The Outer Layer of the

The anterior one-sixth of the fibrous layer of the eye is formed by the cornea. The posterior five-

sixths are formed by the sclera and lamina cribrosa. The cornea is transparent, whereas the sclera, which is continuous within it, is white. The junction of cornea and sclera is known as the limbus. The cornea has five layers anteroposterior^ (Figure 2.3):

1. Epithelium and its basement membrane -stratified squamous type of epithelium with five to six cell layers of regular arrangement.

2. Bowman's layer - homogeneous sheet of modified stroma.

3. Stroma - consists of approximately 90% of total corneal thickness. Consists of lamellae of collagen, cells and ground substance.

4. Descemet's membrane - the basement membrane of the endothelium.

5. Endothelium - a single layer of cells lining the inner surface of Descemet's membrane.

- Epithelium Bowman's membrane

- Stroma

/Descemet's membrane ~ Endothelium

Figure 2.3. The cornea. EQ

- Epithelium Bowman's membrane

- Stroma

Figure 2.3. The cornea. EQ

In the region of the limbus, the epithelium on the outer surface of the cornea becomes continuous with that of the conjunctiva, a thin, loose transparent nonkeratinising mucous membrane that covers the anterior part of the sclera, from which it is separated by loose connective tissue. Above and below, the conjunctiva is reflected onto the inner surface of the upper and lower lids. This mucous membrane, therefore, lines the posterior surface of the eyelids and there is a mucocutaneous junction on the lid margin. Although the conjunctiva is continuous, it can be divided descriptively into three parts: palpebral (tarsal), bulbar and fornix.

The sclera consists of irregular lamellae of collagen fibres. Posteriorly, the external two-thirds of the sclera become continuous with the dural sheath of the optic nerve, while the inner one-third becomes the lamina cribrosa - the fenestrated layer of dense collagen fibres through which the nerve fibres pass from the retina to the optic nerve. The sclera is thickest posteriorly and thinnest beneath the insertions of the recti muscles. There is a layer of loose connective tissue deep to the conjunctiva, overlying the sclera, called the episclera.

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