Anatomy: Internal structures
In simple terms, there are 3 distinguishable tunics (or layers of tissues) making up the wall of the eyeball :
The 3 tunics of an ox eye (posterior)
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- Fibrous coat --- cornea, sclera
- Vascular coat --- choroid, ciliary body, iris
- Retina
Enclosed by the 3 tunics are the lens, aqueous humour and vitreous body. Together with the cornea, they form the dioptric media ----- light is refracted by these transparent structures before they reach the retina.
- Cornea
- Lens
- Aqueous humour & Vitreous body
The 3 Tunics
The anterior 1/6 of the fibrous coat is the cornea, the posterior 5/6 is the sclera. The cornea is structurally continuous with the sclera at the corneoscleral junction.
Light passing through the transparent cornea
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- diameter: ~10.5mm to 12mm
- thickness: 1mm
- 5 layers of living cells: epithelium (outer lining), Bowman's membrane, tough stroma, Descemet's membrane, mesothelium (inner lining)
- transparent, but it will turn cloudy if it is dry, so the moistening effect of tears is needed
- more convex than sclera, very important for refracting light onto retina (see image formation)
- with no blood vessels flowing through
- absorb food and oxygen from aqueous humour
- with nerve endings (we automatically close our eyelids when our cornea is stimulated)
- thickness < 1mm
- the outermost, opaque part of the eye. i.e. the white part we see in between our eyelids.
- tough with collagen fibres, so that it can protect the delicate structures within, provide a surface for anchorage of extrinsic muscles and help maintain the shape of our eyeball
- the optic nerve attaches to the posterior of the sclera
Patients who have liver disease may have a yellow sclera because of raised level of bilirubin ( yellowish bile pigment) in blood.
The vascular coat (or uvea ) consists of the choroid, ciliary body and iris in continuation structurally.
- sandwiched between the outer sclera and the inner retina.
- opaque and deeply pigmented with melanin to absorb excessive light, else internal reflection would form multiple images on the retina.
- contains a blood vessels network to supply oxygen and food to other parts of the eye, especially to the retina. It is less vascular where the retina is thin.
Ciliary body:
The ciliary body is made up of ciliary muscles and ciliary processes.
- Ciliary muscles are the thickenings around the edge of the choroid. The antagonistic action of the circular and radial ciliary muscles is responsible for eye accommodation. It holds the lens in place by suspensory ligaments called Zonular ligaments.
- Ciliary processes are short, black tissues arranged radially. They secrete aqueous humour.
- the opague ring of tissue visible through the cornea, in front of the lens
- made up of connective tissues and muscles with a circular opening, called the pupil.
- Similar to the ciliary body, the muscles are of two types ---- radial and circular. They change the pupil size to control the amount of light entering the eye (i.e. pupillary response). This can be compared to the diaphragm controlling the aperture size in a camera. (see comparison between camera & human eye).
- pigmentation to give a characteristic colour. A brown iris has the most pigment, a blue iris has the least pigment. Blue is due to diffraction.
- The retina is the innermost layer of the 3 tunics. It lines about 2/3 of the choroid layer.
- It is made up of many layers, one of which is a layer of photoreceptors. There are about 125 to 150 millions of them on our retina.
- Photoreceptors are light sensitive. There are 2 types --- rods and cones, so named because of their appearance. They receive light and convert them into electrical signals and transmit them via the optic nerve to the brain.
- There are 2 special areas on the retina --- the macula lutea and the blind spot.
- Macula lutea lies on the central axis of our eye. It has a high concentration of cones, so the visual acuity there is great. It is also important for colour vision.
The fovea in the macula lutea is seen as a tiny pit. Cones are most densely packed there. No rods are present. When we focus on one particular object, our eye will move such that its image falls onto the fovea.
- The blind spot (or optic disc) is the place where the nerve fibres converge to form the optic nerve to leave the eye. It has no photoreceptors and so no image can be formed in that area. The reason why we do not notice this blind spot when we see is because we have 2 eyes with overlapping fields of vision. Besides, we do not specifically focus objects onto the blind spot.
- The retina can come off in some people --- retinal detachment.
Dioptric Media
- The lens is a transparent crystalline biconvex structure suspended from the ciliary body by threadlike ligaments called zonular ligaments.
- It is made up of an elastic capsule, a layer of epithelium and lens fibres. It may be considered as many layers of tissue. The lens is not of uniform hardness. The central harder part is called the nucleus, while the periphery part is called the cortex.
- It is elastic and its shape can be changed during accommodation through changing the tension in the ligaments by ciliary muscles.
- On aging, our lens will become less elastic, making it more difficult to focus accurately. This is called presbyopia.
- Clouding of the lens is called cataract.
Aqueous humour:
- It is located in the anterior and posterior chamber, i.e. the space in front of the lens and behind the cornea. (fig)
- It is fluid-like, a solution of mineral salts, sugars and proteins.
- It is secreted by ciliary processes , goes into the posterior chamber, anterior chamber and is then drained away by the trabecular meshwork near the junction of the iris and cornea. It then flows into the Canal of Schlemm out of the eye.
Vitreous body:
- It is the large bulk of tissue occupying the posterior 2/3 of the eyeball.
- It is located in the vitreous cavity behind the lens.
- It is gel-like made up of hyaluronic acid and is supported by collagen fibres.
- A hyaloid canal exists in the vitreous body, linking the optic nerve with the lens. It assists the transportation of food to the lens.
Both the aqueous humour and vitreous body are transparent, so they help refract light onto the retina. They are also important in providing intraocular pressure to maintain the shape of the eye. However, too much pressure will cause eye problems e.g. Glaucoma. Therefore, it is essential for the drainage system of the aqueous humour to function properly.
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Photograph of cornea is taken from Biology- a modern approach 2 (courtesy of Aristo Educational Press Ltd, HK).
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