Sclera

The sclera is the opaque, white outer membrane that covers approximately five-sixths of the eye’s surface. Together with the transparent cornea, it forms the outermost layer of your eyeball. Whilst the cornea is located at the front of your eye and allows light to pass through, the sclera makes up the rest of the eyeball and provides strength and protection.

The sclera consists of firm connective tissue, mainly collagen fibres, which give the eye its characteristic white colour. The tissue is relatively thick, approximately half a millimetre to one millimetre, and forms a protective shell around the delicate internal structures such as the retina, the lens and the vitreous humour. At the back of the eye, the optic nerve passes through the sclera to the outside via an opening called the lamina cribrosa.

On the outside, the sclera is covered by the conjunctiva, a thin mucous membrane that also lines the inside of your eyelids. On the inside, the sclera borders the choroid, the middle layer of the eye wall which contains, amongst other things, the iris and the ciliary body. The sclera itself contains few blood vessels, which is why the tissue retains its white colour.

The firmness of the sclera is important for intraocular pressure: the tissue maintains the shape of the eyeball, even when the pressure inside the eye fluctuates slightly. In some conditions, or as a result of ageing, the sclera may become thinner or discoloured, but in most people the white of the eye remains firm and white throughout their lives.

Function and importance of the sclera

The sclera fulfils a number of important functions in your eye. Firstly, it gives the eyeball its spherical shape and ensures that the eye retains its structure, even during movement or changes in pressure. Secondly, it protects the delicate internal structures from external mechanical damage. Thirdly, the sclera provides attachment points for the six eye muscles that enable your eye to move in all directions.

Colour and appearance of the white of the eye

In most people, the sclera is bright white, but the shade can vary slightly. In babies and young children, the white of the eye can sometimes have a slight bluish tinge because the sclera is even thinner and the dark choroid underneath shows through slightly. In older people, on the other hand, the white of the eye may take on a slightly yellowish tinge due to the build-up of fat or the ageing of the connective tissue. A sudden change in colour, such as a yellow tinge across the entire white of the eye, may indicate a condition elsewhere in the body and should be assessed by your GP or ophthalmologist.

Red eyes and the sclera

Although the sclera itself contains few blood vessels, the conjunctiva lies directly on top of it. When the blood vessels in the conjunctiva are dilated – for example, due to tiredness, dry air, irritation or mild inflammation – this appears as redness on the white of the eye. The sclera itself is not affected, but the redness is visible on the white surface. If your symptoms persist or you are in pain, it is advisable to contact your optician, optometrist or ophthalmologist.

Contact lenses and the sclera

Soft contact lenses cover the cornea and rest with their edge on the edge of the sclera, just beyond the junction between the cornea and the white of the eye. Hard lenses are smaller and rest mainly on the cornea, although here too the edge of the lens may come into contact with the edge of the sclera. A good fit and proper care of your lenses are important to prevent irritation of the eye’s surface. If you have any questions about your lenses or the comfort of your eyes, please discuss this with your optician or contact lens specialist.