Cross Section

Here is the classical cross section of the human eye. It is often imagined that this is a vertical cross section but it actually shows an eye cut horizontally; imagine the eye cut across the top so you are looking down into the eye. The eye is about the same size as a ping-pong ball. The various structures from the cornea at the front of the eye right through to the retina and optic nerve at the back of the eye, can be observed.

External Eye Structures

This view refers to the external structures of the eye that can be seen when observing another person. The main structures are the eyelids, the eyelashes, the pupil and iris. The eyelids have the function of protecting and lubricating the eye, wiping tears across the eye with every blink and, if something gets in the eye, producing excess tears to wash the foreign object away.

Towards the inside corner of the lower eyelid is a small opening called the punctum (tear duct), which serves as a drain for tears. A similar, but smaller, opening is located on the upper lid. As the tear layer refreshes itself, the pumping action of the eyelids carries tears to the punctum where they eventually drain into the nose.

Cornea

In front of the coloured iris is the transparent cornea - one of the few structures in the body that has no blood supply. The cornea is the ‘window’ to the eye and cannot be seen from straight ahead - we only see the ‘black’ pupil and the coloured iris. It is very sensitive having a complex system of nerves running through it. The cornea is where we place contact lenses and is very easily damaged. However cell production is several times faster than that of the skin and as a result will normally repair itself within 24 hours.

Iris

This is the coloured part of the eye. Whilst it was once imagined that the colour of the eyes consists of  different colours, in fact it is a varying level of melanin - the brown pigmentation - that creates the variety of different eye colours seen in the population. The main function of the iris is to create the pupil. The pupil isn’t a structure as such - it is simply the hole or gap in the middle of the iris. The iris works like the shutter on a camera, controlling the amount of light that enters the eye. In bright light it constricts making the pupils smaller and reducing the amount of light that enters the eye. Conversely, moving into a darker environment, the pupils will dilate making them larger and thus helping more light into the eye to improve vision.

Sclera

The sclera is referred to as the white of the eye and is a very tough tissue which helps the eye keep its eyeball shape.

Conjunctiva

Lying across the sclera at the front of the eye is a tissue known as the conjunctiva, which can be considered like a clear skin across the white of the eye. The conjunctiva contains most of the blood vessels that can be seen in somebody else’s eye. When an eye is irritated (such as due to an allergic or toxic reaction) or gets an infection, it is in fact the vessels in the conjunctiva that we can see dilating making the eye appear red.

The conjunctiva actually runs from the edge of the cornea across the sclera, under the lids (Bulbar conjunctiva), then back down the underside of the eyelids (Palpebral conjunctiva). If you imagine the conjunctiva runs up under the eyelids in all directions, 360 degrees around the cornea - this creates what can best be described as a sack and it is this conjunctival sack that prevents a popular contact lens myth - the loss of contact lenses around the back of the head.

Crystalline lens

The crystalline lens is often imagined as the focusing part of the eye. In fact, this is partially true as, along with the cornea, it does help to focus images providing one third of the eyes’ total power. Although it only provides about one third of the power of the eye, it has the ability to change its shape and it is this ability - known as accommodation - that actually enables us to focus at different distances. In its relaxed state the eye will be focused at distance, but it can make itself more convex (shorter and fatter) to be more powerful and refocus near objects onto the retina.

The structure of the crystalline lens is something like an onion: it has layers which are added to it as we age. This layering of extra tissue on the crystalline lens makes it harden with age. It eventually leads to the condition we know as presbyopia where the eye struggles to focus clearly on near objects without the help of corrective lenses.

Retina

The retina at the back of the eye is like the film in the camera and helps to create the image that we see. It has receptors that convert light into chemical and electrical signals that are then passed onto the brain. There are a number of light sensitive cells, which fall into two types - the rods and the cones. There are more rods in the eye than cones and they provide us with black and white images, enable us to see at lower illumination and also provide us with our peripheral vision. The cones tend to be positioned more in the centre of the retina and provide colour vision. On the retina just to the side of the optic nerve is the area called the macula (or fovea) - the most sensitive point on the retina where most of the cells are collected. The macula gives us our very sharp detailed central vision.

Optic Nerve and Optic Disc

The optic nerve essentially takes the electrical and chemical messages from the retina into the brain. Due to the fact that the optic disc simply carries blood vessels and nerves from the retina into the appropriate areas of the brain, there are actually no receptors here for vision. This is why we have a blind spot and this is one and the same as the optic disc.

Tear Film Composition

The healthy tear film maintains visual clarity and ocular comfort. It is widely regarded as being composed of three layers:

External lipid layer - formed from oily substances secreted primarily by the meibomian glands in the eye-lids, preventing tear spillage and evaporation;

Aqueous layer (bulk of the film) - a saline solution produced by lacrimal glands; provides lubrication, protection and a source of nutrients for the ocular surface;

Mucus layer (sandwiched between the ocular surface and the aqueous layer) - partially dissolved in the aqueous and provides the bond to the ocular surface.