Fibrous and vascular layers of the eyeball
The eyeball wall is built from a tough fibrous outer coat (the white sclera and clear cornea), a vascular middle layer rich in blood vessels and the light-detecting retina inside.
Remember
From outside to inside, the wall is fibrous (sclera and cornea), then vascular (choroid, ciliary body and iris), then neural (the retina).
The wall of the eyeball is built in three layers, and each layer's name tells you its main tissue: fibrous on the outside, vascular in the middle and neural, the light-detecting layer, on the inside. The outer layer is fibrous, a tough protective coat. It is formed by the sclera, which is the white of the eye, and by the cornea, the clear window at the front that bends light. The two are continuous with each other. The sclera also gives the extraocular muscles (the muscles that move the eye) a firm anchor.
The middle layer is vascular, which means it is rich in blood vessels. It has three parts. The choroid is a layer of connective tissue and blood vessels that nourishes (feeds) the outer retina. The iris is the coloured ring around the pupil, and by changing the pupil's size it controls how much light enters the eye. Its smooth muscle fibres narrow or widen the pupil. The ciliary body is a ring of tissue behind the iris that changes the shape of the lens, the structure behind the pupil that focuses light.
The innermost layer is the retina, which contains the light-detecting cells, the rods and cones. It is tempting to place the iris in the outer coat because it is visible from outside, but it belongs to the middle layer.
Key facts
- Outer layer
- fibrous, formed by the sclera and the cornea, which are continuous with each other.
- Sclera
- the tough white coat that gives the eye its shape and anchors the muscles that move the eye.
- Cornea
- the clear window at the front that bends light as it enters the eye.
- Middle layer
- vascular, made up of the choroid, the ciliary body and the iris.
- Iris
- the coloured ring whose smooth muscle changes the size of the pupil.
- Ciliary body
- controls the shape of the lens.
- Inner layer
- the retina, which contains the light-detecting cells.
Exam traps
- The iris is visible from the front of the eye, but that does not put it in the outer fibrous coat: it belongs to the vascular middle layer.
- The choroid lies beneath the sclera, yet it is not part of the fibrous coat: it nourishes the outer retina from the vascular middle layer.
- The outer part of the retina is nourished by the choroid, but the retina itself is the inner layer, not part of the vascular layer.
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