Schlemn’s canal. This continuous flow is very important to maintain the intraocular pressure between 10- 20mmHg and inflates the globe of the eye. Besides, it supplies nutrition such as Oxygen, electrolyte, glucose and vitamin C to the avascular cornea and crystalline lens.

The crystalline lens is suspended by ciliary muscle behind the coloured iris. Its biconvex and transparent structure helps to refract and focus the light on the retina. The lens is flexible and by changing shape, allows for clear vision at a range of distance, known as accommodation process. A loss of the flexibility in focus is inevitable as we get older due to stiffening of the lens, and this is termed presbyopia. The crystalline lens grows through life and the ageing lens can no longer maintain the chemical processes which keep the lens clear. As a result, the lens starts to become cloudy/ cataract.

Vitreous body contains an aqueous component, called vitreous humour. It is a colourless gel-like substance that occupies 4/5 the posterior space of the eye. It contains no blood vessels and 98% of its volume is water. The fluid is substantial enough to fill the eye and gives its spherical shape. This structure is in contact with retina and helps to keep it in place by pressing it against the choroid. Besides, it helps in transmitting light onto the retina.

Neural Components of The Eye

Diagram source: www.antranik.org/the-eye-and-vision

Pupil is an opening in the centre of the iris that allows the light to strike the retina. Circular muscles contract in bright condition and make the pupil size smaller. During the darker condition, radial muscles (dilator papillae) contract and will cause the pupil dilates (enlarge) to allow more lights entering the eye.

Retina is the translucent and thin innermost layer of the eye. It has ten distinct layers of cells on pigment epithelium that attached to the choroid. The light-sensitive layer known as rod-cones. Macula is the centre of the retina with the highest amount of cones and provides the greatest resolving power of the eye. The centre of macula is called the fovea which only contains cone cells. The central retina is cone dominated and the peripheral retina is rod dominated. Cones function best under illuminated conditions and provides colour vision. Rod function primarily in dim light and important in black and white vision. The optical elements focus an image onto the retina initiating series of chemical and electrical events. Nerve fibres send electrical signals to the brain and interprets the signal as visual image.

Optic nerve composed of retinal ganglion cell axons and gill cells. It transmits visual information from retina to the brain, called visual cortex.

Accessory Structures of The Eye

Source: www.ohioeyecareconsultants.com/eyeinfo/dryeyes	Source: www.slideshare.net/sivatejachalla/anatomy-of-the-lacrimal-apparatus

There are accessory organs of the eye with different functions. They are eyebrows, eyelids, eyelashes, conjunctiva, lacrimal apparatus and extra ocular muscles.

Eyebrows protect the eyes by providing shade and directing the movement of perspiration from the head away from eyes.

Eyelids are divided into the upper and lower eyelids. The two eyelids are formed by the folds of skin, muscle, tarsal plate, Meibomian gland, sweat gland and eyelashes. The eyelids function to protect the eye from excessive light and help in spreading the tears.

Eyelashes are the fringe of hairs that extend across the edge of the upper and lower eyelid. They function to protect the eye from the entrance of foreign substances. They enhance touch and promote the blink reflex. Meibomian glands are glands that are arranged vertically within the eyelid near the lashes. The force of an eyelid blink causes oil to be excreted onto the posterior lid margin that helps prevent rapid tear evaporation.

Conjunctiva is a thin and transparent mucus membrane which covers the inner aspect of eyelid and anterior surface of the eye except cornea. Limbus is the junction between the cornea and conjunctiva. Conjunctiva contains glands that produce mucin in tears. It also provides nutrient to the cornea.

Lacrimal apparatus responsible for production of tears and its drainage. It consists of gland and kanalikuli lacrimal, nasolacrimal sac and lacrimal duct. Lacrimal gland situated at top of eye socket. Tears produced here and flow through a few small vessels. There are two tears duct at the top and bottom of eyelid situated nearer to nose called kanalikuli top and bottom which tears flow through it then go to lacrimal sac. Caruncle is a part which divided these two ducts. Nasolacrimal duct situated at the bottom of lacrimal sac where tears flow down and ended at the nose cavity.

Tears is clear alkaline liquid. This structure has three distinct layers; lipid, aqueous and mucin. The sufficient secretion of tears is important to clean and lubricate the cornea and conjunctiva in response to an irritation of the eyes. If excessive tear produced, instead of all the tears draining through the nasolacrimal system, they overflow onto the face. The function of tears are to clean and lubricate the cornea surface so that blinking process is much more easier. It also supplies nutrient and oxygen to the cornea. Besides, lysozyme and antibodies in the tears also formed an important immune system.

There are six extra ocular muscles attached to the eye. Muscles coordination for both eyes allow them to move symmetrically.

References

1. Richard S. Snell & Michael A. Lemp. (1998). Clinical Anatomy of the Eye. USA. Blackwell Science
2. Jack J. Kanski. (2000). Clinical Ophthalmology Fourth Edition. Oxford. Butterworth-Heinemann